THEQUANTUMAGE

HOWTHEPHYSICSOFTHEVERYSMALLHASTRANSFORMEDOURLIVES

BRIANCLEGG

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ContentsAcknowledgements

Introduction

1.Enterthequantum2.Quantumnature3.Theelectron’srealm4.QED5.Lightandmagic6.Superbeams7.Makinglightwork8.Resistanceisfutile9.Floatingtrainsandwell-chilledSQUIDs10.Spookyentanglement11.Frombittoqubit12.It’salive!13.Aquantumuniverse

Index

Abouttheauthor

Science writer Brian Clegg studied physics at Cambridge University andspecialisesinmakingthestrangestaspectsoftheuniverse–frominfinitytotimetravel and quantum theory – accessible to the general reader. He is editor ofwww.popularscience.co.uk and a Fellow of the Royal Society of Arts. Hisprevious books include Inflight Science,Build Your Own TimeMachine, TheUniverseInsideYou,DiceWorldandIntroducingInfinity:AGraphicGuide.

www.brianclegg.net

ForGillian,ChelseaandRebecca

Acknowledgements

Withthanksasalwaystomyeditor,DuncanHeath,forhishelpandsupport,andtoallthosewhohaveprovidedmewithinformationandassistance–youknowwhoyouare.One person Iwould like tomention by name is the lateRichard Feynman,

whose books enthralledme andwho turned quantum theory from a confusingmysterytoanexcitingchallenge.

Introduction

Thechancesarethatmostofthetimeyouwereatschoolyourscienceteacherslied toyou.Muchof thescience,andspecifically thephysics, they taughtyouwas rooted in theVictorianage (which isquiteprobablywhysomanypeoplefind school science dull). Quantum theory, special and general relativity,arguably themost significant fundamentals of physics, were developed in the20thcenturyandyet theseare largely ignored in schools, inpartbecause theyareconsideredtoo‘difficult’andinpartbecausemanyoftheteachershavelittleidea about these subjects themselves. And that’s a terrible pity, when youconsiderthatintermsofimpactonyoureverydaylife,oneofthesetwosubjectsisquitepossiblythemostimportantbitofscientificknowledgethereis.Relativityisfascinatingandoftentrulymind-boggling,butwiththeexception

ofgravity,whichIadmit isratheruseful, ithasfewapplicationsthat influenceourexperience.GPSsatelliteshavetobecorrectedforbothspecialandgeneralrelativity, but that’s about it, because the ‘classical’ physics that predatesEinstein’s work is a very close approximation to what’s observed unless youtravelatclosetothespeedoflight,andisgoodenoughtodealwitheverythingfromtheaccelerationofacartoplanningaMoonlaunch.Butquantumphysicsis entirely different.While it too is fascinating andmind-boggling, it also liesbehind everything. All the objects we see and touch and use are made up ofquantumparticles.Asisthelightweusetoseethoseobjects.Asareyou.Asisthe Sun and all the other stars.What’s more, the process that fuels the Sun,nuclearfusion,dependsonquantumphysicstowork.That makes the subject interesting in its own right, something you really

shouldhave studied at school; but there is farmore, becausequantum sciencedoesn’tjustunderliethebasicbuildingblocksofphysics:itisthereineverydaypractical applications all aroundyou. It has been estimated that around35percent ofGDP in advanced countries comes from technology thatmakes use ofquantumphysicsinanactivefashion,notjustintheatomsthatmakeitup.Thishasnotalwaysbeenthecase–wehaveundergonearevolutionthatjusthasn’tbeengivenanappropriatelabelyet.

Thisisnotthefirsttimethathumanbeingshaveexperiencedmajorchangesinthewaytheyliveasaresultofthedevelopmentoftechnology.Historiansoftenhighlight this by devising a technological ‘age’. So, for instance, we had thestone,bronzeandironagesasthesenewlyworkablematerialsmadeitpossibletoproducemoreversatileandeffective toolsandproducts. In the19thcenturyweenteredthesteamage,whenappliedthermodynamicstransformedourabilitytoproducepower,movingusfromdependingonthebasiceffortofanimalsandtheunpredictableforceofwindandwatertothecontrolledmightofsteam.Andthough it isyet tobe formally recognisedassuch,wearenowin thequantumage.Itisn’tentirelyclearwhenthiserabegan.Itispossibletoarguethattheuseof

currentelectricitywas the firstuseof truequantum technology,as the flowofelectricity through conductors is a quantumprocess, thoughof course noneofthe electrical pioneerswere aware that thiswas the case. If that is a little tooconcealed a usage to be a revolution, then there can be no doubt that theintroductionof electronics, a technology thatmakesconscioususeofquantumeffects,meantthatwehadmovedintoanewphaseoftheworld.Sincethenwehavepiledonallsortsofexplicitlyquantumdevicesfromtheubiquitouslasertothe MRI scanner. Every time we use a mobile phone, watch TV, use asupermarketcheckoutortakeaphotographwearemakinguseofsophisticatedquantumeffects.Withoutquantumphysicstherewouldbenomatter,nolight,noSun…and

mostimportant,noiPhones.I’ve already used the word ‘quantum’ thirteen times, not counting the title

pages and cover. So it makes sense to begin by getting a feel for what this‘quantum’wordmeansandtoexploretheweirdandwonderfulsciencethatliesbehindit.

CHAPTER1

Enterthequantum

Until the 20th century it was assumed that matter was much the same onwhateverscaleyoulookedatit.WhenbackinAncientGreektimesagroupofphilosophersimaginedwhatwouldhappenifyoucutsomethingupintosmallerandsmallerpiecesuntilyoureachedapiecethatwasuncuttable(atomos),theyenvisaged that atoms would be just smaller versions of what we observe. Acheeseatom,for instance,wouldbenodifferent,except inscale, toablockofcheese. But quantum theory turned our view on its head. As we explore theworld of the very small, such as photons of light, electrons and our modernunderstanding of atoms, they behave like nothing we can directly experiencewithoursenses.

Aparadigmshift

Realisingtheverydifferentrealityat thequantumlevelwaswhathistoriansofscienceliketogivethepompousterma‘paradigmshift’.Suddenly,thewaythatscientistslookedattheworldbecamedifferent.Beforethequantumrevolutionitwas assumed that atoms (if they existed at all –many scientists didn’t reallybelieveinthembeforethe20thcentury)werejustliketinylittleballsofthestufftheymade up.Quantumphysics showed that they behaved soweirdly that anatomof,say,carbonhastobetreatedasifitissomethingtotallydifferenttoapieceofgraphiteordiamond–andyetallthatisinsidethatlumpofgraphiteordiamond is a collection of these carbon atoms. The behaviour of quantumparticles is strange indeed, but that does not mean that it is unapproachablewithoutadoctorateinphysics.Iquitehappilyteachthebasicsofquantumtheoryto ten-year-olds.Not themaths, but youdon’t needmathematics to appreciatewhat’s going on.You just need the ability to suspend your disbelief.Becausequantumparticlesrefusetobehavethewayyou’dexpect.As the great 20th-century quantum physicist Richard Feynman (we’ll meet

himagainindetailbeforelong)saidinapubliclecture:‘[Y]outhinkI’mgoingtoexplainittoyousoyoucanunderstandit?No,you’renotgoingtobeableto

understandit.Why, then,amIgoingtobotheryouwithall this?Whyareyougoingtosithereallthistime,whenyouwon’tbeabletounderstandwhatIamgoingtosay?Itismytasktopersuadeyounottoturnawaybecauseyoudon’tunderstand it.You see,myphysics students don’t understand it either.This isbecauseIdon’tunderstandit.Nobodydoes.’Itmight seem thatFeynmanhad foundagoodway to turnoffhis audience

beforehehadstartedbytellingthemthattheywouldn’tunderstandhistalk.Andsurely it’s ridiculous for me to suggest I can teach this stuff to ten-year-oldswhenthegreatFeynmansaidhedidn’tunderstandit?Buthewentontoexplainwhathemeant. It’snot thathis audiencewouldn’tbe able tounderstandwhattookplace,whatquantumphysicsdescribed.It’sjustthatnooneknowswhy ithappens theway itdoes.Andbecausewhat itdoesdefiescommonsense, thiscancauseusproblems. In factquantum theory is arguablyeasier for ten-year-olds to accept than adults, which is one of the reasons I think that it (andrelativity)shouldbetaughtinjuniorschool.Butthat’sthesubjectofadifferentbook.AsFeynmanwenton tosay: ‘I’mgoing todescribe toyouhowNature is–

andifyoudon’tlikeit,that’sgoingtogetinthewayofyourunderstandingit…Thetheoryofquantumelectrodynamics[thetheorygoverningtheinteractionoflightandmatter]describesNatureasabsurdfromthepointofviewofcommonsense.Anditagreesfullywithexperiment.SoIhopeyoucanacceptNatureasshe is–absurd.’Weneed toacceptandembrace theviewpointofanunlikelyenthusiastforthesubject, thenovelistD.H.Lawrence,whocommentedthathelikedquantumtheorybecausehedidn’tunderstandit.

Theshockofthenew

Partofthereasonthatquantumphysicsprovedsuchashocking,seismicshiftisthat around the start of the 20th century, scientists were, to be honest, rathersmugabout their level of understanding– an attitude theyhadprobablyneverhad before, and certainly should never have had since (though you can see itcreeping in with some modern scientists). The hubris of the scientificestablishmentisprobablybestsummedupbythewordsofaleadingphysicistofthe time,WilliamThomson,LordKelvin. In1900hecommented,nodoubt inrounded,selfsatisfiedtones:‘Thereisnothingnewtobediscoveredinphysics.All that remains ismoreandmoreprecisemeasurement.’Asa remark thathewouldcometobitterlyregretthisissurelyuptherewiththefamousclangerofThomas J.WatsonSnr,who as chairman of IBMmade the impressively non-

prophetic remark in 1943: ‘I think there is a world market for maybe fivecomputers.’Within months of Kelvin’s pronouncement, his certainty was being

underminedbyaGermanphysicistcalledMaxPlanck.Planckwastryingtoironout a small irritant toKelvin’s supposed ‘nothing new’ – a technical problemthatwasgiven the impressivenickname ‘theultraviolet catastrophe’.Wehaveallseenhowthingsgiveofflightwhentheyareheatedup.Forinstance,takeapieceofironandputitinafurnaceanditwillfirstglowred,thenyellow,beforegetting towhiteheat thatwillbecome tingedwithblue.The ‘catastrophe’ thatthephysicsofthedaypredictedwasthatthepowerofthelightemittedbyahotbody should be proportional to the square of the frequency of that light. Thismeant that even at room temperature, everything should be glowing blue andblastingoutevenmoreultravioletlight.Thiswasbothevidentlynothappeningandimpossible.Tofixtheproblem,Planckcheated.Heimaginedthatlightcouldnotbegiven

offinwhatever-sizedamountsyoulike,asyouwouldexpectifitwereawave.Waves could come in any size or wavelength – theywere infinitely variable,rather than being broken into discrete components. (And everyone knew thatlightwasawave,justasyouweretaughtatschoolintheVictoriansciencewestillimposeonourchildren.)Instead,Planckthought,what if thelightcouldcomeoutonlyinfixed-sized

chunks?Thissortedouttheproblem.Limitlighttochunksandplugit intothemathsandyoudidn’tgettherunawayeffect.Planckwasveryclear–hedidn’tthinklightactuallydidcomeinchunks(or‘quanta’ashecalledthem,thepluralof the Latin quantum which roughlymeans ‘howmuch’), but it was a usefultricktomakethemathswork.Whythiswasthecase,hehadnoidea,asheknewthatlightwasawavebecausetherewereplentyofexperimentstoproveit.

MrYoung’sexperimentPerhaps the best-known example of these experiments, and onewewill comebacktoanumberoftimes,isYoung’sslits,themasterpieceofpolymathThomasYoung (1773–1829). This well-off medical doctor and amateur scientist wasobviouslyremarkablefromanearlyage.Hetaughthimselftoreadwhenhewastwo,somethinghisparentsdiscoveredonlywhenheaskedforhelpwithsomeofthelongerwordsintheBible.BythetimehewasthirteenhewasafluentreaderinGreek,Latin,Hebrew,ItalianandFrench.ThiswasanaturalprecursortooneofYoung’s impressiveclaimstofame–hemadethefirstpartial translationofEgyptianhieroglyphs.Buthislanguageabilitiesdon’treflectthebreadthofhis

interests,fromdiscoveringtheconceptofelasticityinengineeringtoproducingmortalitytablestohelpinsurancecompaniessettheirpremiums.Hisbigbreakthroughinunderstandinglightcamewhilestudyingtheeffectof

temperatureon theformationofdewdrops– therereallywasnothing innaturethatdidn’t interest thisman.Whilewatchingtheeffectofcandlelightonafinemist of water droplets he discovered that they produced a series of colouredringswhenthelightthenfellonawhitescreen.Youngsuspectedthatthiseffectwascausedbyinteractionsbetweenwavesoflight,provingthewavenaturethatChristiaan Huygens had proclaimed back in Newton’s time. By 1801, Youngwas ready to prove this with an experiment that has been the definitivedemonstrationthatlightisawaveeversince.Youngproducedasharpbeamoflightusingaslitinapieceofcardandshone

this lightonto twoparallelslits,close together inanotherpieceofcard,finallylettingtheresultfallonascreenbehind.Youmightexpectthateachslitwouldproject a bright line on the screen, but what Young observedwas a series ofalternatingdarkandlightbands.ToYoungthiswasclearevidencethatlightwasawave.Thewavesfromthetwoslitswereinterferingwitheachother.Whentheside-to-sideripplesinbothwaveswereheadinginthesamedirection–saybothup–at thepoint theymetthescreen, theresultwasabrightband.If thewaverippleswereheading inoppositedirections,oneupandonedown, theywouldcanceleachotheroutandproduceadarkband.Asimilareffectcanbespottedifyoudroptwostonesintostillwaterneartoeachotherandwatchhowtheripplesinteract–somewavesreinforce,somecancelout.Itisnaturalwavebehaviour.

Fig.1.Young’sslits.

ItwasthiskindofdemonstrationthatpersuadedPlanckthathisquantawerenothing more than a workaround to make the calculations match what was

observed, because light simply had to be a wave – but he was to be provedwrongbyamanwhowaslessworriedaboutconventionthantheolderPlanck,AlbertEinstein.EinsteinwastoshowthatPlanck’sideawasfarclosertorealitythan Planck would ever accept. This discrepancy in viewpoint was glaringlyobvious when Planck recommended Einstein for the Prussian Academy ofSciences in 1913. Planck requested the academy to overlook the fact thatEinsteinsometimes‘missedthetargetinhisspeculations,asforexample,inhistheoryoflightquanta…’.

TheEinsteintouch

That‘speculation’wasmadebyEinsteinin1905whenhewasayoungmanof26 (forget the white-haired icon we all know: this was a dapper youngman-about-town). For Einstein, 1905was a remarkable year in which the buddingscientist,whowas yet to achieve a doctorate andwas technically an amateur,cameupwiththeconceptofspecialrelativity,1showedhowBrownianmotion2couldbeexplained,making it clear thatatoms reallydidexist, anddevisedanexplanationforthephotoelectriceffect(seepage13)thatturnedPlanck’susefulcalculatingmethodintoamodelofreality.Einsteinwasneveronetoworrytoomuchaboutfittingexpectations.Asaboy

he struggled with the rigid nature of German schooling, getting himself areputationforbeinglazyanduncooperative.Bythe timehewassixteen,whenmoststudentshadlittlemoreontheirmindthangettingthroughtheirexamsandgetting on with the opposite sex, he decided that he could no longer toleratebeingaGermancitizen.(NotthatyoungAlbertwastheclassicgeekinfindingitdifficulttogetonwiththegirls–quitethereverse.)HopingtobecomeaSwisscitizen, Einstein applied to the exclusive Federal Institute of Technology, theEidgenössischeTechnischeHochschuleorETH, inZürich.Certainofhisownabilitiesinthesciences,Einsteintooktheentranceexam–andfailed.His problemwas a combinationof youth andvery tightly focused interests.

Einsteinhadnot seen thepointof spendingmuch timeonsubjectsoutside thesciences, but the ETH examination was designed to pick out allrounders.However, the principal of the school was impressed by young Albert andrecommended he spent a year in a Swiss secondary school to gain a moreappropriate education.Next year, Einstein applied again and got through. TheETH certainly allowedEinsteinmore flexibility to followhis dreams than therigid German schools, though his headstrong approach made the head of thephysics department, HeinrichWeber, comment to his student: ‘You’re a very

cleverboy,butyouhaveonebigfault.Youwillneverallowyourselftobetoldanything.’Aftergraduating,Einstein tried togetapostbywritingtofamousscientists,

askingthemtotakehimonasanassistant.Whenthisunlikelystrategyfailed,hetookapositionasateacher,primarilytobeabletogainSwisscitizenship,ashehad already renounced his German nationality, so was technically stateless.Soon,though,hewouldgetanotherjob,onethatwouldgivehimplentyoftimetothink.EinsteinsuccessfullyappliedforthepostofPatentOfficer(thirdclass)intheSwissPatentOfficeinBern.

Electricityfromlight

Itwaswhileworkingtherein1905thatEinsteinturnedPlanck’susefultrickintotherealfoundationofquantumtheory,writingthepaperthatwouldwinhimtheNobel Prize. The subject was the photoelectric effect, the science behind thesolar cells we see all over the place these days producing electricity fromsunlight. By the early 1900s, scientists and engineerswerewell aware of thiseffect, although at the time it was studied only in metals, rather than thesemiconductors that have made modern photoelectric cells viable. That thephotoelectriceffectoccurredwasnobigsurprise.Itwasknownthatlighthadanelectricalcomponent,soitseemedreasonablethatitmightbeabletogiveapushto electrons3 in a piece of metal and produce a small current. But there wassomethingoddaboutthewaythishappened.A couple of years earlier, the Hungarian Philipp Lenard had experimented

widelywith theeffectand found that itdidn’tmatterhowbright the lightwasthatwas shoneon themetal– theelectrons freed from themetalby lightofaparticularcolouralwayshadthesameenergy.Ifyoumoveddownthespectrumoflight,youwouldeventuallyreachacolourwherenoelectronsflowedatall,howeverbrightthelightwas.Butthisdidn’tmakeanysenseiflightwasawave.Itwas as if the sea could onlywash something away if thewaves came veryfrequently,whilevast, toweringwaveswitha lowfrequencycouldnotmoveasinglegrainofsand.Einstein realised thatPlanck’squanta,his imaginarypacketsof light,would

provideanexplanation.Iflightweremadeupofaseriesofparticles,ratherthanawave, itwould produce the effects thatwere seen.An individual particle oflight4couldknockoutanelectrononlyifithadenoughenergytodoso,andasfaras lightwasconcerned,higherenergycorresponded tobeingfurtherup thespectrum. But the outcome had no connection with the number of photons

present – the brightness of the light – as the effect was produced by aninteractionbetweenasinglephotonandanelectron.Einstein had not only turned Planck’s useful mathematical cheat into a

description of reality and explained the photoelectric effect, he had set thefoundationforthewholeofquantumphysics,atheorythat,ironically,hewouldspendmuch of his working life challenging. In less than a decade, Einstein’sconceptofthe‘real’quantumwouldbepickedupbytheyoungDanishphysicistNiels Bohr to explain a serious problemwith the atom. Because atoms reallyshouldn’tbestable.

Uncuttablematter

Aswehaveseen,theideaofatomsgoesallthewaybacktotheAncientGreeks.ItwaspickedupbyBritishchemistJohnDalton(1766–1844)asanexplanationforthenatureofelements,butitwasonlyintheearly20thcentury(encouragedby another of Einstein’s 1905 papers, the one on Brownian motion) that theconcept of the atom was taken seriously as a real thing, rather than ametaphoricalconcept.Theoriginalideaofanatomwasthatitwastheultimatedivision ofmatter – thatGreekword for uncuttable, atomos – but the BritishphysicistJosephJohnThomson(usuallyknownasJ.J.)haddiscoveredin1897thatatomscouldgiveoffasmallerparticlehecalledanelectron,whichseemedtobethesamewhateverkindofatomproducedit.Hededucedthattheelectronwasacomponentofatoms–thatatomswerecuttableafterall.Theelectronisnegativelycharged,whileatomshavenoelectricalcharge,so

there had to be something else in there, something positive to balance it out.Thomsondreamedupwhatwouldbecomeknownasthe‘plumpuddingmodel’of the atom. In this, a collection of electrons (the plums in the pudding) aresuspendedinasortofjellyofpositivecharge.OriginallyThomsonthoughtthatall themassof theatomcamefrom theelectrons–whichmeant thateven thelightest atom, hydrogen, should contain well over a thousand electrons – butlaterworksuggestedthattherewasmassinthepositivepartoftheatomtoo,andhydrogen,forexample,hadonlythesingleelectronwerecognisetoday.

Bohr’svoyageofdiscoveryWhen 25-year-old physicist Niels Bohr won a scholarship to spend a yearstudyingatomsawayfromhisnativeDenmarkhehadnodoubtwherehewantedtogo–toworkonatomswiththegreatThomson.Andsoin1911hecametoCambridge, armed with a copy of Dickens’ The Pickwick Papers and a

dictionaryinanattempttoimprovehislimitedEnglish.UnfortunatelyhegotofftoabadstartbytellingThomsonattheirfirstmeetingthatacalculationinoneofthegreatman’sbookswaswrong.RatherthancollaboratingwithThomsonashehad imagined, Bohr hardly saw the then star of Cambridge physics, spendingmost of his time allocated to his least favourite activity, undertakingexperiments.Towardstheendof1911,though,twochancemeetingschangedBohr’sfuture

andpavedthewayforthedevelopmentofquantumtheory.First,onavisittoafamily friend in Manchester, and again at a ten-course dinner in Cambridge,BohrmettheimposingNewZealandphysicistErnestRutherford,thenworkingatManchesterUniversity.Rutherfordhadrecentlyoverthrowntheplumpuddingmodelbyshowingthatmostoftheatom’smasswasconcentratedinapositive-charged lump occupying a tiny nucleus at the heart of the atom. RutherfordseemedamuchmoreattractivepersontoworkforthanThomson,andBohrwassoonheadingforManchester.There Bohr put together his first ideas that would form the basis of the

quantumatom.Itmightseemnaturaltoassumethatanatomwitha(relatively)massivenucleusandacollectionofsmallerelectronsontheoutsidewassimilarinformtoasolarsystem,with thegravitational force thatkeeps theplanets inplacereplacedbytheelectromagneticattractionbetweenthepositivelychargednucleus and the negatively charged electrons. But despite the fact that thispicture is still often employed to illustrate the atom (it’s almost impossible torestrain illustrators fromusing it), it incorporatesa fundamentalproblem. Ifanelectronweretoorbitaroundthenucleusitwouldspurtoutenergyandcollapseintothecentre,becauseanacceleratingelectricalchargegivesoffenergy–andto keep in orbit, an electron would have to accelerate. Yet it was no betterimagining that the electrons were fixed in position. There was no stableconfigurationwheretheelectronsdidn’tmove.ThispresentedBohrwithahugechallenge.Inspired by discovering reports of experiments showing that when heated,

atoms gave off light photons of distinct energies, Bohr suggested somethingradical.Yes,hedecided,theelectronscouldbeinorbits–butonlyifthoseorbitswerefixed,morelikerailwaytracksthanthefreelyvariableorbitofasatellite.And to move between two tracks required a fixed amount of energy,corresponding to absorbing or giving off a photon. Not only was light‘quantised’, sowas the structureof theatom.Anelectroncouldnotdrift fromleveltolevel,itcouldonlyjumpfromonedistinctorbittoanother.

Insidetheatom

Anatomisanamazingthing,soit isworthspendingamomentthinkingaboutwhat it appears to be like. That traditional picture of a solar system is still auseful starting point, despite the fatal flaw. To begin with, just like a solarsystem,theatomhasamassivebitatthecentreandmuchlessmassivebitsontheoutside.Ifwelookatthesimplestatom,hydrogen,ithasasinglepositivelycharged particle – a proton – as a nucleus and a single negatively chargedelectron outside of it. The proton, the nucleus, is nearly 2,000 times moremassivethantheelectron,justastheSunismuchmoremassivethantheEarth.Andlikeasolarsystem,anatomismostlymadeupofemptyspace.One of the earliest and still most effective illustrations of the amount of

emptinessinanatomisthatifyouimaginethenucleusofanatomtobethesizeofafly,thewholeatomwillbeaboutthesizeofacathedral–andapartfromthevaguepresenceoftheelectron(s)ontheoutside,alltherestisemptyspace.Nowwe need to move away from the solar system model, though. I’ve alreadymentioned that a true solar-system-style atom would collapse. Anotherdifference is that, unlike the solar system, the electrons and the nucleus areattractedby electromagnetism rather thangravity.Andherewe comeacross areal oddity, with a Nobel Prize waiting for anyone who can explain it. Theelectronhasexactly thesamemagnitudeofcharge(ifopposite invalue) to thepositivechargeonaprotoninthenucleus.Noonehasacluewhy,butit’sratherhandy in making atoms work the way they do. The solar system has noequivalenttothis.Gravitycomesinonlyoneflavour.The final reason we have to throw away the solar system model is that

electronssimplydon’ttravelaroundnucleiinnice,well-definedorbits,thesamewaythatplanetstravelaroundtheSun.Theydon’tevenmovearoundonthesortof rail tracks thatBohr firstenvisaged.Aswewilldiscover,quantumparticlesarenever soconsiderate andpredictable as todo something like this.Abetterpictureofanelectron isa sortof fuzzycloudofprobability spreadaround theoutside of the atom, rather than those sweeping orbit lines so favoured bygraphic designers – though that is much harder to draw. More on that in amoment.

BuildingonBohr

Itwouldbe an exaggeration to say thatBohr’s idea for the structureof atomstransformedourviewofphysicsonitsown–apartfromanything,hisoriginal

modelworkedonlyforthesimplestatom,hydrogen.Butbeforelongagroupofyoung physicists –with deBroglie,Heisenberg, Schrödinger andDirac to thefore – had picked up the baton and were pushing forward to build quantumtheory into an effective description of theway that atoms and other quantumparticles like photons behave. And their message was that they behave verybadlyindeed–atleastifweexpectthemtocarryonthewayweexpectordinaryeverydayobjectstoact.LouisdeBroglieshowedthatEinstein’stransformationofthewavynatureof

lightintoparticleswasatwo-waystreet–becausequantumobjectsweusuallythoughtofasparticles,likeatomsandelectrons,couldjustashappilybehaveasif they were waves. It was even possible to do a variant of the two-slitexperimentwithparticles,producing interferencepatterns.WernerHeisenberg,meanwhile, was uncomfortable with Bohr’s orbits modelled on the ‘real’observed world and totally abandoned the idea of trying to provide anexplanationofquantumparticlesthatcouldbeenvisaged.Hedevelopedapurelymathematical method of predicting the behaviour of quantum particles calledmatrix mechanics. The matrices (two-dimensional arrays of numbers) did notrepresent anything directly observable – they were simply values that, whenmanipulatedtherightway,producedthesameresultsaswereseeninnature.ErwinSchrödinger,alwaysmorecomfortablethanHeisenbergwithsomething

thatcouldbevisualised,cameupwithanalternativeformulationknownaswavemechanics that it was initially hoped described the behaviour of de Broglie’swaves.PaulDiracwouldeventuallyshowthatSchrödinger’sandHeisenberg’sapproaches were entirely equivalent. But Schrödinger was mistaken if hebelieved he had tamed the quantum wildness. If his wave equation had trulydescribed the behaviour of particles it would show that quantum particlesgraduallyspreadoutovertime,becomingimmense.Thiswasabsurd.Tomakemattersworse,thesolutionsofhiswaveequationscontainedimaginarynumbers,whichgenerallyindicatedtherewassomethingwrongwiththemaths.

Numbersthatcan’tberealImaginarynumbershadbeenaroundasaconceptsincethe16thcentury.Theywerebasedontheideaofsquareroots.Asyouprobablyrememberfromschool,thesquarerootofanumberisthevaluewhich,multipliedbyitself,producesthatoriginalnumber.So,forinstance,thesquarerootof4is2.Or,rather,2isoneof4’ssquareroots.Becauseitisalsotruethat–2multipliedbyitselfmakes4.Thenumber4has twosquare roots,2and–2.But this leavesabitofagap in thesquarerootlandscape.What,forexample,isthesquarerootof–4?Itcan’tbe2,

norcanitbe–2,asbothofthoseproduce4whenmultipliedbythemselves.Sowhat can the square root of a negative number be? To deal with this,mathematiciansinventedanarbitraryvalueforthesquarerootof–1,referredtoas ‘i’.Once i exists,we can say the square roots of –4 are 2i and –2i. Thesenumbersbasedoniareimaginarynumbers.Thiswouldseemtobethekindofthingmathematiciansdointheirsparetime

toamusethemselves–quiteentertaining,butofnointerestintherealworld.Butinfactcomplexnumbers,whichhavebotharealandanimaginarycomponent,such as 3+2i, proved to be very useful in physics and engineering. This isbecausebyrepresentingacomplexnumberasapointplottedonagraph,wheretherealnumbersareon thexaxisand the imaginarynumberson theyaxis,acomplex number provides a single value that represents a point in twodimensions.Aslongastheimaginarypartscanceloutbeforecomingupwithareal world prediction, complex numbers proved a great tool. But inSchrödinger’swaveequation, theimaginarynumbersdidnotpolitelygoaway,stayingaroundtotheembarrassmentofallconcerned.

Probabilityonthesquare

ThismesswassortedoutbyEinstein’sgoodfriend,MaxBorn.BornworkedoutthatSchrödinger’sequationdidnotactuallysayhowaparticle likeanelectronoraphotonbehaved.Insteadofshowingthelocationofaparticle,itshowedtheprobabilityofaparticlebeing inaparticular location.Tobemoreprecise, thesquare of the equation showed the probability, handily disposing of thoseinconvenient imaginary numbers.Where itwas inconceivable that the particleitselfwouldspreadoutovertime,itwasperfectlyreasonablethattheprobabilityof finding it inany locationwouldspreadout thisway.But theprice thatwaspaidforBorn’sfixwasthatprobabilitybecameacentralpartofourdescriptionofreality.Born’sexplanationoftheequationworkedwonderfully,thoughithadtobe takenon trust–noone could saywhy, for instance, itwasnecessary tosquaretheoutcome.Thereisnothingnewinusingprobabilitytodescribealevelofuncertainty.I

candemonstratethisifIputadoginthemiddleofaparkandclosemyeyesfortenseconds.Idon’tknowexactlywherethatdogwillbewhenIopenmyeyes.Icansay,though,thatitwillprobablybewithinabout20metresofwhereIleftit,andtheprobabilityishigherthatitwillbenearthelamppostthanthatitwillbehalfwayupabeechtreeortakingarideontheroundabout.However,thisuseofprobability in the ordinary world does not reflect reality, but rather the

uncertaintyinmyknowledge.Thedogwillactuallybeinaparticularlocationatall timeswith 100per cent certainty – I just don’t knowwhat that location isuntilIopenmyeyes.If instead of a dog I was observing a quantum particle, Schrödinger’s

equation,newlyexplainedbyBorn,alsogivesmetheprobabilityoffindingtheparticleinthedifferentpossiblelocationsavailabletoit.ButthedifferencehereisthatthereisnounderlyingrealityofwhichIamunawarebeforeIlook.UntilImakethemeasurementandproducealocationfortheparticle,theprobabilityisallthatexisted.Theparticlewasn’t‘really’intheplaceIeventuallyfounditupuntilthepointthemeasurementwasmade.Taking this viewpoint requires a huge stretch of the imagination (which is

probablywhy ten-year-olds copewithquantum theorybetter thangrown-ups),butifyoucanovercomecommonsense’sattempttoputyoustraight,it throwsawaytheproblemswefacewhenthinking,forinstance,ofhowtheYoung’sslitsexperiment could possibly work with photons of light. If you remember, thetraditional wave picture had waves passing through both slits and interferingwitheachothertocreatethepatternoffringesonthescreen.Buthowcouldthisworkwithphotons(orelectrons)?Thisdifficultyismadeparticularlypoignantifyouconsiderthatwecannowfine-tunetheproductionof theseparticlestotheextent that theycanbesent towardstheslitsoneata time–andyetstill,overtime,theinterferencepattern,causedbytheinteractionofwavesofprobability,buildsuponthescreen.

Whereisthatparticle?Thereisaverydangeroustemptationthatalmostallsciencecommunicatorsfallintoatthispoint.IhavetoadmitIhavedoneitfrequentlyinthepast.AndIhaveheard TV scientist Brian Cox do it too, commenting on his radio show TheInfiniteMonkey Cage that the photon is in two places at once. In fact Cox’sbook, The Quantum Universe (co-authored with Jeff Forshaw), even has achapter entitled ‘Being in two places at once’. The tempting but faultydescription is that quantum theory says that a photon can be in two places atonce,soitmanagestogothroughbothslitsandinterfereswithitself.However,this gives amisleadingpictureofwhat is reallyhappening in theprobabilisticworldofthequantum.What would be much more accurate would be to say that a photon in the

Young’sslitsexperimentisn’tanywhereuntilithitsthescreenandisregistered.Up to that point all that exists is a series of probabilities for its location,describedby the (squareof the)waveequation.As thesewavesofprobability

encompassbothslits, thenthefinalresultatthescreenisthatthoseprobabilitywavesinterfere–butthewavesarenotthephotonitself.Iftheexperimenterputsadetectorinoneoftheslitsthatletsaphotonthroughbutdetectsitspassing,theinterference pattern disappears.We have forced the photon to have a locationandthereisnoopportunityfortheprobabilitywavestointerfere.ItwasthisfundamentalroleforprobabilitythatsoirritatedEinstein,making

himwriteseveral timestoMaxBornthatthisideasimplycouldn’tberight,asGoddidnotplaydice.AsEinsteinput it,whendescribingoneof thequantumeffects that are controlled by probability: ‘In that case, I would rather be acobbler,orevenanemployeeinagaminghouse,thanaphysicist.’ItwasfromthecentralroleofprobabilitythatHeisenbergwoulddeducethe

famousUncertaintyPrinciple.He showed that quantumparticleshavepairs ofproperties–locationandmomentum,forinstance,orenergyandtime–thatareintimatelyrelatedbyprobability.Themoreaccuratelyyoudiscoveroneofthesepairs of values, the less accurately it is possible to know the other. If, forinstance,youknewtheexactmomentum(masstimesvelocity)ofaparticle,thenitcouldbelocatedanywhereintheuniverse.

Theinfernalcat

It is probably necessary also at this point to mention Schrödinger’s cat, notbecauseitgivesusanygreatinsightsintoquantumtheory,butratherbecauseitissooftenmentionedwhenquantumphysicscomesupthatitneedsputtingintocontext. This thought experiment was dreamed up by Schrödinger todemonstrate how absurd he felt the probabilistic nature of quantum theorybecamewhenitwaslinkedtothe‘macro’worldthatweobserveeveryday.IntheYoung’sslitsexperiment,evensinglephotonsproduceaninterference

patternasdescribedabove–butifyoucheckwhichslitaphotongoesthrough,the probabilities collapse into an actual value and the pattern disappears.Quantumparticlestypicallygetinto‘superposed’statesuntiltheyareobserved.(Superpositionjustsaysthataparticlehassimultaneousprobabilitiesofbeingina range of states, rather than having an actual unique state.) In the catexperiment, a quantumparticle of a radioactivematerial is used to trigger thereleaseofadeadlygaswhentheparticledecays.Thegasthenkillsacatthatisinabox.Becausetheradioactiveparticleisaquantumparticle,untilobserveditis in a superposed state,merely a combination of the probabilities of it beingdecayedornotdecayed.Whichpresumablyleavesthecatinasuperposedstateofaliveanddead.Whichismorethanalittleweird.

Inreality,themoggydoesn’tseemtohavemuchtoworryabout,atleastasfaras being superposed goes – it can, of course, still die. As the experiment isdescribed, it isassumed that theparticle,andhence thecat, is ina superposedstate until the box is opened. Yet in the Young’s slit experiment the merepresence of a detector is enough to collapse the states and produce an actualvalue for which slit the particle travelled through. So there is no reason toassume that the detector in the cat experiment that triggers the gaswould notalso collapse the states.ButSchrödinger’s cat is sucha favouritewith sciencewriters–ifonlybecauseitgivesillustratorssomethinginterestingtodraw–thatitreallyneedshighlighting.Because it is so famous, thecathasa tendency to turnup inotherquantum

thoughtexperiments.TheoriginalSchrödinger’scatexperimentisallaboutthefuzzyborderlinebetweenthequantumworldoftheverysmallandtheclassicalworld we observe around us. Experimenters are always trying to stretch thatboundary, achieving superposition and other quantum effects for larger andlarger objects.Until recently therewasnogoodmeasureof justwhat ‘bigger’meant in thiscontext–how tomeasurehowmacroscopicormicroscopic (andliabletoquantumeffects)anobjectwas.However,in2013StefanNimmrichterand Klaus Hornberger of the University of Duisburg-Essen devised amathematicalmeasurethatdescribestheminimummodificationrequiredintheappropriate Schrödinger’s equation to destroy a quantum state, giving anumericalmeasureofjusthowrealisticasuperpositionwouldbe.Thismeasureproducesavaluethatcomparesanygivensuperpositionwitha

singleelectron’sability to stay ina superposedstate.Forexample, thebiggestmolecule that has been superposed to date has 356 atoms. The theoristscalculatedthatthiswouldhavea‘macroscopicity’factorof12,whichmeansitbeingsuperposedforasecondisonaparwithanelectronstayingsuperposedfor1012 seconds.There is reasonable expectation that itemswith a factorofup toaround23couldbeputintoasuperposedstate.Toputthisintocontext,andinhonourofSchrödinger,thetheoristsalsocalculatedthemacroscopicityofacat.Theystartedwithaclassicphysicist’ssimplificationbyassumingthatthecat

wasa4-kilogramsphereofwater,andthatitmanagedtogetintoasuperpositionof being in two places 10 centimetres apart for one second. The result of thecalculation was a factor of around 57 – it was the equivalent of putting anelectronintoasuperposedstatefor1057seconds,around1039timestheageoftheuniverse,stressingjusthowunlikelythisis–thoughit isworthnotingthateven the 1023 expectation is longer than the lifetime of the universe.Unlikelythings do happen (if rather infrequently), and quantum researchers are alwayscarefulnevertosay‘never’.

It is these weird aspects of quantum theory that make the field socounterintuitive … and so fascinating. And nowhere more so than whenquantum effects crop up in the natural world. Quantum theory is not justsomething that is relevant to the lab,oreven tohigh-techengineering. Ithasadirect impacton theworldaroundus, from theoperationof theSun that is socentraltolifeonEarth,tosomeofthemoresubtleaspectsofbiology.

Footnotes1.Einstein’sexpansionofGalileo’stheoryofrelativity.Galileohadobservedthatallmovementhastobemeasuredrelativetosomething,butEinsteinaddedthatlightalwaystravelsatthesamespeed.Thisspecialrelativityshowsthattimeandspacearelinkedanddependentontheobserver’smotion.

2. The observation by the Scottish botanist Robert Brown (1773–1858) that pollen grains suspended inwaterdancedaround.Einsteinshowedhowthiscouldbecausedbyfast-movingwatermoleculescollidingwiththegrains.

3.Theelectronisthenegativelychargedfundamentalparticlethatoccupiestheouterreachesofatomsandcarrieselectricalcurrent.

4.Theywouldn’tbeknownasphotonsuntil the1920swhentheyweregiven thenameby theAmericanchemistGilbertLewis.

CHAPTER2

Quantumnature

Becauseofthewaywearetaughtscience,itistemptingtodividethesubjectupintotightcompartments.Physics,forinstance,isabouthowstuffbehaves,whilebiology explains the living side of nature. (As someone with a physicsbackground,Imightcruellysaythatchemistryistheclean-upoperationforthebits in between that neither of the other subjectswants.) But these labels anddivisionsarearbitraryandhuman-imposed.Quantumtheoryhasnointentionofstaying confined in the box labelled physics. Nature makes use of quantumprocesses.

It’squantumallthewaydownAtafundamentallevel,thisisatruismaboutnature.Giventhatatomsandlightaregovernedbyquantum theory,andprettywelleverything innature iseitheratoms or light,1 it is inevitable that quantum processes rule.Quantum physicsdescribeswhyatomsexistandwhy theydon’tcollapse.Soyoucouldsay thatwhenyouwatcharabbitrunacrossafieldorexaminethebeautifulstructureofan orchid you are seeing a product of quantum theory. But that’s just thefoundationlevel,explainingthecomponentpartsofnature.Quantumtheoryalsoappliesatafarhigherlevelthanthebasicsofhowatomswork.Perhaps themost dramatic example of this is the Sun. Because it is so far

away and seems little more than a bright light in the sky, we tend tounderestimate the significance of the Sun to life onEarth. This hasn’t alwaysbeen the case. Earlier civilisations worshipped the Sun as a god for a goodreason.Being closer to the land, theywere aware of theSun’s significance inhelpingtheircropsgrow.Andwithoutartificiallights,theyhadalottothanktheSunforinenablingthemtosee.Inamodernworld,wherewearerarelyfarfroma light at night,whether it’s in our home, a street light or the glow from ourphones,itishardtoappreciatejusthowdarkandscarythenaturalworldatnightcanbe.Sitforawhileinapitch-darkcave,ideallywiththehowlingofwolvesthrowninforfullimpact,andyoucanseewhytheSun’scontributionduringthe

daywassoappreciated.Even our ancestors, though, underestimated the importance of the Sun. Just

imaginetherewasnoSun,thattheEarthwasaloneplanet,wanderingthroughspace.Whatwouldwemiss out on?Therewould be noweather –weather ispowered by the Sun, producing temperature differences to create wind andevaporatingwatertogeneratecloudsandrain.TemperaturesontheEarthwoulddroptobelow–250°C.Thereneverwouldhavebeenanoxygenatmosphere,astherewouldbenophotosynthesis.Butall this is irrelevant ina sense,becausetherewouldbenoEarth.WithouttheSun’sgravitationalpull, thematerial thatcametogethertomakeuptheEarthwouldhaveremainedscatteredinspace.WeoweourexistencetotheSun.

HowoldistheSun?WhenthoseobservingtheSungotpastsimplyregardingitasalightinthesky,theytypicallyconsideredittobeafire.Afterall,whatelseglowslikethat?Butthe ideaof a heavenlybonfirewas itself a problem,becausewe all know thatfires don’t burn for ever.Thiswas a real problemwhen it becameobvious inVictoriantimesthattheEarthhadbeenaroundfarlongerthansuggestedbythetraditionalcreationdate,workedoutfromaddingupBible‘begats’backto4004BC.Twofactorswereresponsibleforthis.Onewasgeology.Byobservingthewayerosionactsatthepresent,geologistswereabletoestimatethatthenaturalformationswe seemust have faced erosion for hundreds ofmillions of years.TheotherVictorianbugbearforageingtheSunwasevolution.Darwinmadeitclearthatthekindofprocesseshehadinmindforevolutionbynaturalselectionwouldalsorequirehundredsofmillionsofyearsforspeciestoevolve.Setagainsttheselongtimescaleswerethephysicists,tryingtocomeupwith

anexplanation forhow theSunworked,notablyWilliamThomson, laterLordKelvin. Kelvin had first considered the possibility that the Sun was simplyburning, but if it were coal – which sounds silly now, but was seriouslyconsideredthen–itwouldlastonlyafewthousandyears,andevenwiththebestenergy/weight reactionavailable, thatofhydrogenandoxygen, itcouldatbesthavealifetimeof20,000years.ThatwasfartooshortforanysensiblemodelofwhatwasobservedontheEarth.AnditwasridiculousthattheEarthshouldbefar older than the Sun. Kelvin also considered whether the Sun could beexternallyheatedbytheimpactofmeteorsincollisionwithit.Buthereckonedthat to achieve the output it did, it would require about two Earths’-worth ofmattertohititeverycentury.Thissteadyincreaseinmassshouldhaveproducedsignificant modifications to the orbits of the planets, which had not been

observed.ThisleftonlyonepossibilitythatKelvincouldthinkof.He suggested that the Sun had formed by a cloud of gas coming together

throughthepullofgravity.Astheatomswerecompressedclosertogether,thiswouldraisethetemperature.Thinkofwhathappenswhenyourepeatedlypushthe handle of a bicycle pump – it warms up. He suggested this compressionheating happened to such a degree that the Sun became intensely hot as itformed,andithadthenspentitsliferadiatingawaythatheat,likeapieceofironheatedinaforge,whichcontinuestoglowandgiveoffheatlongafteritistakenoutof the flames.Kelvin calculated thatwith the immensemassof theSun itcould continue to radiate (though it would be gradually getting dimmer) foraround30millionyears.Inaway,Kelvin’sideawasveryclever.Wedostillthinkthatthiscontraction

under thepullofgravity, creatingheat andpressure, ishowstars like theSunform – but it is not how they keep burning. However, that 30 millionyeartimescaleputKelvinindirectcontradictionwiththegeologistsandDarwin.(Themild-mannered Darwin, rather than challenge Kelvin, was horrified by thepotentialconflict,andremovedreferencestothedurationofevolutionineditionsofOrigin of Species from this point on. Privately he referred toKelvin as an‘odiousspectre’.)Theresultwassomethingofanimpasse.Theonlytheoriesfortheformationof theEarthrequired theSun tobe therefirst.Theonlyway theSun couldwork put its age at less than 30million years, yetmore andmoreevidenceontheEarthsuggestedthatithadbeeninexistenceformanyhundredsofmillionsofyears.Infactwenowknowit’saround4.5billionyearsold.

Thepoweroffusion

The solution to this conundrumwas the discovery of a newway that the Suncould be powered. Under immense temperature and pressure, hydrogen ionscouldfusetoproducethenextheaviestelement,helium.Inthisprocess,energyis given off. Scale this up to the size of the Sun and you have a body that iscapableofproducingenergyforbillionsofyears–theSunseemstobearoundhalfwaythrougha10billion-yearlifespan.Theveryprocessofnuclearfusionisaquantumprocess,providingacentralroleforquantumtheoryintheexistenceofEarth, humans and all of nature, but there is onemore quantum card to beplayedintheexplanationoftheSun’sworkings.Because,whilefusionpoweriscertainlyalikelyprocesstoproducetheamountofenergyrequired,itturnedoutthat even the heat andpressure in the heart of theSun is not enough tomakefusionhappen.

Toproduceheliumrequiresfourprotons–eachahydrogenion,thenucleusofa hydrogen atomwith its electron stripped away – to come together in closeproximity.As it happens, the actual process is a littlemore complex,with anisotopeofhelium,helium–3,forming,thenpairsoftheheliumcomingtogethertoproduce the stablehelium–4and releaseapairofprotons.But theessentialoutcome is that four hydrogen ions have become a helium ion and producedenergyalongtheway.Thosehydrogenions,protons,arepositivelycharged,sotheyrepeleachother.Theclosertheyget, thestrongerthatrepulsionbecomes.They can fuse onlywhen the strong nuclear force takes over.But this has noeffectoutsideveryshortdistances,sotheprotonshavetogetridiculouslyclosetoeachother.Hereiswheretheweirdnessofquantumphysicscomesin.Bearinmindthat

Schrödinger’s equation tells us that, over time, the possible locations of aparticle spread out. So though two protons aremost likely to be kept too faraway from each other to fuse, there is a small probability that they are closeenough already – and in those small number of cases, fusion takes place.Anotherwaytolookatthis,whichgivestheprocessitsname,istothinkoftheelectromagneticrepulsionasabarrier,keepingtheprotonsapart.Afewprotonswillundergoaprocesscalledquantummechanicaltunnelling,whichmeanstheyappear on the other side of the barrier without passing through the space inbetween.Theyjumptotheothersideandfuse.Although this tunnellinghasa lowprobability, therearesomanyprotons in

theSunthatseveralmillionsoftonsofthemfuseeverysecond.Andallbecauseof this strange quantum effect. Without tunnelling, the Sun’s fusion reactionwould not work. The Sun would not be giving off the energy that it does.Meaningnoweather,nooxygenandimpossiblylowtemperaturesontheEarth.Therewouldbenolifewithoutthisuniquelyquantumprocess.

Enzymeenablers

More down to Earth, we are discovering an increasing range of quantumprocesses cropping up in nature, where they might not have been expectedbefore.Awellestablishedexamplethatwasuncoveredinthe1970sistheworkofenzymesascatalysts.Enzymesarelargeorganicmolecules,usuallyproteins,that take part in chemical reactionswithin living things, including the humanbody. For example, enzymes help with the digestion of our food, acting ascatalyststohugelyspeedupchemicalreactionsthatotherwisewouldbetooslowtosupportlife.

Catalystsworkbymakingchemicalreactionsworkmoreeasily,reducingtheamountofenergyneededforthereaction,butthecatalystisnotpartofthefinaloutputof the reaction, so it is freedup tobeusedagain.Acatalystmight, forinstance,changethenatureofachemicalbondorcombinewithonecompoundtoproduceanintermediatesubstancethatismuchmorereactive.Intheactionofsomeenzymes,eitheraprotonoranelectronundergoestunnelling,justliketheprotonsintheSun.Withoutthetunnelling,onlythoseprotonsorelectronswithenoughenergytogetoverthebarrierpreventingthereactiontakingplacewouldsucceed.It’snotthatanewreactiontakesplacebecauseofthequantumeffect,but rather theenzymeproducesamuchfaster reaction thanexpected, typicallythousands of times faster. Without this quantum boost many biologicalorganisms–includinghumans–wouldbeunabletofunction.

It’sallintheDNAAnother place where quantum tunnelling seems to take place is in a hugelyimportantbitof thebiochemistryofevery living thing:DNAmutation.As it’shardtoavoidknowing,DNA(shortfordeoxyribonucleicacid)is thefamilyofmoleculesthatcarriesourgeneticinformation.Itistheinstructionsetbywhichweareconstructed,andpassesonourgenestoouroffspring.Whenacellsplitsinto two so an organism can grow, the DNA itself splits down the middle,unzippingthespiralstaircaseofitsstructuretoproducetwohalves.Thesearen’tidentical,buteachcomplementstheother.Each‘tread’of theDNAspiralstaircaseconsistsof twoorganiccompounds

selected from a set of four, known as bases: cytosine, guanine, adenine andthymine.These always pair the sameway: cytosinewith guanine and adeninewith thymine. (If you think of them as the capital letters usually used torepresentthem,C,G,AandT,thelettersmadeofcurvespair,andthosemadeofstraightlinespair.)SoitiseasytoreproducethemissinghalfoftheDNAfromthebasesintheavailableone.The bond that links together the base pairs until the DNA is ‘unzipped’ is

calledahydrogenbond. It’s the samekindofbond that linkswatermoleculestogether,givingwateranunexpectedlyhighboilingpoint.Hydrogenbondingisanelectricaleffect,wheretherelativelypositivepartofonemoleculeisattractedto the relatively negative part of another molecule. In the case of water, thepositivepart ishydrogen,whichhasonlyasinglenegativeelectronthat is tiedupinthebondtotherestofthewatermolecule,andthenegativepartisoxygen.

Fig.2.A-Tbasepairshowinghydrogenbonds(dotted).

InDNAtherearehydrogenbondslinkingeachpairofbases.Eachpairhasahydrogennucleus–asingleproton–atonesideof thebond.Thisproton isaquantumparticle and thatmeans it can tunnel, in this case across to the othersideofthebondsoitbecomesapartoftheothercompound.So,forinstance,inapairwhereAislinkedtoT,ahydrogennucleusfromtheAsidecouldtunnelthroughtotheTwhileahydrogenfromtheTtunnelsacrossthesecondbondtotheA.Theformulaeforthetwobasesremainthesame,butthestructureisnowdifferent.AndthatmeansthatwhentheDNAunzips,thevariantofAischangedenoughinshapetobondwithCinsteadofT.ThenewcopyofDNAthatresultswill be amutation that could result in a change in the organism for which itcontrolsdevelopment.Thisprocesshasnotbeenexperimentallyconfirmedyet,thoughitisbelieved

tobeastrongpossibilityforthemechanismbehindthiskindofmutation.Andifit is, it means that a specifically quantum process directly causes changes inlivingcells.Thissortofhigh-levelquantumeffectwasnotoriginallyexpectedbecause the ‘messiness’ of awarm,wet biological environment is exactly theopposite of the carefully controlled conditions usually necessary to observequantum effectswithout decoherence, the process bywhich quantumparticlesinteractwiththeotherparticlesaroundthemandstarttoactina‘classical’way,liketheobjectswefamiliarlyexperience,ratherthaninaweirdquantumfashion.

Plantlight

One of themost dramatic and important biological processes that is likely toinvolve high-level quantum effects is photosynthesis, the process by whichplantsconvertlightintoenergy.Aswewillseelateron,anyinteractionbetweenlightandmatter isquantummechanical, justasanything involvinganatomor

electron is, but recent studies of photosynthesis have shown that quantumphysicsprobablyhasamorefunctionalrole.Evenwithoutanyquantummechanicalweirdness,photosynthesisisamarvel

of natural technology. The first hint of something remarkable going on withplantswhenexposedtolightwasanaccidentaldiscoverybyJosephPriestley.Inthe mid-1770s, this trouble-prone Non-conformist minister had settled in theunlikely jobof librarian to theEarlofShelburne inhis statelyhome,BowoodHouse. In return for Priestley’s company and conversation, Shelburne waspreparedtosupportPriestley’scuriosityonthenatureofairanditscomponents.Arguably as a formof entertainment to showoff to visiting guests, ShelburnegaveoverasmallroomnexttohislibrarytoPriestley,wherethescientistcouldundertakehisexperiments.Priestley is usually credited as the discoverer of oxygen, though he himself

wouldnothaverecogniseditsexistence,ashewasasupporterofthephlogistontheory.Thisproposedthattherewasapartofmattercalledphlogistonthatwasgivenoffasthematterburned.Aircouldholdonlysomuchphlogiston,soif,forinstance,youputacandleinabelljaritwouldgooutwhentheairbecamefullyphlogisticated. In reality this was because the oxygen in the air was beingconsumed – phlogiston was a sort of anti-oxygen. Priestley discovered that amouse inside the bell jar would also make the air phlogisticated, causing themousetokeelover,butifheputagreenplantinwiththemouse,itwouldseemtorestorethis‘injuredair’andkeeptheanimalalive.Thisawarenessthatplantssomehowrepairedsomethingthatwasrestrictedor

limited when something was burned or an animal breathed was as far asPriestley got, but towards the end of the 18th century, French pastor JeanSenebierandtheSwissscientistTheodoredeSaussureshowedthatthe‘injuredair’ was carbon dioxide, produced by burning or respiration, and that plantscould convert this gas to oxygen and carbon-based molecules under theinfluenceof light.Wenowknow that photosynthesis from theSun effectivelyfeedstheEarth,actingdirectlyongreenplantsandparticularlyalgae,whichuseupmore than half of the solar energy going to photosynthesis, and indirectlyproviding that energy needed by the animals that eat the plants (or eat theanimalsthateattheplants)inourcomplexfoodchain.Thephysics and chemistry involved in photosynthesis is convoluted,with a

wholechainofreactionstakingplace.Firstthelightpushesuptheenergylevelsofelectronsinspecialcolouredmoleculeslikethegreenchlorophyllinaplant.Thisenergyisconvertedtochemicalformbythephotosyntheticreactioncentre,whichproducesoxygenandincorporatescarbonintotheplant.Oneofthestepsof this intricate process is the fastest known chemical reaction in existence,

takingplaceinatrillionthofasecond.The oddities of the quantumworld come into play in the energy’s journey

fromthatfirstexcitationofanelectroninchlorophylltoitsarrivalinthereactioncentre,whereitgetstoworkconvertingcarbondioxidetosugars(andreleasingsome oxygen in the bargain). The way the energy passes from molecule tomoleculeonthewayinisaresultofquantumparticlesbehavinglikewaves.Theenergised wave of the first excited electron extends into the next molecule,passingon the excitation, and soon.What’smore, thesewavesdon’t seem totakearandomdrunkard’swalk,butrathertheyoverlap,comingintocoherence–thestatewherethewavesallrippletogetherthatmakesalaserwork(seepage129).Thiscoherentbehaviourhadbeenpostulatedforawhile,andtherewassome

weakevidenceofitexistingfromlargeplantsamples,butin2013researchersinSpainandGlasgowdiscovereditatthemolecularlevel,traininglasersonsinglelight-processing molecules to observe the detailed workings of the reactioncentres that convert photons to chemical energy. Experiments also on light-harvesting purple bacteria showed that the ability of the quantum particle toprobabilistically explore all routes and find the best path meant that theconnections change as parts of the organism move, constantly tuning theprocess, meaning that the conversion can reach levels of around 90 per centefficiency, far higher than a solar cell (and possibly with implications forphotovoltaiccelldevelopmentinthefuture).

Thepigeon’scompassAratherlesscertainbutfascinatingpossibilityisthataquantumeffectisbehindone of the marvels of nature – the way that birds like homing pigeons cannavigate, apparently by picking up theEarth’smagnetic field, using a built-incompass.Thismysteriousabilityhasbeen linked tomagneticparticles in theirbeaks, but there is also evidence that may be stronger that the process istriggeredbylighthittingtheretinaofthebird’seye.(Infactthreemechanismshavebeenproposed,anditisentirelypossiblepigeonsusesomecombinationofthem.)Whenlighthitsthereceptorinthebird’seye,itisusedtosplitamoleculeto

formtwofreeradicals.Theseareveryreactivemoleculesthathaveanunpairedelectron (it’s free radicals that are restrainedbyantioxidants fromcausingcelldamage). These electrons can act as tiny magnetic compasses, with theirquantum property called spin influenced by a magnetic field. Typically oneradicalwillbeclosertothenearestatomicnucleus,andhencefeelsthemagnetic

fieldlessthantheother.Thisdifferencebetweenthetwogivesthechemicalsadifferentlevelofreactivity,makingitpossibleforthebirdtogetsomefeedbackfrom the interaction, perhapsby the synthesis of a chemical in the retina.Thetwounpairedelectronsarecreatedentangled,linkedtoeachotherinaquantumfashion,andthiscouldhelpamplifytheeffect.

Ithink,thereforeI’mquantumThemost extreme – andmost contentious – overlap between quantum theoryand biology is the idea that consciousness itself is a quantum phenomenon.Althoughthereisnodirectevidencetobasethistheoryon,somehavesuggestedthat it is not possible to explain the phenomenonof the consciousmindusingconventional classical physics, and that it needs quantum effects likeentanglement to make it possible. One suggestion, with the clumsy name of‘orchestrated objective reduction’, comes from physicist Roger Penrose andmedicaldoctorStuartHameroff.Penrose proposed that the brain is capable of computation that would be

impossibleusingconventionalmechanisms,with theprobabilisticnatureat theheart of quantum theory explaining this extra capability. Hameroff, ananaesthetist, suggested that the cytoskeleton, the structure that supports theneuronsinthebrain,andinparticularmicrotubules,thinpolymersthatformpartof the cytoskeleton, could act as quantum systems, where electrons tunnelbetweenthemicrotubules.Theideathatconsciousnessinvolvesquantumeffectsdoesnotseemtostretch

theboundsofprobabilitytotoogreatanextent,thoughasyetthejuryisout.Wejust don’t understandwhat consciousness is, or themechanismbehind it,wellenough to explore how much it could depend on quantum effects. However,whatcertainlycanbedone is tomakeuseof themathematicsbehindquantumtheorytogetabetterunderstandingofsomeaspectsofhumanbehaviour.

Quantumvoting

Andrei Khrennikov of Linnaeus University, Sweden and Emmanuel Haven atthe University of Leicester took the maths used to describe quantumdecoherence and applied it to theAmerican political system.Specifically theylooked at the voters’ choice between Republican and Democrat in thecombinationofpresidentialandcongressionalelections.Theirideawasthatthemental state of a voter could be considered a superposition of the states‘Democrat’and‘Republican’withacertainprobabilityforeach,andtheytreated

the two elections as if they were entangled qubits (the processing units ofquantumcomputers–seepage231).Thisgavethemavehicleforexploringthedynamicsofthevoters’mentalstateswhenexposedtomediainformation.It’s early days but there is some evidence that this kind of tool could be

valuableingaininginsightsintothewaywethinkandmakedecisions.Evenifthisapproachproveseffective,itisn’tevidencethatthedecision-makingitselfisdependentonquantumphysics.Most likely it is just that themathshappens towork well as a model because both the political situation and the state ofquantumparticlesinvolveprobabilisticmeasurementsofpropertiesthatcanexistonlyinasmallnumberofstates,andthathaveto‘collapse’intoasinglefixedvalue, inthecaseofpoliticswhenavoteiscast.Butit isanotherwaythat thepowerofquantumphysicscangiveusinsightsintobiologicalprocesses.Biologyisagoodexampleofafieldthatgrewfromsimpleobservationtoa

true science, able to explain what was happening in nature. This involvedbuildingagrowingknowledgeofthedetailofwhathappenswithinabiologicalstructure,detailthatwouldeventuallybediscoveredtoincludequantumeffects.Anotherfield that ispurelyquantumbutbeganwithsimpleobservations is therealmofelectricity.It’stimetosingthebodyelectric.

Footnote1.Puristswillpointoutthatitisnottruethatprettywelleverythingisatomsandlight.Afterall,around68percentoftheuniverseisdarkenergy,and27percentdarkmatter.Buttheremaining5percent,thebitwecanactuallyexperience–becausetherestis,bydefinition,unobserved–ispredominantlyatomsandlight.

CHAPTER3

Theelectron’srealm

InalecturehallintheRoyalSocietyinLondon,litbytheflickeringglowofoillamps, a strange performance is under way. It looks like a kind of ritualundertaken by a secret society in a display of bizarre debauchery. A boy issuspended from the ceiling by silk ropes.He reaches out to touch a girlwhostandsjustinreachonatar-toppedbarrel.She,inturn,holdsoutherhandandastreamoffeathersfloatupwardsfromatabletowardsherfingers,asifmagicallyattractedtoher.

Thephilosopher’samberWhat the assembled members were witnessing was a popular scientificdemonstrationofthe18thcenturyknownas‘theelectricalboy’.Theboy’sfeetwere picking up a charge from a hand-cranked device that generated staticelectricity. Exactly what electricity was, no one was sure. But clearly it wassomethingthatcouldpassstraightthroughtheboyonitswaytogivethegirltheability to levitate the pile of feathers. It was typically produced by rubbing arotating disc or sphere, often made of glass, with a suitable cloth like wool,thoughinprevioustimesamberhadbeenemployedtogeneratetheeffect,whichgivesustheword‘electricity’fromtheGreekforamber,elektron.Withoutanyclear explanation of why this was happening, it was thought that this actionproducedsomesortofinvisiblefluidthatcouldpassthroughthebody.Itmightseemsillytoconsiderelectricityassomethingthatbehaveslikewater

– after all, it doesn’t exactly run out of the socket if we leave the toasterunplugged. Yet we do still happily use terms (including plug) that are moreappropriatetoafluid–wespeakofanelectricalcurrent,for instance, just likethecurrentofariver.Aswithanymajordevelopmentinscientificunderstandingtherewerelotsof

peoplealongthewaywhocontributedtoourpictureofelectricityanditscousin,magnetism.WecoulddallywiththelikesofAmpèreandOerstedbuttopickout

thebiggestmilestonesalongtheway,itreallywilltakeonlythreementoleadusfromthebaffledamazementproducedbythequaintdemonstrationsofthe18thcentury up to the 20th-century realisation that electricity was a quantumphenomenon.ThefirstofthesewasMichaelFaraday.

ThewizardofAlbemarleStreet

Faraday stands out inmanyways.These dayswewould expect a physicist tohaveauniversityeducationandanexcellentgraspofmathematics.Faradayhadneither.Backthen,tobeascientistusuallymeantbeingarichdilettante.Faradaystartedpracticallypenniless.Asthesonofablacksmithwhohadtakenthemajorstep of moving from Westmorland to London in search of work, MichaelFaraday probably thought himself lucky at the age of fourteen in 1805 to beapprenticed to a bookbinder. He was learning a trade that should eventuallymake him a decent income, and he had the opportunity to attend a self-improvementgroupcalled theCityPhilosophicalSociety,whichwouldbe thekeytotransforminghislife.FaradaytookcarefulnotesofthelecturesheattendedattheSociety,andwas

allowedbyhisemployertobindthem,producingaleather-backedvolumethatsoimpressedthebookbinderthatheshowedthenotestoarichclientwhowasone of themanywho enjoyed visits to theRoyal Institution. Thiswas a thenbrand-new establishment in Albemarle Street, just off fashionable Piccadilly,that both promoted research and put on lectures to encourage the publicunderstandingofscience.Theclient,aMrDance,gaveFaradayticketstoattendthe RI’s top-of-the-bill performer – Humphry Davy, the ultimate Victorianscientificcelebrity.Thisseemstohave inspiredFaradaysomuchthatwithMrDance’s help he got the opportunity to stand in asDavy’s secretarywhen thegreatmanwas temporarily blinded in an accident. Then, after a return to thebookbinder’s, Faraday becameDavy’s lab assistant and general factotum aftertheincumbentwassackedfordrunkenness.

Wollaston’sfollyFaradayquicklysettledintotheRoyalInstitutionandmaderemarkableprogress,givenhislackofeducation.By1821hehadreceivedapromotion,gotmarriedand moved into the rooms at the Institution previously occupied by Davy.Everythingseemedrosy–yethisfirstbigdiscoveryinthefieldofelectricityandmagnetismthreatenedtoruinhisfairy-talesuccess.DavyhadaskedFaradaytowrite up the current state of knowledge on electricity and magnetism, but

Faraday was always a hands-on person, and rather than simply describe theexperimental results he had seen reported, he reproduced the experimentshimself.Atonepointhehadsetupanelectricalcurrentrunningthroughawirenext to amagnet.Thewire began tomove around themagnet in circles.Thiswasnotintheliteraturehewassummarising.Understandably excited, Faraday rushed to publish his results – only to be

accused of stealing the discovery from an elder statesman of the RoyalInstitution,WilliamWollaston.Wollastonhaddevelopedatheorythatdescribedelectricitymovingalongwiresinacorkscrewspiralmotion.HehadaskedDavyfor help to find evidence of this, but nothing ensued.Now herewas Faraday,claimingtohavediscoveredacirclingmotionassociatedwithelectricity.ClearlyWollaston thought – thoughwith no basis in fact, as therewas no link to hisincorrecttheory–thatFaradayhadstolenhisidea.TheimplicationofcheatinghorrifiedFaraday,whohaddeeplyheldreligious

beliefs.He turned to his oldmentorHumphryDavy for the expected support,knowing that Wollaston’s theory bore no resemblance to his experimentalresults.ButDavysidedwithhis friendandsocialequalWollaston, rather thansupporting the working-class Faraday. The social divide took priority overscientific fact.Thiswas theendofany friendshipbetween the two.When, forinstance,FaradaywaselectedtotheRoyalSocietyjustonepersonopposedhim–Davy. Yet the scientific communitywere very clear that the discoverywasFaraday’s. Not only had he done original work unconnected to Wollaston’stheory,hehadprovidedthebasisfortheelectricmotor.Despitethissupportinthewiderworld,Faradaylargelyabandonedelectricity

andmagnetismfortenyearsasaresultoftheunpleasantness,concentratingonchemistry and taking on an administrative role that enabled him to start theregular9.00pmFridayDiscourses(thesedayssomethingofapantomime,astheaudienceisstillexpectedtoturnupinblacktie)andtheChristmasLecturesforChildren, which in later televised form would be the inspiration for many ayoung20th-centuryBritishscientist.Buttheappealofelectricityandmagnetismdidnotgoaway.By1831Faradaywastemptedbacktothefieldbyhearingofexperimentswhereanelectricalcurrentinonewireseemedmagicallytoproduceasecondcurrentinanotherwire,despitebeingatadistance.

Thenewgeneration

Once he set up a pair of coils of wire and connected up the first of them toproduce a current, Faraday expected to see the other start to produce a

continuousflowofelectricity,butinsteadtherewasabriefsurgeinthesecondcoilwhenheswitchedthefirstonoroff.Heknewthatmagnetismcouldactatadistance,andthatanelectricalcoilcouldactasamagnet.Fromthishemadetheleap todeducing that itwas a changing level ofmagnetism from the first coilthatproducedelectricityinthesecond.Hewassoonabletoreproducethiseffectby moving an ordinary permanent magnet through a coil, inventing thegenerator.Itwasaboutthistimethattheoftenmentionedapocryphalconversationwith

PrimeMinisterRobertPeelissaidtohavetakenplace,whenthepoliticianaskedFaradaywhatusehis inventionwas, toget thereply: ‘Iknownot,but Iwageronedayyourgovernmentwilltaxit.’Faradayhadonemoreessentialcontributiontomakeinthisstory,thoughwe

will meet him again in a later chapter. A more traditional physicist – andcertainly any modern physicist – would have attempted to explain what washappening through the medium of mathematics. But Faraday was nomathematician, arguably the last physicist who could makemajor discoverieswithout it.He had seen howmagnets pull iron filings sprinkled on a sheet ofpaper above them into lines linking themagnet’s twopoles. In thedim,gaslitlaboratory,Faradayimaginedtheselinesglowingaroundamagnet.Whenhemovedawirenearamagnetorpushedamagnet throughacoil, it

wasasifthewirewascuttingthroughthelinesofforce,ashetermedthem.Thecloserthelinesweretogether,themorethewirecutthem,themorecurrentwasgenerated.Thismodelworkedwellwiththebusinessofswitchingonoroffanelectromagnet.When itwasswitchedon, the linesof forceexpandedout fromthe magnet, cutting themselves on the wire. When switched off, the linescollapsedandthereversehappened.Thiswayofimaginingelectromagneticinteractionsasafieldofforcewould

eventually become hugely important in physics, not just in understandingelectromagnetismbutattheheartofquantumtheoryitself.Butasyet,therewasstillmoretodiscoveraboutthenatureofelectricityandmagnetism.Thepersonwhowould takeFaraday’selegantbut innumerateconceptsand turn themintothefirstmodernscientificviewwasJamesClerkMaxwell.

AScottishsavant

Maxwellwasborn inEdinburgh in1831, amemberof a later generation thanFaradayandwithaverydifferentbackground from theolderman.Somehaveargued (ratherweakly) thatMaxwell could be called the first scientist, in part

because the word ‘scientist’ was coined only in 1834, holding the samerelationship to science as an artist did to art. Until then, clumsy terms like‘natural philosopher’ or ‘savant’were likely tobeused.Thismaybe tenuous,but what certainly is true is that Maxwell could be called the first modernscientist,ashepioneeredscientifictheoriesthatweredrivenbymathematics,anapproachthatwouldhavebeentotallyalientoFaraday.WhereFaradaywasputtoworkassoonashewasable,Maxwellhadafree

andenviablechildhood–atleastinhisearlyyears.Hewasallowedtoexploreand experiment in his family’s country house, Glenlair, on the estate atMiddlebie in Galloway, dabbling with everything from crystals to hot airballoons.Thisidyllwasshatteredwhenhismotherdied.Thoughaprivatetutorwas first tried for the eight-year-oldMaxwell, hewas soon sent toEdinburghAcademy,whichmusthaveseemedlikeadescentintohellafterthefreedomhehadexperienced.Maxwellwas smaller than average, had a stutter and a country accent, and

wasmore interested in his books and experiments than sports. Some childrenthriveatboardingschools,butMaxwell,knowntohisclassmatesasDafty,wastheclassictargetforschoolbullies.Hehadtoendurethisuntiltheageofsixteenwhenhehad theblessedreleaseofmoving toEdinburghUniversity,and threeyearslatertoCambridge.TherecommendationtotheMasterofTrinityCollege,fromProfessorJamesForbesofEdinburgh,describedhiminmixedfashion:‘Heisnotalittleuncouthinhismanners,butwithaloneofthemostoriginalyoungmenIhaveevermetwith.’Afterhisgraduationin1854,Maxwellwantedtofollowinthefootstepsofhis

personal hero,Michael Faraday.Hewouldwork on awide range of subjects,producing,amongotherthings,thefirstcolourphotograph;buttheaspectofhisworkthathewillalwaysberememberedforwashismathematicalsummaryoftherelationshipbetweenelectricityandmagnetismdiscoveredbyFaraday.Talkto a physicist about elegant equations that simply capture a description of theworldandmanywillpointtoMaxwell’sequations–originallyeightintotal,butsimplifiedbyOliverHeavysideandHeinrichHertz toproduce fourneat, shortequationsthatremainattheheartofourunderstandingoftheuniverse.

Thomson’stinybodiesThefinalpartofour triumvirate is theMancunianscientist J.J.Thomson,whowouldlaterfailtogetonwithNielsBohr.ThomsonjoinedOwensCollege(laterManchesterUniversity)at theageof fourteen,andsixyears latermovedon toCambridge, where he remained for the rest of his career. Thomson’s driving

interestwasthestructureoftheatom(whichwaswhyitwassosadthathedidn’tget on with Bohr – see page 16), but he worked widely in electricity andmagnetism, and in 1897 his studies of cathode rays would give him theopportunitytoreachforfame(andaNobelPrize).CathoderayswerefirstobservedbyMichaelFaraday(himagain)inthe1830s

whenhepassedacurrent throughaglass tubewithreducedairpressure insideandsawabrightglowin the tube,but theywereproperlystudiedonlyonce itwaspossible to remove themajorityof theair froma tube,notablybyBritishscientist William Crookes, which resulted in these tubes being known asCrookes tubes. Something seemed to travel down the tube between the twoelectrodes from the negative cathode to the positive anode, which was oftenshapedlikeaMaltesecross.Whateverwastravellingseemedtobemovingfastenoughtosometimesovershoottheanodeandcrashintotheglassattheendofthetube,causingittogiveoffadistinctivegreenglow.Just what was travelling down the tubes was the subject of considerable

debate – theywere called ‘cathode rays’ because theywere emitted from thenegatively charged cathode. Crookes himself developed the theory that theywere charged atoms from the residual air in the tube, while others, includingHeinrichHertz,thoughtthattheywereanewkindofelectromagneticwave–avariantoflight.Thomson,however,provedthembothwrongwhenhesucceededinmeasuring themassof thecarrierparticles in these invisible raysandfoundthatthiswasnon-zero–andsothecathoderayswerenotlight–butdiscoveredthattheyhadonlyatinyfractionofthemassofatoms.What’smore,theywereidentical in chargeandmass toother similar electrically chargedproducts likethatfromthephotoelectriceffect.Thomsonconcludedthat‘thecarriersofnegativeelectricityarebodies,which

Ihavecalledcorpuscles,havingamassmuchsmaller than thatof theatomofany known element, and are of the same character fromwhatever source thenegative electricity may be derived’. Thomson’s corpuscles were what wouldbecomeknown as electrons, the namegiven to thembyGeorgeStoney a fewyearslater.ItwassoonrealisedthatwhatThomsonhadfoundwasnotjust thecontent of cathode rays but the constituents of the current in all conventionalelectrical circuits. Electrons flowed through metal wires, just as they passedthroughtheevacuatedtubes.There was a slight embarrassment with this discovery, as it meant that the

traditional way of representing electrical current in circuit diagrams, flowingfrompositivetonegative,wasbacktofront,buttherewasn’talotthatcouldbedone about this.Givenour practical experienceof electricity, youmight thinkthatelectronsgushthroughwiresatextremelyhighspeeds–agoodfractionof

thespeedof light.Afterall,whenweflickaswitch,wedon’thavetowaitfortheelectricitytogettotheotherendaswewouldwithwatertravellingthroughapipe.Butelectricalconduction is rathermorecomplex thanawireprovidingaconduitforagushingcurrentofelectrons.

Aconductor’slifeThemostfamiliarelectricalconductionisthroughametal.Thestructureofthemetalisanarrayofatomslikealatticeworkwiththeouterelectronsofthemetalrelativelyfree,abletodetachthemselvesfromtheirparentatomsandfloataboutinthelattice.Usuallytheirmovementisrandom,floatingaboutuntiltheycollidewithsomethingasaresultofthermalenergy,butifanelectricfieldisappliedtothewirebymakingoneendnegativeand theotherpositive, theelectronsdrifttowards thepositivepole.Thismovement issurprisingly leisurely– theyoftencover only around ametre per second. In effect the electrons travel at aroundwalkingpace.Itmightseemthenthatwhenweswitchonanelectricalcurrentitshouldtake

hourstopassdownalongenoughwire,whichwouldbetrueifthewirestartedout empty and had to gradually fill upwith electrons.But in reality, thewirealready contains electrons along its length. When the circuit is made, theelectrical field is carriedalong itbyanelectromagneticwave, travellingat thespeedoflightinthematerial.Thismeansthatelectronsstarttomovethroughoutthewireatalmostthesametime,sothereisnoneedtowaitforthemtogetfromoneendtotheother.Electrons are, of course, quantum particles exhibiting all the weirdness of

quantum behaviour, so anything involving electricity is inherently a quantumprocess.Ifwethinkofconductioninametal,wealsoseethequantumstructureof the atom coming into play inwhat is known as band structure. In a singlecopperatom, say, theextensionofBohr’sworkon thehydrogenatom tellsusthattherewillbeaseriesof‘orbitals’,fixedlevelsofenergythatanelectroncanoccupy,whileitisnotallowedtobeinthelevelsinbetween.Asmoreandmoreatoms are brought together to form the complex structure of the solid metal,somethinginterestinghappens.Whiletheinnerelectronsremainassociatedwiththeirownatoms,inorbitals

belonging toasingleatom, theouterelectronscan takeonsharedorbitals thatrunacrossatomsinthemetal.Asmoreatomsareaddedin,therearemoreandmore orbitals available, squeezed closer and closer together until the gapsbetweenthemarenegligible.Theyformaband,acontinuousrangethatenablestheelectronstomovefreelywithinthemetal.Thesefreeelectronscarryheat–

whichiswhymetalsaregoodconductorsofheat–andelectricity.

Fromelectricitytoelectronics

The first electronic devices made use of the basic behaviour of electrons.Resistors,forinstance,whichreducetheflowofelectronsthroughacircuit,areusuallymadeofamixofconductingandinsulatingmaterials,reducingtheeasewith which electrons can flow. Another definitive aspect of electronics – theability tocontrolandswitch the flowofcurrent thatenablesus toproduce thelogic gates necessary for computers – is based on the ability to control thedirection of flow of electrons and to switch one current using another. In theearlydaysofelectronics,thesetaskswerehandledbythermionicvalves(knownasvacuumtubesintheUS),whichwerelikeCrookestubesbutmorefunctional(andsignificantlysmaller).Thebestexampleoftheabilityofonecurrenttocontrolanother,theessential

switchingroleattheheartofcomputing,wouldbeinthetriodevalve.HerewehavewhatamountstoatraditionalCrookestube–aglasstubewithmostoftheairremovedandwithacathodeandananode,whicharetwopiecesofconductorinsertedthroughthewallofthetube,sothatelectronscanflowfromthecathodeto the anode. The cathode would normally be heated in a valve, hence thedevice’s typical glow (and the heat it gave off), to give the electrons extraenergy, making it easier for them to flow freely. The switching ability camefromanotherelectrode in the formofagrid that sat in thepathof the flowofelectrons.If thisgridwasgivenanegativecharge,itrepelledtheelectronsandstoppedthempassingthrough,switchingofftheflowthroughthetube.Aswellasactingasasimpleon/offswitch,thetriodecouldalsofunctionas

anamplifier.Asmallamountofcurrentappliedtothegridcouldcontrolamuchbigger current flowing through the valve. So the small variations in the gridcurrentwere amplified intomuchbiggervariations in themain current. If, forinstance,thegridhadanalternatingcurrentwithacomplexwaveformappliedtoit,themaincurrentwouldreplicatethatwaveformbutwithabiggeramplitude,enablingradiosandmusicplayerstoboosttherelativelyweaksignalpickedupfromradiowavesorfromarecording,whereitwouldtypicallybeproducedbyaneedle in a gramophone record pushing on a crystal that produced electricitywhentwisted,aprocessknownaspiezoelectricity.Valvesworked–andarestilloccasionallyused,assomepeoplebelievethey

produce a particularlywarm and attractive sound in the reproduction of audio(though blind testing suggests a lot of this is an audio placebo effect, where

enthusiastshearwhat theywant tohear).Butdevicesbasedonvalvesweren’twithouttheirproblems.Forastart,theglasstubeswerefragile–easilydamagedand anything but safely portable. Theywere also relatively large, the smallestbeing around the size of a thumb and some,where amajor current had to behandled, bigger than awhole hand.What’smore, the need for a heatermeantthat,likealightbulb,theywouldeatupenergyandwouldeventuallyburnoutandneedreplacing.

ComputingwithelectronsBuilding something as complex as a computer using valves was a job on anindustrialscale.Thefirstprogrammableelectroniccomputer,Colossus,usedatBletchleyParkduringtheSecondWorldWartohelpdecodeGermanmessages,had1,500valves in theoriginalversion,and2,400in theMark2.Bythe timetheUSbuiltENIAC,astepuponColossusinbeingtrulygeneralpurpose, thevalve count had risen to over 17,000. These massive machines bore noresemblancetocomputingaswenowknowit,whichwaswhyitwasn’tentirelystupid when Thomas Watson, the head of IBM, remarked that the worldprobablyhadademandforaboutfivecomputers.TherewasnevergoingtobeamarketformanyENIACs.ENIAC weighed around 27 tonnes, was 30 metres (100 feet) long and

consumed150kilowattsofelectricity.Thevastmajorityofthiselectricalpowerwent to heat, so this monstrous device was pumping out enough warmth torequire the building to be constantly cooled (leading to awhole generation ofcomputer rooms with special air conditioning). Because of the inevitable andregularfailureofthevalves,thelongestENIACeverranwithoutbreakingdownwasjustunderfivedays,andamoretypicaltimebetweenfailurewastwodays.We might moan about misbehaving modern computers, but they areastonishinglyreliablebycomparison.Although a fair number of valve-based computerswere built, originally for

militaryanduniversityuseandlaterforbusinesses,theyremainedatinycornerof the growing applications of electronics in everyday life. Before longpracticallyeveryhousehada‘wireless’–aradiothatusedvalves,givingitthecharacteristic‘warmup’timeaftertheequipmentwasswitchedon.ButlessthantenyearsfromENIAC’sfirstuse,asearlyas1954,thetransistorstartedtotakeoverfromthevalve,bothindomesticelectronicsandcomputing.Wewillcomebacktohowitworks,butatransistormanagedtodothesame

jobasthetriodevalve,controllingoneelectricalcurrentwithanother,butusingasolidchunkofmaterial.Therewasnoheater(hencenoneedtowarmup),no

needforadelicateglassenclosure,novacuumtomaintain.Andtransistorscouldbemademuch smaller than the equivalent valve –manywere smaller than afingernail.ThefirsttransistorwasinventedbyJohnBardeen,WilliamShockleyandWalterBrattain,making the trio rare physicsNobel Prizewinners for theinventionofapieceoftechnology,ratherthanascientificdiscovery.ThefirstcomputertousetransistorswasbuiltattheUniversityofManchester

in1953– its92 transistorsavanishinglysmallcountwhencomparedwith the1019transistors(that’s1with19zeroesafterit)turnedouteachyearnow,butitwasthestartofatransformationoftheelectronicsindustry,movingproductionfromexpensivehand-craftedmanufacturingtocheapindustrial-scalemassproduction.

Integrationrules

The last step in the development of modern electronics was the move tointegratedcircuits.Theearlyradios,computersandotherelectronicgearbasedon solid-state electronics consisted of individual components like transistors,resistorsandcapacitorssolderedontoprintedcircuitboards.Theseboardsweresimplyasheetofplasticwherethewiringbetweenthecomponentswasreplacedbylinesinametalfilmonthesurfaceoftheplastic.Theshapeofthecircuitwasetchedontothemetalbystartingwithaboardcoatedwithacompletemetalfilmand painting the parts to be kept with a resisting chemical, then dipping theboardintoacidthatateawaytheunprotectedmetal.Eveninthe1960s,consumerelectronicsandcomputers(thenstillverymuch

an industrial tool)weredependenton thesecircuitboardswith theirmassesofindividualcomponents.Itmeantthatsimpledeviceslikearadiocouldbemademuchsmallerthantheyhadbeenbefore,abletobecarriedaroundinthehandorfitted in the dashboard of a car. But computers, requiringmany thousands oftransistors, would still need hundreds of circuit boards, so they had to beaccommodatedinroom-sizedcabinetsandneededseriouscoolingsystems.Themainframecomputersof the1960smayhavebeen smaller thanENIAC–andpacked in significantly more power – but they were not exactly miniature orsuitableforthehome.To produce the kinds of electronics we are familiar with today, where

computersbecameadesktopbox,oratablet,orevenapocket-sizedcomputerintheformofasmartphone(suchphonesareinrealityapowerfulcomputerwithadded features like the radio transmitters required for calls and Bluetooth),integratedcircuitshadtobeemployed.Devisedinthelate1950sandbecoming

practically available from the mid-1960s, these put all the elements of anelectronic circuit – the transistors, resistors and so on – onto the surface of asinglechipofsilicon.

AhopelessconductorBothsolid-state transistorsand the later integratedcircuitsweremadepossibleas a result of the discovery of the flexibility of what otherwise might haveseemed a useless type of substance.Manymaterials are either conductors likemetals,whichelectricityflowsthrougheasily,orinsulatorslikeceramics,whichprevent electricity from flowing at all, both valuable in electrical circuits.Butthere is a third class of material, semiconductors, which allow limitedconduction,oftenundertheinfluenceofasecondaryinput,thatwouldmakeallthesesolid-stateelectronicspossible.The operation of semiconductors is a purely quantum effect that, unlike

ordinary electricity, couldnot evenbeunderstoodwithout a graspof quantumtheory. Valves were quantum devices, but they could be built and operatedwithoutrealisingwhatwasgoingon,usingasimplemodellikethecontrolofaflowofwater.Withtheintroductionof transistors,wesawthefirst technologyemerging that required an understanding of quantumphysics to design it. Theelectronicdevice,originallyahybridthatusedquantumprocessesbutcouldbe(mis)understood in a classical fashion,was dropping its classical heritage andgoingentirelyweird.Ifwegobacktotheideaofenergybandsinconductors,aninsulatorhasabig

gap (known by the inspired name ‘band gap’) between the ‘valence bands’wheretheboundelectronsstaywiththeatomsandtheconductionbandswhereelectronscanmovefreely.Thismeansitisdifficultforanelectrontogetfree.Inasemiconductorthatgapisnarrower,butthesubstancestillactsprimarilyasaninsulatorwithoutsomeassistance.Forsomekindsofsemiconductor,thatkicktoget it conducting isprovidedby lightenergy.Selenium, for instance, conductsbetterwhen light is falling on it.But for the kinds of semiconductors used intransistors and integrated circuits, the boost usually comes from doping, theadditionofimpuritiestothesemiconductor.Whencurrentflowsinthehigherconductionbandinasemiconductor,afew

oftheelectronsfromthevalencebandarealsokickeduptotheconductionband.Duetoacomplexpeculiarityofthewayelectronsbehave,thoseatthetopofthevalence band behave oddly and move ‘against the current’, so they will beflowing in the opposite direction to the electrons in the conduction band,carryinganygaps in theelectronswith them.Thesegapsareknownas‘holes’

andaretreatedasiftheywereparticlesintheirownright.Thenetresultisthatelectrons flow in one direction in the conduction band and holesmove in theoppositedirectionatthetopofthevalenceband.Thisprocessisstronglyaidedbythedopingagents,becausetheyprovideanextrabandlevelthathasamuchsmallergapthanusual.Dopingdramatically increases thenumberof freeelectronsavailable toplay

with.Therearetwotypesofdopingagent,nfornegativeandpforpositive.Anatom of an n-type doping agent adds a spare electron compared to what isavailableintheoriginalsemiconductor,whileanatomofap-typeagenthasonefewer electron than usual. This may seem a disadvantage, making thesemiconductormorelikeaninsulator,butthemissingelectronprovidesahole,effectivelyoperating as apositively chargedparticle that canmovearound (inrealityelectronsarestillmoving,butaswehaveseen,theholemoveswiththem,andthisissometimeseasiertodealwithmathematically,asasinglemovingholeistheequivalentofmanymovingelectrons).So,forinstance,silicon,theprimesemiconductorofmodernelectronics,mighttypicallybedopedwithphosphorustocreateann-typesemiconductor,orboronforap-type.

Fromsemiconductortocircuit

Astraightforward traditional transistorwasoften constructedof a sandwichofthreesectionsofsemiconductor,correspondingtothethreeelectrodesinatriodevalve–thesematerialswouldusuallybeeithern,pandndopedmaterialsorp,n, p. By applying a voltage between one side and the central segment of thesandwich, the arrangement of extra band levels means that the small voltageappliedactslikeavalve,controllingtheflowofelectronsthroughfromonesideofthesandwichtotheother.In an integratedcircuit, amore complexarrangementknownas aMOSFET

(metaloxidesemiconductorfieldeffecttransistor)istheusualequivalentofthatbasic transistor, producedbygrowing a layer of silicondioxide1 on topof thesilicon wafer and spraying on a fine layer of metal or a substance known aspolycrystalline silicon, producing a more complex layered effect that stillproduces the essential action of the transistor, but in a much more compactarrangement.Even working alone, transistors are valuable because the ability of a small

varyingvoltageappliedtothecentralsegmentofthesandwichtocontrolamuchhigher voltage across from side to side produces amplification. But for acomputer,transistorsarelinkedtogethertoformunitscalledlogicgates.Tosee

whytheseare requiredweneed to takeabriefstepawayfromelectronicsandintotheVictorianmathematicsofBooleanalgebra.

Symbolsoflogic

ThemanbehindthismathematicalodditywasGeorgeBoole,borninacobbler’sshopinLincolnin1815.Althoughhisfatherwasashoemaker,hehadaninterestin maths and engineering, teaching young George the basics of mathematicshimself. George was educated only to the age of sixteen, never attendinguniversity–hewent straight tobe an assistant schoolmaster inDoncaster, butcontinued his education by reading, building a considerable expertise inalgebraic methods. After running his own school for a while, Boole wasappointedtothechairofmathematicsatQueen’sCollege,Cork,wherehetaughtfor the rest of his life, which is why he is sometimes described as an Irishmathematician.Five years after taking up the post, Boole published a book on the

mathematical theories of logic, which turned logic into a kind of algebra thatcould manipulate concepts with symbols. As we will see in a moment, hisapproachisattheheartofthewaycomputerswork,butwealsouseitatamorevisiblelevelwhenspecifyingarequesttoasearchenginelikeGoogle.ImaginethatIputinthefollowingrequest:

(CarsANDtrucks)(redORblue)(NOTFord)

The words I have highlighted in capitals (which is traditional for Booleanalgebra)areeffectivelythekeyinstructionsonhowtoprocesstherequest–theycontrol the search engine. So the ‘AND’ tells the computer that each resultshould refer to both cars and trucks. It isn’t enough to have just cars or justtrucks, both must be present. The middle section uses ‘OR’, which tells thesearchenginethataslongastheresultfeatureseitheroneofredorblueitisfine–itdoesn’thavetoincludeboth,althoughitcan.AndthefinalsectiontellsthesearchengineIwanttoexcludeFordsfrommyresult.Thebracketsarejusttherefor clarity of what goes with what. Interestingly, though search engines wereoriginally strictly Boolean, Google appears not to use Boolean controls anymore:when I tried this search inGoogle Images, aroundhalf the resultswereFords.Googlenowseemstohaveaverylooseinterpretationof‘NOT’,possiblyoverriddenbyadvertisingbudgets.

MrBoole’sgatesCombinationsofsimplelogicalinstructionsareusedtosetupalltheoperationsrequired for the internalworkingsof a computer thatwenever see.There, thecontrolsarereferredtoas‘gates’.So,forinstance,anANDgateisonethattakestwoinputsandreturns1(representedbyanelectricalcurrent)ifbothinputsare1, while it returns 0 (no current) in any other circumstances. Using Booleanlogic,theANDgatereturnsaresultonlyifbothinputshaveavalue.Bycontrast,anORgatereturns1ifeitherofitsinputshasavalue.AndtheNOTgatesimplyreversesasingleinput, turning0into1and1into0.TherearealsocompoundgateslikeaNANDgatethatproducesthereversedoutputofanANDgate.Ifyouhadameanstoproducethesegateselectronically,youcouldassemble

themtoprovideallthemainfunctionsofacomputer–andtransistors(orforthatmattervalves)dojustthis.Ifyouputtwotransistorsinseries,forinstance,theresultisanANDgate,becausecurrentwillflowthroughonlyifbothtransistorsare‘open’totheflowofcurrent.Ifeitheroneisclosedoff,representinga0,thecurrentdoesnotflowthroughthewholestructure–effectivelyputtinga0out.Onlybyhavingbothtransistorssetto1,making1AND1,doyougetanoutputof1.Similarly,anORgatecanbeproducedusingtwotransistorsinparallel,soifeitherisopentoallowingacurrenttoflow(i.e.setto1)thencurrentwillflowthroughthegate,producinganoutputof1.The basic building blocks of electronics can be put together with as much

flexibility as Lego bricks, combining to produce everything from an audioamplifier to a computer.When I was young there were popular toy kits thatenabled the young scientist to do this, plugging actual components liketransistors and resistors into a pegboard circuit.But there ismore building onthisquantumframeworkneededtofillouttheworkingsofeverydevicewehaveproducedsince.Threepiecesof technology inparticulardemandourattention:memory,screensanddigitalcameras.

Afadingmemory

Fromtheearliestdayscomputershaverequiredtwokindsofstorage–aworkingmemoryinwhichtostorebitswhiletheyaremanipulatedbythoselogicgates,andlonger-termstorage.Conventionalelectronicdeviceswerefinefortheshort-termusageofworkingmemory,butforlong-termstoragetherewastheproblemthat as soon as the valves, or later transistors, lost their power, the memorydisappeared.Earlycomputersusuallyreliedoninformationthatwaspunchedasa series of holes in paper tapes or cards, but for much of the lifetime of the

digitalcomputer–andstill toaconsiderableextent–themostfrequentlyusedformsoflong-termstoragehavebeenmagnetic.Here,informationisstoredonametallic surface as a series of magnetic domains – the orientations of smallpiecesofmagneticmaterials – originally ondrumsor on tape, but universallynowontheplattersoffast-rotatingdiscs.The1990ssawanewmodeofstoragebecomecommonplace–opticalstorage

of data on CD-like technology. But this has proved a surprisingly short-livedphaseinthedevelopmentofcomputing.Nowthelong-termstorageofchoiceisoftenflashmemory.Thishastheadvantageofthespeedofreadingandwritingof computer memory – all forms of disc are inevitably slower – without thefragility of a high-speed rotating disc. Anyone like me who has dropped acomputer with a hard disc knows that this is not a goodmove. Because it issolid-state, flash memory can also be made much smaller than a mechanicaldevice,withtensofgigabytessqueezableintoachipthesizeofafingernail.As isusual inelectronics,all themeansof storagesincepunchedpaperand

cardboardwerephasedouthavebeentechnicallyquantuminnature,butjustasthat quantummechanismbecamemore explicit in going fromvalves to solid-state, so themove frommagnetised surfaces to flashmemory brings quantumphenomena to centre stage in its operation.Flashmemorywas invented at theJapanese firm Toshiba around the beginning of the 1980s and was originallyprimarilyhiddenawayandonly indirectlyaccessible,used tohold informationthatwasrarelychanged,liketheBIOSinstructionsthatacomputeruseswhenitfirststartsup.Thiswasbecausetheearlyflashmemorywasexpensiveandslowtoreadandwrite,sonotidealforeverydayrapidmemoryuse.Thefirstof thenewgenerationof flashmemorychipscame intouseduring

the 1990s, used in removable memory cards, and these days it provides thestorage in phones and compact computers, where we take it for granted thatmanygigabytesofinformationcanbesafelystoredonasmall,portabledevice.Thistypeofflashmemoryismuchquickertoaccessthantheearlierversion.Itdoeshavealimitation–itcan’taccessasinglelocationatatime,pullinginorwritinghundredsorthousandsofbitssimultaneously–butthisiseasilyworkedaround,issometimesusefulandisalwaysfaroutweighedbythespeedofaccess.Flashmemory,liketheconventionaltypeofstorage,makesuseoftransistors,

butthesearespecialvariantsofMOSFET(seepage63)devicescalledfloating-gatetransistors.Thegateisthetransistor’sequivalentofthegridinatriode,theelectrode thatcontrols the flow through thevalve. Ina floating-gate transistor,there are twogates: a traditional ‘control’ gate, andbeneath it, a floating gatewhich iselectrically isolatedso itcanholdacharge indefinitely,providing thedatastorage.When the floatinggate ischargedup, it screens thecontrolgates

away from influencing the flow through the transistor, giving it a permanentswitchingrole.The floating gate is totally isolated by insulators, acting as a screen by

induction – which leaves the problem of how to change the value of thisinaccessiblememorybycharginguporerasingthechargeonthegate.Anditishere that a purely quantum effect takes over. Charge is added to the gate, orwiped from it, by quantum tunnelling (see page 34), the process by which aparticle– in thiscase,anelectron–canpass fromonesideofabarrier to theother without passing through the space in between. Without explicitlyexploiting thisweirdquantumeffect, this typeof floating-gate transistor couldnotfunction.Withinanydeviceusingflashmemoryforstoragethereisawholebundleoftunnellinggoingon.

Seeingthedata

Foralongtimetheinformationfromcomputerswaspresentedtotheworldjustasitwasstoredonpaper,firstaspunchedtapesandcardsandlaterastheoutputofautomatedtypewritersknownasteletypes,buttherewasalreadyatechnologyfor outputting visual information that dated back to Victorian times and thatwouldbecomethestandardforcomputersandtelevisions–thecathoderaytube.As we have already seen, cathode rays were really streams of electrons, firstdiscoveredwhentheycausedtheendofanevacuatedglasstubetoglow–butitwasn’t long before the original experiment was improved on in two specificways.Thefirststepforwardfromtheoriginal‘Crookestube’wastocoattheglassat

theendofthetubewithaphosphor.Glassitselfismildlyphosphorescentwhenhitbyastreamofelectrons,hencetheoriginalghostlygreenglowthatCrookesandtheotherexperimenterssaw,butaphosphorgivesoffamuchbrighterlight.Inaphosphor,theincomingelectronssmashintotheatomsinthelatticeofthematerial.Someof theirkineticenergyisabsorbedbytheelectronsorbitingtheatomsofthephosphor,boostingthemfromthefixedvalencebandtothehigher-levelconductanceband,wheretheycandriftthroughthelatticeuntiltheyreachspecially introduced impuritiescalledactivators.Capturedby theactivator, theelectrondropsdowninlevelandgivesoffenergyintheformofatinyflashoflight–ascintillation.Theearly screensworked inblackandwhite,usingamixof zinccadmium

sulphideandzincsulphidesilver,whilecolourscreenshavecollectionsofthreedots of phosphors, peaking in the blue, green and red regions to produce the

primarycoloursoflight.(Ifyouthoughttheprimarycolourswerered,blueandyellow, as they still teach in primary schools, you were misled. These aresimplifications of the secondary colours magenta, cyan and yellow. These‘opposites’ofthetrueprimariesarethekeycolourswhenusingpigments,andasyouwereprobablytaughtthemwithpaintratherthanlightitself,youweretoldafibtokeepthingssimple.)With the rightphosphors, theglow fromacathode ray screencanbebright

andwellcontrolledincolour,butatraditionalCrookestubemechanismsimplylightsupthewholeoftheendofthetube(exceptfortheshadowthatiscastbytheanode).To turnacathode ray tube intoaTVorcomputer screen it isalsonecessarytocontrolatightbeamofelectrons,movingitacrossthesurfaceofthescreentopaintanimageonthephosphorsurface.Theimage,whetheritiswritingonacomputerscreenorapictureonaTV,is

built up by rapidly sweeping the beam across the surface of the screen andrelyingon the phosphor continuing to glow just long enough to still be activewhen thebeamgetsback togive it anotherkick.Thedirectionof thebeam iscontrolled by pairs of electrically charged plates, which steer the stream ofelectronsasrequired.Ofcourse,electronsarefiredonlyatthebitsofthescreenwhere a glow is required.Gaps are left to leave the black in between. (Morepreciselyit’susuallythegreyinbetween.ATVcan’tshowanythingdarkerthanthe colourof the screenwhen switchedoff, butourbrain,well experiencedatfoolingus,turnsitintojetblackwhenrequiredfor,say,aspacescene.)The really rather crude Victorian technology of the cathode ray tube

dominatedthewaywelookedatimagesandtextelectronicallyfromtheearliestdaysofTVright throughto the1990s.Itwasonlythenthat theother typesofdisplaybegantotakeover.Thetroublewithcathoderaytubesisthattheywerebig, bulky and heavy (not to mention needing dangerously high voltages tooperate). Because the ‘gun’ that produced the electrons had to be far enoughbackfromthefrontofthedevicetobeabletosweepitselectronbeamacrossthewholescreen,thetubeshadtobeatleasthalfasdeepastheywerewide.Withtheearliestscreens,whichwerejustafewinchesacross,thiswasn’tmuchofaproblem,butastheimagegotbiggerandbigger,thedepthofthetubebecameasignificantembarrassment.

ElegantdisplaysEnter the flat screen.Flat screens rely on three technologies.The earliest, andstillprobablythemostpopular,wasLCD–theliquidcrystaldisplay.Notonlydidthisdoawaywiththefatbacksideofacathoderaytube,itusedsignificantly

lessenergytoproduceanimage–andwascapableofbeingmademuchlarger.WheretheoldTVsandmonitorsmadeupapicturebycontrollingthewaylightisgeneratedfromthephosphors,anLCDhasauniformilluminationallthetimeacrossthescreenandinsteadcontrolshowmuchlightgetsthroughtotheviewerusingakindoffilter.Thesecrettothewaythisworksistheliquidcrystalitself.Liquid crystals were first discovered way back in 1888, a strange type of

substance thatcouldflowlikea liquidbuthadsomeof thecharacteristicsofasolidcrystal.Theparticularliquidcrystalsusedinscreenshaveaclevertrickuptheirsleeve.Intheirnaturalstatetheytwistlightthatpassesthroughthem.Lighthas a property called polarisation,which can be thought of as the direction inwhich thewave of lightmoves from side to side compared to its direction oftravel.(Ifyouprefertothinkoflightasphotons–seethenextchapter–thenthisisjustapropertyofaphotonthathasaparticulardirectionatrightanglestothedirectionoftravel.)Whenlightpassesthroughtheliquidcrystal,thedirectionofpolarisationistwisted.InanLCDscreen,alargesheetofthecrystalisplacedbetweentwopolarising

filters, which are like sieves that only allow through light polarised in aparticulardirection.Thefiltersareatrightanglestoeachother.So,forinstance,if the back filter works horizontally, only horizontally polarised light getsthrough.Thisthenhitsthefrontfilter,whichonlyletsverticallypolarisedlightthrough.Soitblocksthehorizontallypolarisedlightfromthebackfilter.Result– nothing gets through. A dark screen ensues. But put the liquid crystal inbetweenand it rotates thepolarisation, turninghorizontallypolarised light intovertical.Thismeans the light from the illuminatedpanelbehind thehorizontalfiltershinesoutofthefrontofthescreen.So far, so good. But here’s the clever twist (literally). When an electrical

currentisputacrosstheliquidcrystal,itsmolecules,whichhadbeentwistedinspirals, straighten up. In this new arrangement, the liquid crystal no longerrotates the polarisation of the light. So the screen goes dark.Apply a current,dark,switchoffthecurrent,light.Ifthatwereallthereweretoit,ascreenwouldonlybeanygoodforproducingMorsecode,butinpracticeadisplayisdividedupintomanythousandsoftinysegments,eachofwhichhasakindofelectricalgridtocontrolit.Colourscreenshaveeachofthesesegmentsorpixelsdividedinto three, corresponding to the primary colours.The grid enables a particularsegmenttobeaddressedandallowsjustitspartofthecrystaltohavetherotationeffect switched off and on – the result being that the screen can build up acomplexpicturedependingonhoweachsegmentorpixel(acompactversionof‘pictureelement’)isswitched.Thismeans that a screen can now display a complex image in colour. The

computerIamwritingthisbookonhasa2,550×1,440display–2,550pixelswideby1,440deep–whichmeans that the screenhasa totalof3,672,000ofthesesegments,combiningtomakeaverydetailedimagethatisquitedifficulttodistinguishfromarealview.Althoughtherearenowawholehostofcompetingtechnologies,eachdependingonavariantontheliquidcrystaltechnology,theyallworkbroadlyonthisprinciple,typicallywithaseparatetransistorcontrollingthecurrentappliedtoeachpixeltomanagethewaythedisplayshowsanimage.

ViewingthefourthstateofmatterAn alternative to LCD that was more popular before it was possible tomanufacture really large LCD displays is the plasma screen. These look likeLCDs but are often significantly brighter. This brilliance comes at the cost ofconsiderably higher power consumption and shorter lifetime than an LCD. Aplasmascreen is really likeahugematrixof tiny fluorescent lightbulbs.Eachlittle cell on the screen’s surface contains a noble gas like neon and a smallamountofmercury.Themercuryisvaporisedbyanelectricalcharge,creatingaplasma,acollectionof ions–atomswithelectronsmissingoradded.Unlikeagas,aplasmaisveryconductive.Electronsflowthroughtheplasmaandbrieflyexcitetheelectronsattachedtothemercuryatomswhichthendropbackdown,givingoffultravioletlight.Whenthishigh-energyformoflighthitsaphosphoratthefrontofthecellitmakesthephosphorglowwithvisiblelight,creatingtheimage.Plasma displays are arguably in decline these days, but LCDs also see

competitionfromLEDscreens.Thesehavepixelsmadeupoftinylight-emittingdiodes(hencethename),eachsmallerthanapinhead.Thesediodesmakeuseofaquantumeffect ina semiconductorwhereelectronsplunge intoholes,givingofflightastheydoso.ThemostimpressivethingaboutLEDpanelsisthatthereisnolimit to thesize thatcanbeconstructed–theyhavebeenbuilt40metresacross for outdoor displays. For TVs and computer screens OLEDs (organiclight-emitting diodes) are the most common form of LED, using an organiccompound as the light-emitting layer. LEDs are increasingly popular becausethey can be used to produce thinner, lighter screens thanLCDswith a highercontrastratiothantheiroldercounterpart.

Quantumsnaps

Another technology that has been totally transformed since the 1990s is themeansof takingphotographs.Thischange is reflected in theway theEastman

Kodakcompany,onceoneofthebest-knownbrandsintheworld,wasforcedtogointoprotectivebankruptcyin2012afterthetotalcollapseofthemarketforitsfilms.Thefilmcamerasthatwereusedallthewaythroughthe20thcenturyhadchanged only incrementally from Victorian photographic equipment, butquantumtheoryturnedthetechnologyonitsheadbycapturinganimageusingelectronics.Thedigitalcamerahasrevolutionisedphotography,transformingitsbusiness

modelbecausethereisnolongeraconsumableinvolvedintakingpictures.Itisironic that the digital camerawas actually invented atKodak in 1975, but thecompanyinitiallysuppressedtheproduct,realisingthatitwouldhaveanegativeimpact on its core film business. This was as short-sighted as the reaction ofVictorian gas lighting companies which, faced with electric lighting, decidedthatallthatwasnecessarytoseeoffthecompetitorwastodevelopabettergasmantle.Kodakhaspaidaheavypriceforitsconservativestance.Asaresultofthemovetodigitalwenowtakevastlymorephotographsthanweusedto–andofcourse,thankstotheincorporationofcamerasintheubiquitousmobilephone,manyofuscarryacameraatalltimes.Light entering a digital camera is typically passed through amosaic of tiny

coloured filters, because the sensors used to detect the light don’t distinguishcolours.Thesensorsthemselvesworkintwopossiblemechanisms.Theearliesttechnology, used from that first camera in 1975 and still very common, is thecharge coupled device (CCD).This is effectivelymade up of an array of tinycontainers, each of which can hold electrons. As photons hit a region of thedevice,knockingelectronsfree,thenumberofelectronsinsidethecellbuilds,sotherewillbeahigherelectricalcharge inacell if ithasbeenexposedtomorelight.Whentheimagehasbeencaptured,thevoltageofeachcellismeasuredtoproducethedatathatwillbeturnedintoapicture.The alternative approach to CCDs is a ‘complementary metal oxide

semiconductor’ (CMOS) sensor. In effect, the CMOS sensor is an integratedcircuitwithanarrayoflight-sensitivediodesandamplifierswhichreactdirectlyto the incoming light, rather thanbuildingan imageover time.CMOSsensorshavetendedtotakeoverthemarketformostordinarycameras,astheyoperatefaster and are cheaper to make than CCDs. However, CCDs are still used insome applications like high-quality video cameras, as they capture a wholeimage,whereCMOSsensorsusuallycapturearowatatime,whichcanresultinoddvisualeffectswhencapturinghigh-speedmovingimages.

Theubiquitousinteraction

Thesequantumtechnologiesarenowallaroundus.Wetakethemprettymuchforgranted.TherecanbefewhousesindeedwithoutaTVoraradio,acomputeror a phone. If we take a trip to the laundry room, even the humble washingmachinehascomputingpowerandascreen.NotonlywasFaradayrightthatonedaythegovernmentwould taxhis invention,but thosefirststeps ingeneratingandusingelectricityweretheopeningstepsinthetransformationofourday-to-daylives.Electricity,ofcourse,existedlongbeforewedevelopedtechnologybasedon

it. All living things have an electrical component to their internal operation.There is also electricityon the loose innature,most dramatically in lightning.But there is another quantum effect that ismuchmore obvious in the naturalworldandinsomewaysisevenmoreremarkablethantheelectricaleffect.Thisquantum marvel is responsible for the way we see, and why the Earth ishabitablethankstotheSun.Itisevenattheheartoftheattractionandrepulsionbetween electrically charged particles that lies behind the structure of solidmatter, all mechanics and the way that we can sit on a chair rather than fallthroughit.Itistheinteractionbetweenlightandmatter.

Footnote1.Alsoknownassilica,themaincomponentofsandorquartz.

CHAPTER4

QED

Practicallyeverythingweexperienceistheresultoflight,matterorboth.Lightismuchmorethanaphenomenonthatenablesustosee.ItislightfromtheSuncrossingthevacuumofspacetotheEarththatcarriestheenergythatkeepsourplanetwarmenoughtobehabitable.Different frequenciesof lightperformthecooking in microwave ovens, support our radio, TV and mobile phonecommunications,andenablemedicstoproduceX-raysandCTscans.Andatafundamental quantum level, photons of light are the carriers of theelectromagnetic force that is responsible for themajorityof thedirectphysicalexperienceswe have, from being able to touch something to not disappearingthroughthefloor.

Firefromtheeyes

Lighthasfascinatedhumanbeingsforaslongaswehaveanyrecordofwhatwethought and puzzled about – and no doubtwas amystery and awonder longbeforethen.Inevitablythefirstassociationswemadewithlightwereintermsofsight. The prevailing idea in Ancient Greek times was that sight was madepossiblebyaspecialfireintheheadthatprojectedabeamfromtheeyetotheobject being seen. (Built-in water chambers were thought to prevent this firefromburningus.)Thismightseemacrazyconcept,becauseitimpliesthatthereis no need for an external light source to be able to see something – yet theGreeksrationalisedthisproblembysayingthattheSunhadaroleinsight,butonlyinfacilitatingthebeamsfromtheeye.IfthisnowappearsclumsyandinneedoftheapplicationofOccam’srazor,it

came about by an approach that put philosophical structures above what wasobserved.Because sightwasunderstoodas something thatwedo to theworldaroundus,itseemedinconceivabletotheGreeksthatitcouldbedrivenpurelyby an external source.Theyweren’twilling to accept thatwe are just passivereceptorsoflightthatisalreadyoutthere.AgoodexampleoftheverydifferentunderstandingtheyhadoflightcomesfromtheGreekmathematicianEuclid.He

pointedoutthatifwearealookingforaneedleontheground,thelightfromtheSunisalwaysfallingonit.Butwedon’tnecessarilyseeit.It isonlywhentheviewing ‘light’ from the eyes falls on it that it comes into view. The Sunfacilitated,theybelieved,buttheeyes’beamsproducesight.By the Middle Ages, Arab and European scholars alike had set aside this

tortuousGreekreasoningandsawlightassomekindofflowthatcamefromasourceliketheSun,reflectedoffanobjectasajetofwaterspraysoffawall,andthatcamefromthatobjecttooureyes,enablingustoseeit.TheyrealisedthattheMoondidnotshinewithitsownlightbutsimplyreflectedthedominantlightoftheSun.Asthepictureoflightassomethingthatflowedfromplacetoplaceinstraightlinesbecomemoresophisticated,itwasexpandedbytheintroductionof technology that helped us to manipulate that flow. The earliest opticaltechnology had beenmirrors, polishedmetal or stone that reflected light in amoreconcentratedfashionthananordinaryobject,butitwasthedevelopmentofthelensthatturnedlightintoamoreprofoundvehicleforexploringtheuniverse.Theword ‘lens’ isderived from theLatinname for the lentil, reflecting the

similar shapeof a convex lens to the ediblepulse. If lightwas something thattravelled in straight lines (without worrying too much what that ‘something’was),lensescouldbeusedtobendthatflow,tomanipulatelight,concentratingandmagnifyingtheoutcome.Soonthelenswasmakingitpossible toexaminevery small items that were not discernible to unaided eyesight, or to take indistantviews,notonlyontheEarthbutoutintheheavens.Lightwasbecominga more versatile phenomenon, harnessed to human needs – yet very littleprogresswasmadeinunderstandingwhatlightwas.

Mechanicallight

French philosopher René Descartes, in 1664, was one of the first to give arationalscientificexplanationforthewaylightmoved,thoughhisconceptwaseasy enough to counter. He believed that all of space was filled with anintangiblesubstancehecalledtheplenum.(Thismightseemstrangeenoughtocallhistheoryintoquestion,althoughtimeandagainscientistshavecomebacktoasimilarconcept,whetheritwastheluminiferousether,whichwewillsoonseebeingusedtoexplainlight’smotion,orthemodernconceptofquantumfieldtheory,whichisoneverysignificantwaycurrentlyusedtomodelthenatureofreality.)Descartes imagined that when something gave off light it set up a kind of

pressure(ashecalledit,a‘tendencytomotion’)intheplenum.So,forinstance,

whenwelookupatthenightskyandseeastar,itwas,Descartesthought,asifthere were an immensely long rod connecting the star to our eye. The starpresses on one end of the rod; the other end of the rod presses on the eye,producing the effect of vision. This model implied that light travelled at aninfinitespeed,whichhasbeenasubjectofmuchdebateforcenturies.Descartesknewthatitwascertainlyveryfast–Galileohadattemptedtomeasurethespeedbysendingaservant toadistanthillwitha lanternatnight, trying to time thelight’s two-way journey when he signalled to the servant and the servantsignalled back. He found exactly the same timing was recorded when theyperformed theexperimentstandingnext toeachother.All thedelay theywereable to spot was caused by their response time. But whether light wasinstantaneousorjustspeedywasbeyondtheexperimentsoftheday.Itwas IsaacNewton, bornnearly50years afterDescartes,whomade some

striking discoveries about light, and replaced Descartes’ theory with a morepractical-soundingone–thoughNewton’sownideasweredisputedandwouldsoonbedismissedforover200yearsbeforetheywerefoundtobeclosertothetruththananyonecouldhaveimagined.Newtonthoughtthatlightwasastreamofparticleshe called ‘corpuscles’.Thesemeant that light could travel throughspacewithoutanyinvisibleplenumorether,makingNewton’smodelpleasinglysimplecomparedwithmostoftheopposingtheories;andaccordingtohim,lightwouldbeexpectedtotravelatafinite,ifextremelyquickspeed.

UnweavingtherainbowNewtondidnotattempttomeasurethisspeed,buthedidperformanumberofexperimentsthatopenedupourunderstandingoflightandcolour.WhenhewasatCambridge in 1664, aged 22,Newton attended a fair and bought a toy thatwouldprovehighlyeducational.Atthetimetheuniversity’sprivatepoliceforce,the proctors, attempted to keep university members under control (and awayfromthecity’staverns),whichmeantthattheStourbridgeFair,justoutsidetheboundsoftheproctors’remit,wasanannualopportunityfortheacademicstolettheirhairdownandhavea littlefun.Itwasthere,fromastallsellingtoysandtrinkets, thatNewtonbought a prism, ablockof glasswith a triangular cross-section.Thereasontheprismwassoldasatoyisthatithadlongbeenknownthatit

would produce a pretty rainbow pattern when light shone through it. Newtonwasdeterminedtodiscoverwhatwasgoingon.Themostpopulartheoryofthetimewasthatimperfectionsintheglasscolouredthelightasitpassedthrough–somethingthatseemedquitelikely,asthequalityofglassatthetimewasfairly

poor (so poor, in fact, that when the German writer and science enthusiastGoethe tried to reproduce Newton’s experiments he couldn’t even see arainbow). After playing with the prism for a while, Newton was inspired toobtainasecondonetotestoutthistheory.He argued that if the spectrumwas produced by imperfections in the glass,

thenifhepickedoutaparticularcolourandsentthatthroughhissecondprism,thecolourwouldbemodifiedoncemore. Itwasn’t– it stayed thesame.Evenmoreimpressively,heconsistentlyobservedthatthedifferentcolourswerebentby different amounts by the prism. (There is some doubt about how muchNewton achievedwith theverypoor-qualityprismshewasworkingwith, andhowmuch itwas a case that hewas convincedwhat the result should be andreported it as such. The fact remains, though, that he got it right.) TheobservationshemadeinspiredNewtontoexplainthatwhitelightwasmadeupofall thecoloursof thespectrum,whichweresplitout todifferentdegreesbytheeffectofpassingthroughtheprism.Onceheunderstoodthewaythatthecolourswerealwayspresentinsunlight,

itopenedNewton’seyestotheexplanationofwhyweseeanobjectasbeingaparticular colour.When, for instance,white light from theSunhits a red postbox,thepainttendstoabsorbthecoloursinthebeam,butitdoesn’thangontothe red light. So when the light reflects back to our eye, it is only the redcomponent that is left and the box appears red. Although there was someresistance to Newton’s ideas, particularly from his arch-rival, Robert Hooke,Newton’sviewpointsoontriumphed.ButNewtonhadlesssuccessinpersuadingothers of his notion that lightwasmade up of corpuscles.Unlike the rainbowfrom the prism, he had no clear experiment to support his notion, merely asignificant distaste for the alternative theory, and to be fair, therewas a goodreasonforhissuspicion.

Ripplesintheether

WithDescartes’ theoryoutof theway, the alternative to lightbeingaparticlewasthatitwasawave,liketheripplesthatspreadoutonasmoothpondwhenastone isdropped into it, thoughoperating in threedimensions rather than two.Thiswastheopinion,forinstance,oftheDutchscientistChristiaanHuygens.Itwasalreadywidelyacceptedthatsoundtravelledasawavethroughtheair,andthe(admittedlylimited)similaritiesbetweensoundandlight,particularlyintheiranthropocentriclinkagethroughthesensesofhearingandvision,helpedsupporttheideathatlightalsotravelledthisway.

Newton, however, quite reasonably struggledwith the concept of thewave,becauseoftheextrarequirementitplacedonnature.Particlescanhappilypassthroughemptyspace.Butwavesneedamedium.Awaveisfundamentallyjustamovementwithinasubstance.Somethinghastodothewaving.Forsounditwasobvious enough that themediumwas air, andbefore long thiswasmadeveryclearwhenaringingbellwasput ina jarandtheairwassuckedout.Thebellcould no longer be heard, because there was no air for the sound to travelthrough.Butthebelldidnotdisappear–itcouldstillbeseen.Sothelighthadnoneedforairtocreateitswaves.Huygens imagined something like Descartes’ plenum, but instead of being

rigid, he thought that it was composed of lots of little compressible chunks,ratherlikespacebeingfulloftinyrubberballs.Lightwouldpassthroughitasaseriesoflittlewavelets,movingfromballtoball.Initially therewas littleexperimentalevidence tosupporteither theory,with

muchargumentoverexactlywhatwashappeninginrefraction,whenlightbentas it moved from one substance to another. But at the very start of the 19thcentury the apparent death blow for Newton’s idea was Young’s experimentwithtwoslitsthatwehavealreadymet(seepage9).Iflightwascorpuscles,asNewtonsaid,Youngexpectedtoseetwobrightbarsonthescreen,oneforeachslit.Butinsteadwhatwasseenwasaseriesofdarkandlightfringes.Thisjustdidn’tmakesenseforparticles.However,iflightwereawave,aswehaveseen,interferencebetweenthetwowavesshouldcausethefringesthatareobserved.

Whichwaytowave?Youngalsoguessed(correctly)thatthefrequencyofthewave–thetimeittooktogetfrompeaktopeak–wasdifferentdependingonthecolourofthelight.Infact,thatthiswaswhythereweredifferentcoloursoflight–andthiswasalsoreflected inhis experiment,becausechanging thecolour shifted thepatternonthe screen, aswould be expected if thewavelengthwas changing.ButYoungalsomadeasuggestionthatmanytooktobeasteptoofar.Ithadgenerallybeenassumedthatlighthadtobeawave,likesound,thatmadeitsoscillationsbackand forth in the samedirectionas thewave travelled, rather than jerking fromsidetoside,likeawaveonthesurfaceofapondorinarope.ButYoungthoughttherewassomeevidencethatlightwassuchatransversewavethatwiggledsidetosidecomparedwithitsdirectionoftravel.This presented a real problem for the theory, because transversewaves can

existonlyontheedgeofsomething.Ontheedge,thewavecanstickoutofthematerialunopposed,butifit triedtotravelthroughthecentreofthematerialit

wouldsoonbedampeddownbycollisionwiththestuffaroundit.Yetwhateverit was that was waving to enable light to travel, known by now as theluminiferousether,thelighttravelledcomfortablythroughthemiddleofit.Howcouldthewaveripplefromsidetoside?Althoughnoonecouldexplainhowthiswassustainable,Younghadheardabouttheeffectcalledpolarisationwhereyoucould have apparently different ‘types’ of light associated with a direction atright angles to thebeam’s travel.This,Young thought, couldonly sensiblybeexplainediflightwasaside-to-sidewave.Worryingabouthowthiswaspossiblewouldhavetowaitforabettertheory.Moreandmoreexperimentsconfirmedthatlightdidtravelasawave,though

uncomfortablytherewasnootherevidencefortheexistenceoftheetherneededforittowavein.Andtheetherhadtobeastrangematerialindeed,fillingallofspace, totally undetectable to the touch, capable of vibrating andyet infinitelyrigidsothattherewasnolossofenergyaslightwavespassedthroughit.

Faraday’sspeculationIn 1846, Michael Faraday would give the first suggestion of why the ethersimply wasn’t needed. Although he seems to have been a reserved person ineveryday social life, Faradaywas a great science communicator and regularlylecturedat theRoyal Institution.According to legend, fellowphysicistCharlesWheatstonewasduetogiveaFridayeveningdiscourseattheInstitution.Thesestiffaffairsweredecidedlyintimidating,especiallyas,bytradition, thelecturerwas expected to rush out onto the platform and begin his talk with nointroduction.Onthefatefuleveningof10April1846,Wheatstone issaid tohave losthis

nerve at the prospect of addressing such an audience and rushed off, leavingFaraday,themanresponsiblefortheevent,tofillinandextemporise.It’sagoodstory, but probably not true. The reason this is thought to be legend is thatFaraday had aweek’s notice to fill in for JamesNapier, whowas the personactuallyduetospeak.FaradaydidgiveabriefpaperonWheatstone’sinventionofanelectricclock,butthenwentontospeakonhisownthoughtsaboutlight.Aswehaveseen,Faradayhadcomeupwiththeideaofafield,extendingout

fromaconductor,andproducing,forinstance,electricityasmagneticfieldlineswerecut.Now,hesuggestedthatlightwasavibration–awave–inthoselinesof force.Not amechanical vibration inmatter, but awave in an insubstantialforce field. As he put it in his lecture, his theory ‘endeavours to dismiss theaether, but not the vibrations’. If the lines of forceweremore than imaginaryaidsbuta real field (whatever thatwas), then itcouldcarryvibrationswithout

theneedforamagicalether.

Theinterplayofwaves

Michael Faraday never pretended to be a mathematician, and his ideas weremore a visual representation than a detailed theory. But when James ClerkMaxwell (seepage50) cameupwith amathematicaldescriptionof the linkedeffects of electricity and magnetism, he also provided the final piece in thejigsaw of understanding the nature of light, filling in the gaps of Faraday’shypothesis.Strangely,Maxwellneverdiddismisstheideaoftheether(showingthateventhegreatestscientistcanbemorethanalittleconservativeinhisorherideas).Infactitwasbyconsideringtheetherasafluidthathecameupwithhisshow-stoppingresult.Faradayhadshownthatmovingmagnetismproducedelectricity,andmoving

electricityproducedmagnetism.Maxwellrealisedthatamagneticwavemovingat just the right speed would produce an electric wave that produced themagneticwaveandsoon…but itwouldcontinuetohaul itselfupbyitsownbootstrapsonlyifitmovedatthespeedoflight.‘Thisvelocityissonearlythatof light’,Maxwellcommented,‘that itseemswehavestrongreasontobelievethat light itself (including radiant heat and other radiations if any) is anelectromagnetic disturbance in the form of waves propagated through theelectromagneticfieldaccordingtoelectromagneticlaws.’Atthisstageinourunderstandingoflight,itcouldn’tbefurtherfromquantum

theory.Although, aswehave already seen, itwas theunderstanding that lighthad to come in little packets that would form the origin of quantum theory,Maxwell’s electromagnetic waves in the ether were entirely smooth andcontinuous. No packets here; nothing strange. Maxwell’s assumption that theetherstillexisted(ignoringFaraday’sdismissalof theneedfor it)wasstill thepredominant view towards the end of the 1880s, when American physicistsAlbertMichelsonandEdwardMorleysetouttostudytheether.TheyplannedtousetheEarthitselfaspartoftheirlaboratory.

Analtartotheether

Theetherwasconsidered theuniversalconstant, something thatwas fixedandthat everything moved through, giving a frame of reference to measureeverything against. If the ether were not there, as Galileo had shown in theconcept known as relativity long before Einstein came on the scene, all

movementwouldhavetobeconsideredasrelativetosomething,becausetherewas no ‘preferred frame’, nothing that defined what being motionless trulymeant.Theetherprovidedthatreference.AstheEarthrushedthroughtheetheronits journeyaroundtheSun, itshouldresult inaneffectiveflowof theetherpasttheplanet,whichmeantthatlightshouldtravelatslightlydifferentspeedsdependingonwhetheryoumeasureditinthedirectiontheEarthwasmovingoratrightanglestoitstravel.MichelsonandMorley’sexperimentalset-uplookedmorelikesomethingthat

belonged in amedieval cathedral than a highly technical experiment in a late19th-centurylaboratory.Onabrickbasewasacircularmetaltroughfilledwiththealchemists’favouritesubstance,mercury.Awoodenstructurefloatedonthisliquidmetal, on top of which was positioned a large stone slab. So carefullyconstructedwastheequipmentthatonceitwassetinmotionatastatelyrateofturningonce every sixminutes, itwould keepgoing for hourswithout furtherintervention.Ontopofthestoneplatformwasanopticalarrangementthatsplitabeamof

light in half, sent the two portions off at right angles, then recombined them,where a microscope made it easy to watch the interference fringes producedwhen thewavesof light interacted. If therewasanydifference in thespeedoflight in the twodirections, itought to result in the fringesshiftingas thegreatslab ponderously rotated. The outcome was a huge anti-climax. Nothinghappened.Whathadsetoutasanexperimenttomeasureapropertyoftheetherendedupindismissingitsexistencealtogether.Somewould remain uncomfortable that there seemed to be nothing for the

waveof light towavein,whileothersrevertedtoFaraday’spositionthat therewasnolongeraneedforanether,becausethewavewassimplyadisplacementin the insubstantial electrical and magnetic fields. However, once Planck’sdevelopmentofthequantumandEinstein’stheorylinkingittothephotoelectriceffecthadbeenestablished,therewasevenlesstobeworriedabout,asnoonethought that particles needed an ether and it seemed that lightwas, at least insomemysteriousway,asmuchaparticleasitwasawave.

Amightyspectrum

Bythetimethequantumtheoryoflightwasstartingtobeaccepted,‘light’hadgonefrombeingatermforthestuffwesawwithtocoveringahugespectrumofelectromagneticradiation,runningallthewayfromverylong-wavelengthradio,throughmicrowaves, infrared,visible,ultraviolet,X-raysandgammarays.The

bitwecanseeisjustaverynarrowslicesittingaroundthemiddleoftherange.All thosedistinctionsareman-made.There isnoboundarybetween,say, radioandmicrowaves,orultravioletandX-rayswheresomethingdrasticallychanges.Thereisnodifferenceotherthanfrequencyorenergybetweentheradarweuseto keep aircraft flying safely and the deadly gamma rays emerging from anuclearblast.Ifwe thinkof light as awave,we simplyhave an electromagneticwaveof

shorterandshorterwavelength,orhigherandhigherfrequency,aswemoveupthe spectrum.With a quantumhat on, the picture is even simpler, going fromlow-energyphotons tohigh-energyphotons–andall thedifferingcapabilities,like, for instance, the way X-rays pass through flesh and damage DNA, arenothingmorethanareflectionofthoseincreasingenergylevels.

ThedanceoflightandmatterWhat was emerging with the development of quantum theory was anunderstandingofthewaythatlightandmatterinteracted.Theoriginalquantumideahadcomefromthewayhotmatteremittedlight,whileEinstein’sworkhadbeenbasedonlighthittingmetalandknockingoutelectrons.Itmightseemtheinteractionof lightandmatter isonlyasmallpartof thephysicalworld,but itprovedtobeoneofthemostimportant.Alllightisproducedbymatter.Itistheinteractionoflightandmatterthatheatsourplanetandenablesustosee.Anditwould be discovered that even as fundamental an interaction as theelectromagnetic repulsion between your bottom and a chair that prevents yousinking through it is a result of a stream of never-observed photons passingbetweentheatoms.Our understanding of this interaction between light and matter, which was

given the name QED, short for quantum electrodynamics, was the work of agood number of people, but one inevitably stands out. The groundwork camefromthereclusiveBritishphysicistPaulDirac,butthebest-knownapproachthatpulled together QED came from one of his contemporaries. He was thephysicists’physicist,themanwhompracticallyanyworkingphysicistwouldputatthetopoftheir‘mostadmired’list–theAmericangeniusRichardFeynman.

TheFeynmantouch

Feynman’s popularity combines a number of factors. He was outgoing and agreat communicator in a field where many workers were (and still are)introverted – but he also had a way of seeing problems in physics with a

different eye tomanyofhis colleagues,not relyingonexistingwaysofdoingthings.ThisviewseemstohavebeeninstilledintheyoungRichardatanearlyagebyhis father,Melville.Born in thedyingmonthsof theFirstWorldWar,RichardwouldbechallengedbyMelvilletolookbeyondthelabelstowhatwasactuallyhappening.FeynmanstudiedatMITbeforemoving toPrinceton.While searching fora

topicforhisdoctorate,hecameacrossapaperonquantumtheorybyDirac. ItsetFeynmanthinkinghowtogeneralisewhat theBritishphysicisthaddone toproduce a simpler description of quantumevents. Feynman took a very visualapproachtohismathematics,andmadeuseoftheideaofparticle‘worldlines’.InFeynmandiagrams theseareplotsof thepositionofaparticleagainst time,with time as one dimension andposition in space the other.Traditionalworldlinediagramsalwayshavetimegoingupthepageandpositionhorizontally,butFeynmandiagramscanbedrawnineitherorientation.Sowithtimegoingupthepage,astationaryparticlewouldbedescribedbya

vertical line,whileonemovingat a steadyspeedwouldbeadiagonal straightline. In the real world, of course, particles can move in three dimensions ofspace,butthatwouldmakeaFeynmandiagramfour-dimensional,whichisabitof a pain to draw. To keep things simple we consider just the one arbitraryspatialdimension,butacknowledgethattherestaretheretoo.Quantum theorymade it clear that a particle didn’t have a definite location

withtimeortrajectory.SoFeynmanimagineddrawingeverypossibleworldlinethatlinkedtheparticle’sstartandendpoints.Ifyoucouldpulltogetheralltheselines, each with its own probability attached, you would have a completedescriptionofhowtheparticlebehaved.Therewereinfinitelymanywaysforaparticle to get from A to B, but Feynman realised this didn’t have to be aproblem.Afterall,thewholepointofintegralcalculuswastoprovideawayofproducing a finite end product from adding together an infinite collection ofsmallquantities.Andanyway,manyofthepathswouldinpracticecanceleachotherout,orwouldbeofsuchalowprobabilitythattheycouldbeignored.AtthetimeFeynman’sideawasn’tapracticaltoolforcalculation,butitwas

enoughforhisthesis.Hesettleddowntoheadtowardsacomfortableconclusionto his PhD when the SecondWorldWar intervened. In December 1941, thesamemonthastheJapaneseattackonPearlHarbor,Feynmanwassecondedtowork on a top-secret project, to produce a bomb based on a nuclear chainreaction.Theatomicbomb.

Quantumdestruction

WhileworkingwithateamlookingatwaystoseparatetheuraniumisotopeU-235 from the much more common and chemically indistinguishable U-238,Feynmancompletedhisdoctorate, inpartbecausehewasrequiredtocompletethisbeforehegotmarried.Hehadbeenengaged tohis fiancéArlineforsometime, but the urgency was a result of her advanced tuberculosis. They weremarriedinhospitalinJuly1942.Meanwhile,Feynman’sworkonuraniumseparationwasmaderedundantbya

bettermethod,andhewasmoved toLosAlamos inNewMexico tobecomeamoredirectpartoftheManhattanProject.HeagreedtogoonlyifArlinecouldbe found a hospital nearby. In the end, the closest they could achieve wasAlbuquerque,60milesaway.AsFeynmanworkedonthecomplexcalculationsrequired to specify the detailed construction of the bombs,Arline gotweaker,dyinginJune1945.Justonemonthlater,withFeynmanpresent,thefirstnucleardevicewasexplodedintheTrinitytestatAlamogordo,200milessouthofLosAlamos.FeynmanmadeahugecontributionatLosAlamos,bothinthephysicsandin

keepingmoralehighamongthescientists,whoinevitablyclashedculturallywiththemilitary.One of hisways to do thiswas to constantly challenge the pettyregulationsthatbesetmilitarylife,gettingareputationforbeingabletobreakinandoutofthebaseunobservedandofbeingabletoaccesslockedfilingcabinetsandsafes.Butitwasoncehegotbacktocivilianlifethathewasabletopickuptheworkthatwentintohisdoctorateandcarryitforwardtodeveloptheultimatetheoryoftheinteractionoflightandmatter,quantumelectrodynamics–QED.

QuoderatdemonstrandumThe theory was developed in different but parallel ways by Feynman, JulianSchwingeratHarvard,and,independently,bySin’ItiroTomonagainJapan–yetit was Feynman’s version that made the theory approachable as a way ofunderstanding the interaction of photons of light and electrons. In essencepracticallyeveryinteractionoflightandmattercomesdowntoacombinationofverysimpleelements.Eitheranelectronlosesenergyandgivesoffaphoton,oran electron absorbs energy and a photon disappears. Photons are born aselectronsdropdowninenergyanddieastheyjumpupinenergy.Althoughthisseemsasmallpartofreality,infactQEDcoversaremarkably

largeswatheofoureverydayexperience,anddoessostunninglywell.Ofallthephysical theorieswehave,QED’spredictions areby far the closest towhat isactuallyobserved.AsFeynmandelightedinpointingout,QEDsowellmatchesobservation, it is as if we had a theory that predicted the distance fromNew

YorktoLosAngelestotheaccuracyofthewidthofahumanhair.Thisapparentperfection,henoted,leavesuswithaproblem,becauseQEDdoesnotdescribethe common-sense world we think we experience. Instead it piles on thequantum weirdness with particles acting as if they considered every pathpossible at the same time and even sometimes acting as if they travelledbackwards in time.And yet the theory is remarkably precise in predicting theactualoutcomesweobserve.Feynmanwouldusehisworldlinediagrams,throwingin,wherenecessary,an

additionalcomplexityofanarrowattachedtoaparticlelikethesecondhandofawatch. This arrow rotated steadily with time, while the size of the arrow (ormoreaccuratelyitssquare) indicatedtheprobabilityoffindingtheparticleataparticular location. This compass-needle arrow represented a property of aparticle called its phase and provided the interface between a particle and awave. The phase represented the position in thewavemotion at that point intimeandspace. Its regular rotationwith timeenabledaparticle toproduce theobservedwave-likeeffects.Once he had photons on his diagrams equipped with their phase arrows,

Feynman realised that every problem classical physics had thrown at the ideathat light was a stream of particles – the way, for instance, it reflected off asurfaceorinterferedwithasecondbeamoflight–wasanecessaryoutcomeofthisstructure.Onceyourparticleshadphaseitwasn’tnecessaryto thinkabouttraditional waves. This doesn’t mean physicists don’t still talk about waves.Theyaresometimestheeasiestwaytodescribewhatishappeningortocalculatetheoutcome.Buttheywerenolongeressential.It’simportanttobearinmindatalltimesthatphysicsisn’taboutdescribingreality.Itisaboutprovidingamodelthatpredictsoutcomesthatmatchwhatisobservedascloselyaspossible.

Lightis…lightIrememberearlyoninmyexplorationinphysicsaskingaprofessor:‘Yes,butwhatdowe think light really is? Is it awave,or is it aparticle?’Hegroaned.Sometimes,Feynmanseemedtocomedownfirmlyonthesideofparticles.Hewrote:‘Iwant toemphasizethat lightcomesinthisform–particles.It isveryimportant toknow that lightbehaves likeparticles, especially for thoseofyouwhohavegonetoschool,whereyouwereprobablytoldsomethingabout lightbehaving likewaves. I’m telling you theway itdoes behave – like particles.’Feynmanwasrightinthecontextofwhathewassaying.HisapproachtoQEDworked by considering light as a particle, and it worked well. Waves aren’tnecessary.Butdespitehisdramaticwordinghewasn’tsayinglightliterallywas

aparticle.Apart fromanything,QED is aquantum field theory, treating light in some

respectsasneitheraparticlenorawavebuta fieldwithasetofvaluesacrossspace and time. In fact that is howmost working physicists will see light inpractice.Buteventhoughsomephysicistswouldprobablyarguetothecontrary,theideaoflightasdisturbanceinafieldisnotatruedescriptionofrealityeither.Lightisn’taparticleanditisn’tawaveanditisn’tadisturbanceinafield.It’s

light. It operates at a quantum level that we can never directly observe ordescribe.Lightbouncingoffamirrorisn’tlikeatennisballhittingawallorlikeawavehittingablockage.Thosearelarge-scaleitemsthatwecanusetogiveamental picture that represents what is going on, but they aren’t what light isreallylike.Andlightisn’ttheoutcomeofadisturbanceinafield–thatisjustamathematicalapproachthathappenstoproducereliableresults.Allthreearejustrepresentations – models, as scientists call them – that enable us to makepredictions.Sometimesthewavemodeliseasiertouse,sometimestheparticlemodel.Fromamathematicalviewpointthefieldapproachismostuniversal,butisoftendifficulttovisualise.Eachissometimeshelpful.Noneisatruepictureofreality.

Onreflection

To get a proper understanding of the revolutionary nature of Feynman’sapproach,andhowthebehaviourofthesespecialparticlescannotonlydeliverthe same results as considering light awave, but can also better predictwhatreallyhappensinsomecircumstances,let’slookataverysimpleopticalset-up,onethatwasfamiliarasfarbackastheMiddleAges–abeamoflightreflectingoffamirror.Atschoolyouwereprobablytoldthelightbeamor‘ray’travelsintowardsthemirroronastraightline,bouncesoffthemirrorandheadsoutinastraightlineatthesame(butopposite)angleatwhichitarrived.Thisisausefulsimplification,butitisbynomeansourbestmodelofwhatishappening.Tobeginwith, lightdoesn’tbounceoff themirror likeaballbouncingoffa

wall.An incomingphoton isabsorbedbyanelectron in themirror,and thenasecondphoton isemittedby theelectron. It isn’t thesamelight that leaves themirror as originally headed towards it. Butmore significantly, the photon cantakeanyrouteitlikestogetfromAtoB.Forinstance,insteadofbouncingoffthe middle of the mirror at a symmetrical angle it can head in at a muchshallowerangle,reachingmuchfurtheracrossthemirrorbeforeitreflects,thenshoot off at a much sharper angle. Every path it can take in this fashion has

prettywellexactlythesameprobabilityofoccurring.Sowhydoweseeabeamoflightreflectfromthemiddlewithequalangles?

Fig.3.Manypathsreflection.

Asithappens,theoutcomeisthesamewiththemanypathsandthetraditionalray bouncing symmetrically. If you do the maths and add together all thepossible paths, taking into account the compass arrows of phase, most of thepathswillcanceleachotherout,resultinginthepaththatweexpectedinthefirstplace – reflecting at equal angles. However, we can’t throw away the morecomplexpicture. Ifyoutakeamirrorwhere light is reflectingfromthemiddleandchopout themiddlesectionyouwon’tseeareflection.Butputaseriesofdark lines on one side, leaving only the paths where the photon would havesimilar phases, and the reflection starts again – at an angle that seems totallycrazyforreflectionasweexpect it tobehavefromourexperienceofbouncingballs.A photon does, in a probabilistic sense, take every single path for the

reflection;wejustdon’tgenerallyseetheoutcomebecauseofphasescancelling.

Fig.4.Missingmirrorreflection.

TakingtheeasyrouteThere’sanotherrevelationfromthewaylightreflectsfromamirrorthatseems

to relate to a very fundamental aspect of the universe – it tells us that theuniverseislazy.Thisissometimescalledtheprincipleofleastaction.Ittellsus,forinstance,whattrajectoryaballwilltake.‘Action’inaphysicssenseisinthiscase the difference between the potential energy – the energy due to the ballbeingsuspendedinagravityfieldthatpullsittowardstheEarth–andthekineticenergy of its motion. The path the ball takes is the one that minimises this‘action’.Similarly, lightobeys theprincipleof least time, taking the route thatwillgetittoitsdestinationintheshortesttime.Itmightseemthatthiswillalwaysinvolvetravellinginastraightline–andit

oftendoes.Butwhere,forinstance,lightmovesfromairtoglassorairtowater,it bends in the process called refraction. In this particular circumstance theprincipleofleasttimeissometimescalledtheBaywatchPrinciple,aslifeguardsunderstand the concept that seems to guide the path of light. Rather than runstraight towardsadrowningperson,a lifeguardwill run furtheron thesand tocutdown thedistance theyneed to travel through thewater, because even thefastestswimmerisslowerinwaterthanonland.Similarly,lightwilltravelalonga route that takes it further through air and less far throughwater or glass, aslightisslowerinwaterandglass.Itfollowsthepaththatwilltaketheleasttime,producingthatrefractingbend.NowwhenyoulookatallthedifferentpathslightcantakegettingfromAto

Bwhile reflectingoffamirror, thepaths thatarenear thecentreof themirror(numbered5 to9 inFig.5)are theones taking theshortest time–andasyoumoveawayfromthecentre, tobeginwith, thepathlengthincreasesonlyquiteslowly.Thatmeansthephotonsfollowingthosecentralpathshavephasearrowspointinginroughlythesamedirection,sotheyreinforceeachother.Asyougetfurther away from the centre, the length of path increases more and morequickly, so there ismorechance for thearrow tobe facing in averydifferentdirectionandforthephasesofaphotontocancelout.RichardFeynmancametohis approach to QED from thinking about the principle of least time and itsimplicationsforphotons.

Fig.5.Differentpathsforreflection.

This is alsowhywe say that light travels in straight lines, andwhywe canusuallygetawaywithconsideringonlystraight linesfromAto themirrorandmirror toB. In reality, there is a probability for a photon to take every singlepathbetweenA andB, includinggoing in the opposite direction, travelling toParis,takingatriproundtheEiffelTowerandreturninginawigglyflightlikeadrunken fly.But these routes runcounter to theprincipleof least time,andassoonasthephotonwigglessignificantlyfromthestraightlineitgivesthephaseclockachancetogetfarenoughawayfromthatofastraight-linephotontohavesomething to cancel it out – but all the very nearly straight-line routes havephaseclockspointinginalmostexactlythesamedirectionandaddtogether.Thestraightlineisnotthe‘actual’route,itisjusttheonethatisleftbehindwhentheotherclockshavecancelledeachotherout.

Magicmirrors

Intherealworld,reflectionisoftenrathermorecomplex.(Infactitisatruismthat the realworld isnearlyalwaysmorecomplex thanaphysicsexperiment.)Thebasicreflectionexperiment isusuallydonewith lightofasinglecolour tokeepthingssimple.Thecolouroflightdependsontheenergyofitsphotons,but

it also corresponds to thewavelength of the lightwhen thought of as awave,whichmeans that for lightofdifferent colours the littlephasearrows rotate atdifferentspeeds.Thismeansthatdifferentcolourswillhavetheirphasesaddingupconstructively for reflectionatdifferentanglesaway from thecentre. Ifweforcethephotonstogoatastrangeanglebyselectingjustsomeofthepossiblephases,by takingaway thecentre andusinga seriesofdark linesknownas arefractiongrating (this is the samesituationasFig.4above,butwithmultiplecolours), thedifferent colourswill split out.Shinewhite light ononeof thosespecialmirrorswiththedarklinesandyouwillseearainbow.Thisisanexperimentanyonecandoathome,becausewehavelotsofdiscs

withtinyslotsintheirsurfacethatprovideasimilareffecttothosedarklines–CDsandDVDs.Holdadiscatanangleinwhitelightandyouwillseerainboweffects that are the result of exactly this kind of reflection in an unexpecteddirectionpredictedbyQED.

Everywhichway

WecanalsouseFeynman’sapproachtotakeasubtlydifferentwayoflookingatthequantumversionof the two-slitexperiment,whether it involvesphotonsorelectrons.ThewaywelookedatitinChapter2wastosaythataphotondidn’thave a location.All that existedbefore, for instance, it hit the screen andwasregistered, was a wave showing its probabilities, which is why in passingthroughtheslits,thewavecouldcauseinterference.Feynman’swayoflookingatitwouldbetosaythatwecanthinkofthephotontakingeverypossiblepathbetweenitssourceandthepointonthescreenwhereitisdetected.Thinkofthatforamoment,andconsiderhowmindbogglinglyoutrageousthis

is.WilliamofOckham,who came upwith the ‘Occam’s razor’ ideawe havealreadymet(‘Ockham’isspelleddifferentlyforhistoricalreasons),wouldhaveafit.Hisprinciplesaysthatwithnothingelsetoguideus,weshouldgoforthesimplesttheory(tobeprecise,‘entitiesshouldnotbemultipliedunnecessarily’).YethereinFeynman’simaginationisanentitybeingmultipliedaninfinitesetoftimes.We are not just saying that the photon takes every simple straight-linepath,likepassingthroughbothoftheslitsinascloseaspossibletostraightlinesbefore ending up on the screen.We are saying, for instance, that the photonheadsoffintospace,takesatriparoundtheSunandcomesbacktolandonthescreen.However,aswehaveseen,thevastmajorityofthesepathswillcanceloutor

have disappearingly low probabilities. Nonetheless, if we imagine the photon

takingeverypath,thenitcandoeverythingtheprobabilitywavecan,includingproducinganinterferencepattern.It’snotstrictlyaccuratetosaythatthephoton‘does’ take every path, as there are probabilities attached to the paths and theword‘does’seems to imply100percentcertainty,puttingus indangerof theerroneousstatementthatthephotonisinmorethanoneplaceatthesametime–but this is a problem that English has in being applied to quantum problems,rather than anything wrong with the picture. The outcome – and Feynman’smethod–isoftendescribedas‘sumoverpaths’(orformathematicians,whogetembarrassed by using such simplewords, ‘path integral formulation’) becauseweliterallyconsiderthequantumparticletohavetakeneverypossiblepathandaddtogethertheresults,bearinginmindthephasesandprobabilities.

Fig.6.AfewFeynmandiagramsforsimpletwo-electroninteraction.

Theglueoftheuniverse

As he developed the diagrams thatwould be permanently linked to his name,and that are massively useful tools in quantum physics to this day, RichardFeynman brought in the particles that photons would interact with. Not onlywouldtheseprocessesexplainhowlightwecanseeinteractswithmatter, theyalsoworkedforlightwecan’tsee–so-calledvirtualphotonsthatneverescapefromtheirhomeparticles.Thesewouldbeimmenselyvaluableinunderstanding,forinstance,howanatomworks.Niels Bohr had used Planck and Einstein’s work on quanta to suggest that

electronswerefixedinorbits that theycouldonlyleapbetween,intheprocessgivingoffor receivingaphoton.But theywerealso limited fromapproachingtooclosetothenucleusoftheatom.Afterall,electronsarenegativelychargedandthenucleusispositivelycharged.Theelectronsshouldfeelthestrongurgetoplungeintothenucleus.Butkeepingthematarm’slengthisaconstantflowofvirtualphotons travellingbetween theelectronsand thenucleus. Inasense,everyatominsideyouandeverythingaroundyouisaglowwithlightthatneverescapes,actingasthebinding‘glue’ofmatter.

Anarrayoffields

Oncewe have the picture of a photon taking every possible route, and of thelittleclock indicating thephase (directionof thehand)andprobability (sizeofthehand,ormorepreciselythesquareofthesizeofthehand)ateverypointinthespacetheparticlecanbeconsideredaspassingthrough,wehavereturnedinawaytoFaraday’sview–butwithamathematicalrealityheneverenvisaged,described by Schrödinger’s equation and the other equations of quantummechanics.Faradaythoughtofelectricityandmagnetismintermsoffield,andour infinite array of clocks is nothingmore or less than an alternativeway ofdescribing a field. QED is called a quantum field theory because it takes the‘field’ approach to describing what is going on – and such theories are verycommon in modern physics, because they can often be the most practicalmathematicalwayofexplainingtheoutcomesweobserve.It’seasytothinkthatfieldsdon’treallyexist–afterall,theyareverystrange

things, mathematical entities that stretch throughout the universe havingdifferentvaluesatdifferentplaces.Onlyamathematicianortheoreticalphysicistcouldlovethem.Theyreekofoverkill.Theyhavenoneoftheeasyacceptabilityoftheideaofaparticle,withitsapparentresemblancetoasmallballtravelling

from A to B. But we have to bear in mind that quantum particles bear noresemblancewhatsoever to a ball in theway they behave.Using a field is nomore right or wrong than using a particle. They are both entirely equivalentmodels–butthefieldapproachisoftenthemostpracticaltocalculateresults.

Thecolourproblem

Feynmandiagramsareveryuseful tounderstandvisuallywhat ishappeninginan interaction between particles, but they also became the mainstay ofcalculationsof thewayparticleswouldbehave, assigningprobabilities to eachdiagram to produce an overall outcome. In principle this is a lengthy task, asthereisaninfinitesetofdifferentinteractions,buttheprobabilitiesdropoffveryquickly with the more obscure combinations, adding more and more virtualparticles, which means that generally speaking it is possible to go to only acoupleofextralayersofdetailandignoretherest.AndFeynmandiagramshavebeenusedthiswaytothepresentday.Buttherearesomeissuesthatremain.Specifically,problemsarisewhenextendingthescopeofthediagramsbeyond

electrons and photons to include the interactions of nuclear particles. Therelatively massive particles that form the central part of atoms, neutrons andprotons,aremadeupoftripletsoffundamentalparticlescalledquarks,whicharelinkedbyaflowofgluons,thequarks’equivalentoftherolethatphotonsplayinelectromagnetism.TheparalleltheorytoQEDforquarksandgluonsisquantumchromodynamics,orQCD.Thenamecomesfromthefact thatgluonscomeinthree different ‘flavours’, distinguished by the colours red, green and blue.(Gluons aren’t actually coloured – the concept is meaningless, as they don’tinteractwithlight–thecolournamesarepurelyarbitrary.)Thisaddedcomplexityofcolourcomparedwiththecolourlessphotonmeans

that the Feynman diagrams for QCD become significantly more complicated,and the sheer number that have to be considered spirals out of control. Justdealing with an event where two gluons collide to produce four other (lessenergetic) gluons, a very commonprocess in a particle collider like theLargeHadronCollider, requires220diagramstobeassessedbefore thecontributionsbecomenegligible,resultinginacalculationwithmillionsofterms.Therehadtobeabetterwaytohandlequarksandgluons,butonewouldnotemergeuntilthe21stcentury.

AjewelofphysicsAhinthadcomeinthe1980s,whenStephenParkeandTommyTaylor,working

at theFermiNationalAcceleratorLaboratory in Illinois,managed to come up(usingmorethanalittleguesswork)withasimplemathematicalexpressionthatreplacedallthediagramsandcalculationtermsforthattwo-gluoninteraction.Ittookovertwentyyearstogetbeyondthisfirstleapofinspiration,butinthemid-2000s, by using a mathematical tool called twistors developed by BritishphysicistRogerPenrose,awayofmappingspaceandtimeintoadifferentkindof space using complex numbers, an increasing number of shortcuts weredeveloped–butatthisstagenooneknewwhytheyworked.Recently it has been discovered that these shortcuts were dependent on a

conceptwiththeimposingnameofapositiveGrassmannian.Thiswasabitofmathsdeveloped in the19thcentury that isamulti-dimensionalversionof theinsideofatriangle.Inthetriangleweareoperatingintwodimensions,andthespace inside is a region bounded by intersecting lines. The more generalGrassmannianismulti-dimensional,withthespacedemarkedbytheintersectionofplanes.Andforparticleinteractions,aGrassmannianofthesamenumberofdimensionsas thereareparticles involvedis required todescribe thescatteringprocess.The final part of the journey to calculating the scattering amplitude (which

describes the way the particles interact), traditionally the role of Feynmandiagrams, is to create a new object, a so-called ‘amplituhedron’. This is astructurethathasbeendescribedasa‘jewelofphysics’inthatitbringstogethertheGrassmannianstructurestoproduceasingleobjectwhosestructurehasthesolutioncodedintoit,asitsvolumeequalsthescatteringamplitude.Theamplituhedronforaneight-gluons interaction, for instance,canbe sketched with a handful of lines, but is the equivalent of 500 pages ofcalculations. It’s early days – this theorywas developed only in 2013 – but asingle amplituhedron can correspond to hundreds of pages of calculations andmay provide a way to reach values that simply weren’t accessible using theconventionalFeynmandiagramcalculations.

CHAPTER5

Lightandmagic

Oncewe have the insight ofQED, every interaction between light andmatterbecomesaquantumaffair.

Lettherebelights

Thinkof thehumble lightbulb.ThomasEdisonandJosephSwan,whosharedthecreditsforinventingthelightbulb,reallyweren’tbotheredexactlywherethelightwascomingfrom,theyjustwantedaworkableproducttoreplacethedirty,dangerous gas jet lighting. Swan (who bore a startling resemblance to BrianBlessed),working inNewcastleuponTyne,got there firstbya fewmonths in1879,butthemoreastuteEdisonknewthepowerofpatents,andassoonashisownpatentwasestablishedhesuedSwanforinfringement.Alltoooftenthehistoryofinventionislitteredwithcaseswherethewinnerof

litigation was simply the one with the most money, but this time the courtsrecognisedthevalidityofSwan’sclaimofprecedenceandfoundagainstEdison.As part of the judgement, Edison had to concede that Swan independentlyinventedaworkingbulbaheadofhisrival,andgrudginglysetuptheEdisonandSwanUnitedElectricLightCompany.It’snotthatEdison’scontributionwasn’tsignificant, both in independently devising a bulb based on a carbon filamentandkeepingitworkingforconsiderablylongerthanSwandid,butitdoesseemalittleunfairthatmostpeoplewouldnametheinventoroftheelectriclightbulbasEdison.Whatever theirclaims–andnot tomention the longdrawn-outbattleEdison

would have with Westinghouse, pushing the benefits of his direct currentelectricalsupplyoverWestinghouse’salternatingcurrentsystemdevisedbythegreatNikolaTesla–all thatEdisonwasreally interested inwaskeepingahotfilamentglowingwithoutitburningaway.Yettheprocessbehindthatinventionwasjustasmuchaquantumone–theemissionofphotonsaselectronsthathadabsorbedenergydroppedbacktoalowerlevel–asanysophisticatedelectronicdevice.

Thewindowpuzzle

There was, at the time Edison and Swan were working, no real puzzle toincandescentlight,butonceitwasknownthatlightcameintheformofphotons,QEDprovednecessarytoexplainaneffectthathadbaffledIsaacNewton:partialreflection.Thesimplestexamplehastobeaglasswindow.Apieceofglassletsthroughsomeofthelightthathitsit,butreflectssomeofitback.Ifyoustandinfrontofaglassshopwindowyoucanusuallyseebothwhat’sontheothersideandyourself reflected in thewindow.Light both passes through the glass andreflectsoffit.Specifically,lightfromyouisreflectingoffit,soyoucanseeyourreflection.But people inside the shop can also see you through the glass – sosomeofthelightfromyouispassingthrough.BecauseNewtonthoughtoflightasbeingmadeupofparticles,orcorpuscles

ashecalled them, thisproveda realproblemforhim. If lightwaswaves,youcould imagine a part of the wave going through the glass and a part beingreflected.Butparticleslikephotonsdon’tsplitintotwolikethis.Eitherawholeparticle reflects or awhole particle passes through.And the questionNewtoncouldn’tanswerwaswhyaparticularparticlewould‘decide’topassthroughorreflect.Whatmadethedifference?Becauseclearlysomedidonethingandsomedidtheother.Anobviousassumptionmightbethatithassomethingtodowiththesurface

of the glass – perhaps that itwas rougher andmore scratched in some placesthanothers.TheglassofNewton’sdaywas,afterall,anythingbutuniform.ButNewton could dismiss this with the throwaway line that it couldn’t be true‘BecauseIcanpolishglass’.Newtonspentafairamountoftimemakingoptics,whichmeantpolishingthesurfaceoflensesandmirrors.Ashepolishedupthesurfacehemadefinerandfinerscratches,changingthesurfaceandreducingthepossibilityofreflectionsbeingcausedbyunevennessinthesurface.Andyettheglasshappilycontinuedtoreflect.Itishandythatthispartialreflectionprocessdoeswork.Technically,adevice

thatsplitsabeamofphotonsinthiswayiscalledabeamsplitter,andwhiletheeffectmightbeirritatingsometimes(forinstance,whenthereflectionisoffyourlaptop screen or a car windscreen), it is also very widely used in opticalexperiments, in head-up displays, in devices that make very precisemeasurements and some forms of camera, and even in special reflectors inspotlights, used to reduce the amount of infrared in the spotlight beam,whichcancausetheequipmenttooverheat.Thisbeamsplittingisapurelyquantumeffect.TherewasnopointinNewton

everstretchinghisbraincells inanattempttofindaruleoranexplanationforwhyparticularcorpuscles(orphotonsaswewouldsay)reflectwhileotherspassthrough, because there is no reason behind the selection, other than theprobabilistic nature of quantum theory. We can say that a photon has, forinstance,a10percentchanceofreflectingoffaparticularsurface,butwecanneverknowwhataparticularphotonwilldountiltheoutcomehasoccurred.

Thicknessmatters

Something even stranger (unless you take a quantum viewpoint) is that thereflectionfromthefrontofapieceofglassdependsonhowthick theglass is.Changethethicknesswhileleavingeverythingelsethesameandthepercentageof photons reflecting will alter. You can imagine some kind of wave-basedexplanationforthis,wherethewaveisalreadypassingthroughthewholesheetofglass–butitisveryhardtounderstandifyouthinkofphotonsaslittlepointparticles.Howcan theypossiblyknowthedistance to thebackof thesheetofglasswhenthey‘decide’whetherornottoreflect?Luckilyforus,weknowmoreaboutquantumparticles,andspecificallythat

theirprobabilitywave spreadsoutover time. Just as theprobabilitywaveofasinglephotoncanbeinfluencedbythepresenceofbothslitsintheYoung’sslitsexperiment, so the probability wave passes through the whole sheet of glass.Thismeansthereisaprobabilityforthephotontoreflectfromthebackof theglass,andtocomebacktothefront.Atthispoint,justaswithYoung’sslits,thephases of a photon reflecting off the front and reflecting off the back willcombine, depending on howmuch their ‘clocks’ have had a chance to rotate.Thereisonlyonephoton,butasusualitdoesn’thavealocationandallwecanworkonisthevariousprobabilities.Bycombiningthesquaresofthesizeofthearrows,wereachtheprobabilityoftheactualreflectionhappening.Asthethicknessoftheglassincreases,theamountofreflectionincreasesup

to amaximum, then decreases all theway down to practically nothing aswereachapositionwhere the twophasearrowsprettywellcanceleachotherout.Aswegetthickerglassstill,thepercentagereflectingwillincreaseagain,uptothemaximum, then reduce, and soon. In realitywhat is happening is that thephotonhasachanceofinteractingwitheveryelectroninthebodyoftheglass,which will absorb it and then reemit it in a new direction, a process calledscattering.Butas ithappens thevariousprobabilitiesandphasearrowsaddupand cancel out in such away that the effect is as if the photonwas reflectingfromeitherthefrontorbackoftheglass.

This effect has adecorative application in the realworld.Wheneverwe seesomething iridescent– think, for instance,of the rainbowcoloursyousee inathinfilmofoilontheground–whatishappeningisthat,becausethephotonsofdifferent-coloured light have clocks that rotate at different speeds, theprobabilityofreflectionforaparticularthicknessofoilisdifferentfordifferentcolours.Asthethicknessvariesslightlyacrossthefilm,youseedifferentcoloursreflected.When you see a piece of iridescent jewellery or pottery glaze, it ismakinguseofthisdramaticquantumeffect.

Thequantumcheat

Another example ofQED in action is the lens, a piece of technologywe useeveryday in spectaclesand telescopes, in camerasand in the lens that isbuiltintooureyes.Whatthelensdoesischeatontheprincipleofleasttime.Imaginewelookattwooftheinfinityofroutesaphotoncantakegoingfromsomethingyou are looking at – this page, for instance – to the retina of your eye. Onepossiblerouteisastraightlinefromtheobjecttotheretina.Anotherpossibilityis for the photon to head upwards away from that straight line, then suddenlychangedirectionandheadbacktohitthesamepointonyourretina.

Fig.7.Lens:shorterpathsspendmoretimeinglass.

Fromtheprincipleofleasttimeweknowthatthephotonhasamuchhigherprobabilityofbeingfoundonthestraightline,becauseitisthequickestroute.Ifit took the route heading up a fair way then changing direction, it would beeasilycancelledoutbythephasearrowofanotherclock.Butnowlet’sputabit

ofmagicmaterialinmid-airthatslowsphotonsdown.Wewillmakeitthickestatthecentre,wherethestraightlinepassedthrough,andthinneratthepointourotherroutechangeddirection.Soweareaddingmoretimetothestraightroutethantothelongerroute.Theoutcomeofaddingthismagicmaterial is that thestraight-linepathnow

takesthesametimeasthedivertedpath.Bothroutesnowtaketheleasttime,andbecausetheytakearoundthesametime,thephasearrowsoftheirphotonswilladdup.Thephotonwillhaveasignificantprobabilityoftakingbothroutesandarrivingatthesamepoint,sowehaveincreasedthechancesoflightturningup.We’ve focused the light. Using just the principle of least time and our littlephasearrows,wehaveinventedthelensfromscratch.Andanylens–includingthelensinyoureye–isshowntobeapowerfulquantumdevice.

Poweredbylight

ItwouldbeimpossibletonumbereveryquantumdeviceweusethatdependsonQED.Clearlythisappliestoeverysortoflighting,butthinkalsoforinstanceofthesolarpanel.AswesawinChapter2,photosynthesisisaprocessdependingonaquantuminteractionof lightandmatter,andaformofquantumcatalysis,butthereismoreconscioususeofquantumeffectsinthetechnologyweusetogenerateelectricityfromlight.Aswehaveseen,thephotoelectriceffect,whereaphotonblastsanelectronin

ametal up to the conduction layer, was one of the key discoveries to inspirequantum theory.To be practical, though, thematerial used in solar cells (alsocalledphotovoltaicorPVcells)isasemiconductorlikesilicon,wheretheeffectof pushing the electron up is to produce an electron/hole pair. The cells arestructuredlikeadiode,sothecurrentcanflowinonlyonedirection,andjoinedtogether in a large matrix to produce sufficient electrical energy to beworthwhile.There are a whole range of photovoltaic cell technologies, from the

traditional, chunky silicon-based solar cellpanels to cells that areprintedonafilm.Theprintedcellsaremuchcheaper thana traditionalstructure,butalsoalotlessefficient–theyturnlessoftheincominglightenergyintoelectricity.Atypical solar cell has around 25 per cent efficiency,with the best (but not yetcommercially practical) current cells getting close to 50 per cent.By contrast,thefilmcellscanbeaslowas1to2percentefficiency–butcanbeproducedforafractionofthecost.Therewassomethingofamediastirin2013whenitwasannouncedthatsolar

cellsproducemoreelectricitywhenmusic isplayed to them.Entertainingly, itturnedoutthatthecellspreferredpopmusictoclassical.Thiswasspecificallyavariantofthefilmstyleofcellbasedonzincoxide.Thesecellsusuallymanageonlyaround1.2percent,butbybuildinginlittleoxide‘hairs’thatvibratewithsound,theefficiencycanbenearlydoubled.There’smoreenergyproducedfromhighfrequenciesthanlow,sopopmusicwithitsgreaterpreponderanceofhighfrequencies wins the race. However, there is no danger of solar farms withspeakers blasting out the latest chart sounds to encourage generation.Significantlymoreenergyiswastedpumpingoutthemusicthaneverprimesthecellstogreateractivity.Thistechnologywould,however,beusefulinanaturallynoisyenvironment–nearanairport,forinstance.

Themovingquantumwrites…One other example to show the breadth of QED in action. However you arereadingthisbook,youwillbemakinguseofthequantuminteractionoflightandmatter. It could be that you have a paper version, the oldest of our portabletechnologiesformakingalargeamountofinformationaccessible.Writinginallits forms is arguably the most transformational technology we have everdeveloped.Andeventhoughwereceivealotofinformationthesedaysbyotherroutes – for instance, watching videos – there is still a massive amount ofinformationthatreachesusinwrittenform,whetheronourelectronicdevicesorthroughmoretraditionalmeans.Asaformofcommunication,writingformspartofanactivitythatiscommon

across living things – but it is a very special way of going about it, becausewriting takes away the limitations of space and time that restrict naturalcommunication. I have books onmy shelf (often via intermediary translators)written by the prehistoric Israelites responsible for the Old Testament of theBible,AncientGreekphilosopherslikeArchimedesandAristotle,themedievalscribes of theAnglo-SaxonChronicle,Galileo,Newton and, yes,BrianClegg.Thereareprobablymorebooksonmyshelvesbringingmecommunicationfromdeadpeoplethanliving–andcertainlyveryfewofthem,withtheexceptionofTerryPratchett(andBrianClegg),liveneartomyhome.Computers and the internet stretch this lack of locality even further.Emails

canarrive fromanyplaceon theglobe.The internetallowsme toburrowintoinformationfrommanyplacesandtimes.Andwritingmakespossiblefarmorethantheirritatingspamemailthatfillsmyinbox.Withoutwrittendocumentswecould not have developed the structures of science – our only means ofunderstanding the universewould be orally transmittedmyth.With nowayof

transmitting ideas across distance and centuries we would be constantly re-inventing thewheel (literallyandmetaphorically).Andall this isbasedon theQEDtechnologyofwriting.

Fromstartobrain

Onatraditionalprintedpage,gettingtheinformationfromthepapertothebraininvolves what seems a simple enough process, but nonetheless involves apowerfulquantumchainofevents.Let’s imagineyouarereading thisbookbydaylight.Millionsofyearsago,high-energyphotonswerereleasedfromnuclearreactions deep in the Sun. Over the years they have been working their wayoutwardsthroughthestar’sdensestructure,beingconstantlyabsorbedandnewphotonsemitted.Eventually theyreachedtheSun’ssurface,which isheated toaround 5,500°C,making it glowwhite, spraying outmany billions of photonseverysecond.Those photons cross space largely untroubled before they reach the Earth’s

atmosphere. Here they will be scattered by the gas molecules in the air(primarily nitrogen and oxygen), absorbed and re-emitted in new directions.High-energyphotonsaremorelikelytobescattered,sotheunscatteredphotonsweseeintheSun’sdiscappearalower-energyyellow,whilethescatteredbluephotons give a colouration to the whole sky. A mix of direct and scatteredphotonsarriveatthepageofyourbook,producingawhitelight.Thephotonsareabsorbedbytheatomsofthepaperandink,increasingtheenergylevelsoftheatoms by boosting electrons to a higher orbit. In the black ink, much of thisenergywillgotoheatingupthemolecules,butforthewhitepapermanymorephotonswillbere-emittedtowardsyoureyes.There ismorequantumprocessingatworkas thephotons’pathsareshaped

bythelensoftheeye,absorbingandre-emittingphotonsaccordingtotherulesofQED.Therewillbeabsorptionandre-emissionas thephotonspass throughthe jelly-like inner substance of the eye, until they reach the retina. Here thephotons hit special cells that contain a collection of photoreceptor molecules,wheretheabsorbedphotonssetacollectionofelectronsontheirjourneytotheopticnerveandfinally into thebrain,where themixedsignalsare transformedintoavisualimageandthestrangemarkingsonthepapercanbeinterpretedasameansofcommunication.

Theelectronicword

Somuchforthepaperbook,butthechancesareincreasinglyhighthatyouwillbe reading this bookon an e-reader.When I first startedwriting therewasnosuchthingasane-book,butnowtheyaccountforoverathirdofallsalesofmybooks and that proportion is likely to increase. Many of those e-readers aresoftwarerunningonacomputerortabletusingaconventionalLCDscreen(seepage72).Iusuallyreade-booksonaniPad,forinstance.Butdedicatede-readerslikeaKindleoraNookinsteadmakeuseofe-ink.This is a passive visual technology. Where an LCD pumps out photons

towards your eyes, an e-ink reader sits there like a paper page, waiting forphotonsfromtheSunoranotherlightsourcetocarrythemessagetowardsyourbrain.Althoughe-inkdisplaysaremuchslowertorefreshthanLCDandlimitedintheirgraphiccapabilities,theyhavetwobigadvantages:theydon’tuseenergytokeep the image inplace,only to set itup; and theycanbemademuch lessreflective,idealforuseinnaturallight.A typicale-inkdisplayusesa thin layercontainingoil inwhich floatbright

white titanium dioxide particles. The pixels that make up the display arecontrolled by a pair of electrodes, top and bottom, with the top electrodetransparent.Whentheelectrodeischargeduptoattractthewhiteparticlestothetop, the pixel showswhite, butwhen the particles are attracted to the bottomelectrode they fall to thebottom.Dependingon the specific technology, eitherthe blackpigment is present in the oil, or there is a set of blackparticles thathavetheoppositechargetothewhiteandsonowdrifttothetop.Eachpixelhasa corresponding transistor and capacitor on a thin film forming an ‘activematrix’, the same addressing technology used in most LCDs, which activelyensuresthatthepixelhasthecorrectvalue.

AspecialrayFrom the electric light bulb to electronic paper, the possibilities for quantumoptical technologies are endless.And todate, thesedeviceshave reliedon thesimpleinteractionofphotonsandmatterwiththeassumptionthatallphotonsareequal.Butadiscoveryfromthe1950swouldmakeitpossible toproducelightthatmadeuseof thosephotons inaveryspecialway.Justaselectronicsmadeuse of quantum theory to transform the way we control electricity, this newtechnologywoulduseexplicitknowledgeof thequantumnatureofphotons toproduceakindoflightthathadneverbeenseenbefore.Alightthatcouldbeusedtodecodemusic,toperformmedicaloperations,or

eventokill.

CHAPTER6

Superbeams

In the coldwintermonths of early 1973, two seventeenyear-olds quietly tookoveranunusedstoreroomintheirschool.Notuntypicallyforteenageboys,theyputupsignsonthedoorproclaiming,‘Donotenter!Dangerofdeath!Extremelyhighvoltages!’–butunlikethewarningsignsthatadornthebedroomdoorsofmany teenagers, theseweremore than for show.The pair reallywere dealingwithtechnologythathadthepotentialtokill.

Lightschool

I was one of those seventeenyear-olds.My school then encouraged would-beOxbridge candidates to take theirA-levels a year early, leaving us free in ourfinalyearatschooltoconcentrateontheentranceexams.Buttheseweretakenat theendof thefirst term.If theschoolcouldencourageus tostayonfor therest of the academic year, they got significantly more money from thegovernment. So they tempted us to remain by putting on all sorts of exoticclasses– andbyallowing science students to takeona challengingproject.Afriend,DavidBall,andIdecidedthatweweregoingtobuildalaser.Thiswas,atfirstsight,decidedlychallenging.Itwas,afterall,littlemorethan

adecadeafter theworld’sfirst laserwasbuilt.Butwewerefollowingdetailedinstructions published in a 1970 Scientific American that guided the readerthroughtheprocessofbuildingadyelaser.Wedidn’tmakeit.Intheend,weranoutoftime.Ourdeviceneverworked.Butwegotsomeexcellentexperienceofbuilding scientific equipment. We constructed the dye chamber, with its endmirrors,onehalf-silvered, suspended fromfinelyadjustablemetalmounts.Webuilt a high-powered flash tube that zapped the remaining gas in a thicktransparenttube,partlyevacuatedbyattachingittoanairpump,withtheoutputofapairofhugecapacitors.Weevengotthemirrorsaligned,oneofthetrickiesttasks in building a successful laser (though I have to confess we cheated byusinganoff-the-shelflasertoguidethealignment).Theflashtubewedidgetworking–arealscientificdevicebuiltfromscratch.

Our flashes were dramatic in the extreme, giving us the masterful feeling ofhavingtamedindoorlightning,andthegreatgreycapacitors,eachthesizeofanoil can, borrowed from theUniversity ofManchester Institute of Science andTechnology,werethereasonforthesign.Thechargetheygeneratedwouldhavekilled instantly ifyou touched the terminals. (Ican’t imagineschoolsallowingtwostudentstoplaywithsuchdangeroustechnologyunsupervisedthesedays.)Butwhetherwewouldeverhavegot the laser itself to function Idon’tknow.Certainly it proved a non-trivial task for the early workers in the field, asbecomes obvious once you explore the history of this most iconic quantumdevice.

TheRussianamplifier

Oneof thepointsoforiginof the tangled taleof thedevelopmentof the lasercame in Russia in October 1954. Alexander Prochorov and Nikolai Basovpublished a paper in the Russian language Journal of Experimental andTheoretical Physics, describing an as-yet theoretical process based on aprediction that Einstein had made in 1916, shortly before he became totallyengrossedwithgeneral relativity.TheRussiansbelieved thatusing theprocessEinsteindescribed,asuitablematerialwouldactasanamplifierformicrowaves(lower-energy photons than visible light), multiplying up the weak stream ofphotonstomakeamorepowerfulbeam.Thisprocesswouldbecomeknownasmicrowaveamplificationthroughthestimulatedemissionofradiation–makingthetheoreticaldevicea‘maser’forshort.WhatEinsteinhadrealisedwasthatwhenhitbylightoftherightwavelength

–whichinBasovandProchorov’sexperimenthappenedtobeinthemicrowaveregion–anelectroncouldabsorbtheenergyofaphotonandsitinasemi-stablestate,likethecockedhammerofagun.Ifasecondphotonhappenedtohitthatsameelectronwhile itwas still in this state, then the electronwouldemit twophotons, amplifying the incoming light as the photon triggered the electron’sdouble leap downwards. If this took place in a reflective chamber, where thephotonscontinuedtopassthroughthemedium,hittingelectronsrepeatedly,theresult would be to build up a whole collection of atoms with electrons in anenergisedstatereadytodropbackdowntogetherinachainreactionproducingastreamofphotons,synchronisedbytheprocesssothattheirQEDphasearrowswereallpointingthesamewayandmovingtogether,makingthem‘coherent’.ForEinsteinthiswaslittlemorethananinterestingpieceoftheory,thoughin

the 1930s a Russian scientist, Valentin Fabrikant, took things further in his

imagination.Hewonderedwhatwouldhappenifyouhadamaterialwherethemajorityoftheatomswereinanexcitedstate,aninversionoftheusualsituation.If you then sent photons of the same energy through the material, it shouldtrigger a mass emission, amplifying the original light (whether visible or,perhapsmoreusefullyatthetime,radioorthemicrowavesofradar)intoamuchmore powerful beam. It was this concept that Prochorov and Basov haddescribed in their paper, and that Prochorov described in ammonia gas at aconference organised by the Faraday Society in Cambridge in 1955. It was aconceptthatleftonememberoftheaudiencestunned.

That’smymaserHisnamewasCharlesTownes,andtheshockcamebecausehehadalreadybuiltan ammonia maser. Unaware of the publication in a journal still largelyconcealed from theWestby the IronCurtain,Townes thought thathewas thefirst to work on masers – and he certainly was in terms of constructing thedevice. But he did not publish details of his work on themaser until August1955,givingProchorovandBasovpriorityon the theory.TowneshadworkedonmicrowavesforaBellLabsradarprojectduringtheSecondWorldWar.Thepowers-thatbe wanted more and more detailed radar readings, which meantusing microwaves of a higher and higher frequency, all the way up to 24gigahertz (waves thatoscillated24,000,000,000 timesa second).Followinguponstudiesshowingthatthegasammoniawaseasilyexcitedbylightofaroundthis frequency, Townes had hoped that he had the solution to producing suchpinpointradar,onlytodiscoverthatwatervapourintheairreadilyabsorbedthisfrequency,makingituselessforradar.Therewasn’talotofpointindevelopingaradarsystemthatwasstoppedbyanunavoidablecomponentofair.Afterthewar,TownesmovedtoaprofessorshipatColumbiaUniversityand

continuedhisstudiesoftheinteractionofthesehigh-frequencymicrowaveswithmolecules like ammonia. Working with a postdoc called Arthur Schawlow,Towneshadoriginally consideredworkingwith ammoniamolecules thatwereexcited by giving them extra rotational energy, which would have required avery high-frequency source, but settled instead on the more conventionalexcitationofvibration,forwhichthosemicrowavesaround24gigahertzwouldbeideal.(Inallcases,excitationisfundamentallyaboutpushingelectronsuptohigher energy levels, but when this happens in a molecule it can result in aquantisedrotationorvibrationbecauseofthechangeintheenergyofoneofitscomponentatoms.)BynowworkingwithJimGordon,comethespringof1954,Towneshad achieved aworkingmodelofwhat theynamedamaser, andwas

ready to tell the world about it by the time he had the shock discovery ofProchorov’sworkinCambridge.Itwouldbe appealing if the storyof every scientific discoverywere anice,

neattaleofanindividualorteamworkingagainstalloddsandproducingaresultthatnooneelsehasthoughtof,butinrealitythereareusuallymanycontributorsto a development (in what I’ve described above I’ve already omitted severalotherminor breakthroughs along theway), and it isn’t uncommon for two ormoretotallyunconnectedteamstohitonsimilarresultsataboutthesametime.In thisparticularcase, thedistinction isquiteclear–Prochorovpublishedfirston the theory (and publication is required to be considered to have got therefirst), butTowneswas firstwith aworkingdevice.Before long, though, therewould be a messier dispute over the maser’s far more significant successor,whereeventothisdaytheprioritiesarenotconsideredcutanddried.

Amaserforlight

Themaserwasan impressivedevice,andeven thoughtheammoniamaserdidnot produce the wonder precision radar that Townes had hoped for, itsunparalleled precision in the frequency of its radiation had immediateapplicationsinimprovingthedesignofatomicclocks.Butitwasswiftlyrealisedthattheconcepthadthepotentialtobefarmoredramaticwhenitwasappliedtovisible light. The racewas on to design a visiblemaser – and race itwas, asAmerican physicists Art Schawlow and Gordon Gould would enter into aferociouspatentbattleoverwhocameupwiththeconceptfirst.SchawlowandGouldwerebynomeans theonlyones towork towards the

dream goal of an equivalent of a maser working with visible light. Others,including Ali Javan, Donald Herriott, John Sanders, Herbert Cummins, IsaacAbella,GeoffreyGarrett, PaulRabinowicz, Steve Jacobs, IrwinWieder, PeterSorokin,WolfgangKeiser,Mirek Stevenson, RonMartin, Valentin Fabrikant,Fatima Butayeva, Charles Asawa, Ben Senitzky, Elias Snitzer and WilliamBennett, would all make steps that proved helpful to the three main players.Though the individual names in this list are likely to be meaningless to thereader,itgivesafeelforthedegreeofparallelworkinginanattempttogettherefirst,thefeverishactivitybehindthescenes,someinthesameestablishmentsasSchawlowandGould,whileotherteamslabouredatuniversitiesandcompaniesacrosstheworld.Another name we have alreadymet, working directly with Schawlow, was

Charles Townes himself. After discovering the difficulties of simply moving

existingmasertechnologytohigherandhighermicrowavefrequencies,hemadethemental jumpof thinking itmight be easier to shift a gooddistance up theelectromagneticspectrum.In1957hebegantoconsiderwhathewouldalwaysrefertoasan‘opticalmaser’.Thetitlemayseemalittleclumsy,butthemaserwashisbabyandhewantedtoensurethatanyfuturedevelopmentwouldbetiedclearly back to its roots and his device. Townes picked up on an idea calledopticalpumping,whichwasawaytouseilluminationwithselectedfrequenciesof light to push electrons in gas molecules up to a higher level to improvemicrowave transmission.Townes felt that this techniquecouldalsobeused tostartoffthechainreactionofanopticalmaser.

Belldiscoveries

Laterthatyear,TownesmovedfromColumbiaUniversitybacktoAT&T’sBellLabs, where he would work with his brother-in-law Schawlow, forming oneteaminthelaserrace.Bell’sgreatnessissomethingofafadedmemorynow,butthecompany’snear-strangleholdontheUStelecomsmarketmeantthatitcouldafford to throw a large amount of money at basic research and development.Thiswasarguablytheidealplacetoworkonanopticalsuccessortothemaser–afterall,justtheyearbefore,itwasBellscientistswhohadwontheNobelPrizefortheirworkonthetransistor.Townes,Schawlowandtheirteambegan,inaverycareful,structuredway,to

examinethepotentialofvariousvaporisedmetalsassubstancestostimulateintoemission.Theseinitiallyseemedattractiveas theyweregoodcandidates togetinto the necessary excited state, but the practical side of dealing with metalvapoursprovedtrickierthanexpected.Atthesametime,theteamhadtodevisesome kind of chamber for the stimulated emission to take place in. Themicrowavesinthemaserhadrequiredonlysimplerectangularmetalboxeswithholes around the size of themicrowavewavelength (a few centimetres) to letthem escape. But it was hard to see how visible light, with its much smallerwavelength,couldbesuitablyconfined.Youcan’tstorelightinametalbox.ItwasArtSchawlowwhocameupwiththesolutionthatbecameprettymuch

universal in early lasers. He suggested using a Fabry-Perot cavity, a chamberthatwasopenonthesides,allowingthelightthatwouldstimulatethemediumtogetin,andthathadmirrorsoneachend,sothattheemittedlightbarrelledbackandforthbetweenthereflectivesurfaces,constantlyre-stimulatingthemediuminside.Aslongasthemirrorswerehighlyreflective,exactlyparallelandmanywavelengthsacross,theyshouldformthebasisofanexcellentchamber.

AgemofalaserUp to now, all the ideas for opticalmasers had focused on gases (metallic orotherwise)asthematerialtoproducestimulatedemission,butBellLabs’successwiththetransistorwasapowerfulreminderoftheimportanceofconsideringthesolid-state. Schawlowbegan to look into using somekind of transparent solidinstead.Syntheticrubieswerealreadyusedinmasers,sothisseemedanobviousstarting point.With the systematic scientific viewpoint prevalent at Bell, thismeant studying the different excitation states of the chromium atoms that areadded to aluminium oxide to produce rubies, something that wasn’t properlyunderstoodatthetime.Aftersomeconsiderableeffort,rubiesweredismissed.Itwasdifficulttogetenoughofthechromiumatomsintoanexcitedstate,readytobe triggered, and the team’s research suggested that rubies would absorb toomuchof the lightenergy,wasting itasheat rather thanre-emitting it togetaneffectivestimulatedemissionreaction.By theendof1958,TownesandSchawlowhadpublishedapaperoutlining

their optical maser concept in Physical Review Letters, and, after a lot ofgrumbling from theBell PatentOffice,which found it hard to believe that anopticalmaserhadanypracticaluse,filedpatentsontheuseofopticalmasersincommunications. Almost inevitably, Bell Labs saw the world in terms ofcommunications, because this was their speciality. The scientists working onopticalmasers considered them an exciting prospect because using the opticalregionmeantyoucouldpackfarmoresignalsintoa‘pipe’,providedyoucouldproducelightonatightbandoffrequencies.Thiswasexactlywhattheconceptoftheopticalmaserpromised.AsfarasBellwasconcerned,theirscientistswerewell ahead of the race – but they had no idea of the competition that alreadyexisted.

TheGouldstandard

While atColumbia,Townes had asked for somehelp froma graduate studentcalled Gordon Gould. They had discussed optical pumping and the use of aparticular typeof lamp.ButGouldhadalreadybeen thinkingabout anopticalversion of a maser, and the comments from Townes spurred him into action.Theywereverydifferentcharacters:Townesthestraitlaced,solid,establishmenttype – Gouldmore of a revolutionary, given to working in frantic periods ofeffortthendoingverylittlefordaysatatime.WhereTownes’backgroundwasunimpeachable, Gould had dabbled with Marxism, a youthful dalliance thatwouldcomebacktohaunthim.

Gould independently came up with the idea of using parallel mirrors toproduce repeated passes of light through a medium, building excitation, andwhereTowneshadmerelyseentheopticalmaserasavariantofhislow-powermicrowave device, Gould realised there was a potential for pass after passthrough the medium to build up more and more stimulation, eventuallyproducing an intensely concentrated beam of light. He believed it should bepossibletocreateabeamthatwouldeasilyproducetemperaturesashighastheSun’s surface – around 5,500°C – or that could even compress atomic nucleiuntil they fused as they did in themassive temperatures and pressures of theSun’sinterior.

ThelaserisnamedItwasNovember1957.Gouldknew the importanceof establishingpriority insuchacompetitivescientificenvironment.Hewroteupcarefulnotesinaformthat could be used as evidence for a patent, conjuring up a new term for thedevice that would rapidly displace ‘optical maser’. Gould headed his notes:‘SomeroughcalculationsonthefeasibilityofaLASER:LightAmplificationbytheStimulatedEmissionofRadiation.’Thenormalthingtodowouldbetohavethisdocumentsignedoffbyacolleague,butGouldwasawaretherewasalreadycompetitionatColumbia.Waryoflosinghislead,heinsteadtookhisnotebookto a notary public (by coincidence also called Gould), a low-level official inAmericawhocanadministeroathsandwitnessdocuments.ThenotarystampedanddatedGould’spagestoestablishjustwhenhehadcomeupwiththeidea.InprincipleGouldwasnowall readytogethispatentapplicationinseveral

monthsbeforeTownes,butatthispointhemadeacrucialmistake.Hisparentsgot him an introduction to a patent lawyer in January 1958, but Gould cameawayfromthemeetingwiththeincorrectideathatheneededtobuildaworkingmodelbeforehecouldbegrantedapatent.(Workingmodelswererequiredforperpetualmotionmachines for obvious reasons, but usually a patent could begrantedonappropriatepaperworkalone.)GouldhadalreadyworkedbrieflywithacompanycalledTechnicalResearchGroup(TRG)thatspecialisedindefencecontracts.Henowgotajobthereworkingonatomicclocks,hopingtospendhislunchtimesandeveningsonhislaserproject.Tobeginwith,Gouldkeptthistohimself,buthewassoonrequiredtosigna

documentwaiving his patent rights on inventions hemadewhileworking forTRG.Heexplainedheneededanexistingideatobeexcludedfromthiswaiver.Thecompanyagreed,andwantedtohearmore.Initiallytheydoubtedthevalueofalaser,butGouldwasapassionateadvocateandmanagedtosellthemonthe

prospect.TRG’sfounder,LawrenceGoldmuntz,suggestedthatbuildingalasercouldbeaprojectthatwouldgainPentagonfunding,meaningtheycouldswingheavy-dutyresourcesbehindit,eventhoughGouldwouldkeepthepatentrights.Gould piled on the possibilities, talking up a device that he said could send atightcommunicationsbeamallthewaytoMarsorpunchholesinmetalwiththesheerconcentratedenergyofitsbeamoflight.OnedayafterthepublicationofTownesandSchawlow’spaper,on16December1958,theTRGproposalwentofftothePentagon,askingforahefty$300,000offunding.

Militarymight

Until this point in history it had been difficult to interest the conservativeUSmilitary in leading-edge technology, but the proposal arrived at an opportunemoment.InresponsetotheshockoftheSovietSputnikbecomingthefirstman-made satellite, flaunting apparent Soviet technological superiority, the USgovernmenthadsetuptheAdvancedResearchProjectsAgency.ARPAwastofund and manage precisely the kind of crazy possibilities that Gould wasclaimingforhislaser.Gouldrespondedbydreamingupotherpotentialmilitaryusesforhisdevice,fromactingasanultra-preciseradar,withitsmuchshorterwavelength resulting in far greater precision than traditional equipment, to thesciencefictionvisionofprojectingabrightglowingspotonadistantpersonorpiece ofmilitary hardware to help target incomingweaponry. Perhaps,Gouldsuggested, in a precursor to Ronald Reagan’s 1980s ‘Star Wars’ StrategicDefenseInitiative, thepowerof laserscouldevenbeused todestroy incomingmissiles.Thispossibility led to theAirForcegiving theproject the codenameDefender.NotonlydidGould’senthusiasticandvisionaryideasinspiretheARPAteam,

it resulted in a shocking outcome. Most companies are used to governmentagenciesattemptingtotrimtheirbudget.It’sthekindofgameeveryoneexpectswhenbiddingforgovernmentcontracts.Inresponsetotherequestfor$300,000,ARPAcamebackwithapprovalforaspendofpractically$1milliononthelaserproject, requestingthatTRGwent intooverdrive, testingdifferent technologiesin parallel rather than carefullyworking through the options one at a time. ItseemedasifeverythingwasabouttotakeoffforGould–thatitwouldbeplainsailingfromnowon.Butthemilitarybureaucracyhadnointentionoflettingthathappen.

Yourbrainisclassified

Thepowers-that-bedecided thatwithall itspotentialmilitaryapplications, theTRG laserproject shouldbeclassified, a commonenoughmove.Gould’s firstconcernwas thathispotential for apatent– thewhole reason thathecame toTRG in the first place – would be scuppered, as you can’t properly patentsomething that has restricted access. To get around this, TRG split theinformationonthelaserintotwo,separatingtheunderlyingtechnologyfromtheapplications, enabling them to file a patent application in April 1959. Gouldprobably thought theworstwas over.True, he had a history of dalliancewithleft-wing ideas that would inevitably be considered suspicious to an Americathat was still reeling from the McCarthy era, but he was assured by TRG’sARPAcontactsthathissecurityclearanceshouldbesimple.Unfortunately, thenewlyformedARPAhadnotyetdevelopedtheexpertisetomasterthecomplexmilitarygameofsecuritylevels.Gould’ssecuritypositioncametoaheadwhenGouldandGoldmuntzhada

meetingwithRAND(anacronymforResearchANdDevelopment), the think-tank set up at the behest of the US government in the 1940s. As Gould andGoldmuntzsettleddowntodiscussGould’sproposalwithRAND’sscientists,anofficialtookthedocumentaway,sayingthatGouldwasnotallowedtoseeit,ashedidnothavehighenoughsecurityclearance.This,remember,wasaproposaldescribing Gould’s work and ideas. He wasn’t allowed to look at his ownproposal.Similarly,GouldwasimmediatelyrestrictedfromenteringthebuildingatTRGwherethelaserworkwasbeingcarriedout.Itisapowerfulreminderofhow different times were, that two of the genuine problems in obtainingclearance for Gould were that he had lived with his wife before they weremarried, and that two of the referees for his security clearance had beards.Sporting facial hair was considered a sign of being subversive. You couldn’tmakethisup.

RubiesreturnWhileGouldandtheBellLabsteamwerestillstrugglingwiththebureaucracy,andBell’s teamcontinuedwithpainstakingresearch,a thirdplayerentered thefield. The Hughes Aircraft Corporation, founded by the reclusive HowardHughes, was a major US defence contractor in the 1950s and like Bell wasprepared to indulge in fundamental research in the hope of developing newproducts. One such project was work on an improvedmaser with the aim ofusing it to achieve more precise radar, and it was for this that the HughesCorporationhiredin1956,amongothers,TheodoreMaiman,ayoungmanwithan ideal combination of an electrical engineering degree and a doctorate in

physics. Herewas someonewith both the theoretical and practical training tomakethelaserareality.Maiman learned a lot about workingwith rubies in a project exploring the

potentialofaruby-basedmaser.Thiswasafiddlybusiness–toworkatthekindof energy levels required formicrowaves, a rubyhad tobe cooledwith liquidheliumor liquidnitrogen–but itmadeMaimanverymuchthemasterof rubymanipulation.Moreenthusiasticaboutresearchthanpureengineering,Maimanput a lot of effort into studying theway that the chromium atoms in the rubycouldbepusheduptohigherenergylevels.Hepickedupontheopticalpumpingwith visible light that had been written up by Townes and wondered if thiswould be helpful for a ruby maser … but it also got him to thinking aboutproducingavisiblelightoutput.

SecurityfolliesMeanwhile, themost likelycontender toget there first,GordonGould,despitethehelpofcorporatelawyers,wasstillfightingtogethissecurityclearance.Akey problem was Gould’s reluctance to name former friends who had beenfellow communists. His sense of loyalty made the paranoid administrationsuspicious that despite his capitalist veneer and willingness to work for themilitaryindustrial complex, Gould still harboured overly liberal left-wingleanings.UnfortunatelynotallofGould’soldfriendswereashigh-mindedashewas.One,HerbertSandberg, inanattempt toclearhimself,namedGouldasaknownsecurityrisk.ThisbombshellcameasitlookedasifGouldwouldgettheclearance, sending everything back to square one in the nightmare game ofsecuritysnakesandladders.By now there were as many as twenty people working on the TRG laser

project,supposedlyGould’sbaby,yethewasn’tevenallowedintothebuildingwhere theyworked.They couldnot ask for advice fromhimdirectly, onlybyaskingindirect,evasive,hypotheticalquestions.Itwasliketryingtocarryoutacarservicedownaphoneline.ThingscametoaheadwhenGould’snotebookswereconfiscated,becausehedidn’thavethesecurityclearancerequiredtoreadthewordsthathehadwritten.

Errorcorrection

The opposition could have got there easily before Maiman, but Gould washampered by the security issues, while Schawlow was convinced that rubieswouldnotworkbecausehebelievedtheyweretooinefficientatconvertingthe

lightenergythatwasusedtostimulatethemintocoherentbeamsoflaserlight.Thisconvictionwasprimarilybasedonanincorrectvaluefor theefficiencyofthe process given by another researcher at a conference in June 1959, whichSchawlowdidn’tbothertocheck.Thissmall lapsewasenoughtolosehimtherace.AsdidGould,Schawlowcontinued to focuson thebetter-understoodbutmore operationally tricky use of gases and particularly metal vapours asmaterialsforstimulation.MaimanhadalsoheardabouttheproblemswithrubiesfromSchawlowataconferenceinSeptemberthatyear,buthefeltthattherewassomethingnot rightwithSchawlow’sconcerns,givenhisownexperiencewiththematerial.Maimanhadtwoparticularissues.OnewasthatSchawlowhadsaiditwould

notbepossibletogetenoughelectronsexcitedinrubybecausethecrystalwouldbebleachedby the strong light – butMaimanknew that in theoptics of suchcrystals,bleachingwasnotaboutbeingwashedout,butwasatermmeaningthatmostoralloftheelectronswereexcited–exactlytherightstateforstimulatedemission.Thisseemedapositivebenefitratherthanareasonforignoringrubies.Maiman also had a problem with the efficiency calculations used to argueagainst ruby’s effectiveness. Something seemedwrong, and it nagged at him.His superiordidn’t agree,persuadedbySchawlow’s talk,butMaiman insistedthatrubywasworthago,andwontheargument.The thing thatparticularly frustratedMaimanwas theefficiencyproblem. If

mostof the energyof thephotonsbeingpumped into the rubydidnot endupproducing stimulated emission, where was it being absorbed or being re-radiated?Ifyouputenergyin,ithastoendupsomewhere,yetnooneseemedtoknowexactlywhereitwasgoing.BynowSchawlowhadabandonedrubyatBellLabs, believing that itwouldonlypossiblywork if cooledwith liquidhelium,whichprovedanexperimentalnightmare.Maimanknewthatrubywasastrongabsorberofbothyellow/greenandblue/violetlight,yetaccordingtothefiguresfrom the conference, only about 1 per cent of this energy was resulting instimulatedemission.Thereseemedtobetwomainwaysthattheenergycouldescape.Onewasby

simple scattering, the effect that produces the blue sky and enables us to seeobjects. In this, rather thandropdown toan intermediate levelbeforeemittingtheredphoton,astimulatedelectrondropsbackallthewaytoitsinitialenergylevelandputsoutthesameenergyoflightthatitabsorbed.Theotherpossibilityforusingupenergyisthattheenergyfromtheelectronistransferredtokineticenergyintheatomsinthesubstance.Thematerialheatsup.Intryingtoquantifyhowtheenergywaslostviathesealternativeroutes,Maimanhopedtodiscoverthekindofmaterialthatwouldworkbetterthanrubyinalaser.Butinsteadhe

gotashock.In his experiments, Maiman discovered that there was relatively little

scatteringorheating.Hisfirstguessattheactualefficiencyofrubywasaround70 per cent, and it was later measured more accurately in the high 90s.Somehow, theoppositionhadgot thekeymeasurementwildlywrong andhadbased their entire strategy on this mistake. It didn’t help at this point thatMaiman’smanagersseemedmorewilling tobelieve theBellLabs results thanhis own.Unable to get permission to proceed officially, he startedwork on arubylaserwithoutinforminghisboss.

Aflashsolution

The problemnowwas to get a bright enough light to illuminate the ruby andpumpthoseelectronsup,readytobetriggered.Arclampswerebrightenough,but generated toomuchheat, damaging the ruby, soMaiman’s team turned toindustrial versions of cinema projector bulbs. These were fiddly technology,requiring cooling systems to avoid them burning out, but they were certainlyintense.All the ideas for laser design to this point had assumed that the laserwould be a continuous beam, but Maiman’s assistant, Charlie Asawa, alsolookedatthepossibilityofusingpulsesoflighttostimulatethelasingmaterial.Thisseemedappealing,asitgavethelampachancetocoolbetweenflashestoavoid overheating.And herewaswhere one of those examples of serendipitythatoftencomeupinscienceoccurred.One of Asawa’s friends was an enthusiastic photographer and had recently

bought thelatestsnapper’sgadget.Backthen,night-timephotographyrequiredflashbulbs,one-shotbulbsthatburnedoutastripofmagnesiumorzirconiuminahigh-oxygen atmosphere to produce an intense flashof light, thenhad to bethrown away. But Asawa’s friend had bought one of the newly developedelectronic flashes, which built up an electrical charge in a capacitor, thenreleased it inonegoacrossa lowpressure tube typicallycontainingxenongas,producing a blindingly bright flash that could be repeated as quickly as thecapacitorcouldberecharged,withoutchangingthebulb.Thelightfromanelectronicflashseemedidealintermsoftheenergylevels

provided, was easily controlled and required none of the messy coolingmechanisms of the specialist projector-type bulbs. Admittedly, no one hadthoughtofalaserproducingitslightinpulsesratherthanasacontinuousbeam–anditprobablywouldn’tbeanyuseforapplicationslikecommunications–butatleastitwoulddemonstratethefeasibilityofvisiblelight-stimulatedemission.

Maimandecided togive itago–only tohave theHughesCorporationputanunintentionalspannerintheworks.In early 1960, the company moved their labs from Culver City to a new

locationinMalibu.Whilethismighthavebeenanattractivelifestylerelocation,it proved highly inconvenient timing, as just when the prize seemed in sight,Maimanhadtopackuphislabandmove.Luckilyforhim,bothSchawlowandGouldwerehittingpracticalproblemswiththeirgaslasers,sufferingeverythingfrommetaldeposits turning the tubecontaining theglassopaquetodifficultiesgetting an appropriate light source to do the pumping. Gould was also stillhands-off,unabletocontributedirectlytohisownproject.HehadsatthroughasecurityhearingatthePentagoninApril,butitprovedinconclusive.

Lasersgolive

ThesamemonthasGould’shearing,Maimanwasfinallyabletogetstartedonapracticalrubylaser.Hemadeuseofthethencommonspiralformofflashtubes,slippinga thincylinderofrubyinside thespiral.Eachendof therubyrodhadsilvermirroring,butoneendhada smallhole scraped in thesilver so that thelaser light could emerge. The whole thing was enclosed in a reflectivealuminium casing, which both protected the scientists’ eyes from the intenseflashlightandreflectedlightbackinwards,ensuringthat themaximumamountof the flash was directed in towards the ruby rod. Compared with the large,clumsy,bench-sizedapparatusrequiredforthegaslaserexperiments,thewholethingwassmallerthanafist:compactandneat.Withanexperimentaldesignconstructed,therewasonlytheneedtoseeifit

worked.Thiswasn’taseasyasitsounds.Rubiesfluorescenaturallywitharedlightthatisnotthecoherent,tightbeamofstimulatedemission.Justbecausethedeviceproducedared lightdidnotmeanitwas lasing.SoMaimanhadto testfor a sudden sharp spike ina tight frequency rangeof a spontaneousemissioncascade,ratherthanthelazier,spacedoutglowofordinaryfluorescence.By16May1960heandhisteamwerereadytogiveitago.The procedure was far more like a Hollywood version of a scientific

experiment than is usually the case.Maiman started off with a relatively lowvoltageon the flashlightandproduceda red spotofconventional fluorescenceonawhitetarget.Asheandhisteamgraduallycrankedthevoltageup,makingthe lightmoreandmore intense, thespotgotgraduallybrighter.Butsuddenly,with one more increase in voltage, the output spiked, the length of the flashmeasuredon anoscilloscope collapsed and the intense red spot reflected from

thescreenlitupthedarkenedroom.Successhadcomefromanunexpecteddirection.Itwasn’talloveron16May,

though. Maiman had to get his results published, and was shocked to get arejectionfromPhysicalReviewLetters,inpartbecausehehadmadethemistakeof referring to the device as an optical maser, and the Review’s editor haddecidedbythenthatmaserswereoldnews.Instead,MaimanmanagedtogetashortletterintotheprestigiousjournalNatureandscheduledalongerpaper(thatwould eventually be published elsewhere) with the slow-moving Journal ofAppliedPhysics.Aware that timewas tickingby, theHughesCorporationalsoarrangedapressconference,whichon7Julybroughtthelasertotheattentionoftheworld’spress.Intermsofpresentation,thiseventwassolidlyfocusedonthedown-to-earth

potential applications of a laser like enhanced communications, but the pressknewthatwhat theywereseeinghad thepotential foradramaticheadlineandpushedforacommentonwhetherthiswasaprototyperaygun.Maimantriedtoputthisoff,butwasunabletosaythatitwouldneverbeusedasaweapon.Thiswas enough for the press, and toMaiman’s horror the headlines blared: ‘L.A.mandiscoverssciencefictiondeathray.’

Whogottherefirst?Therewasnodoubt thatMaimanhadmade the first laser, though therewas acertainamountofcarpingfromBellLabs(stubbornlystillreferringtothedeviceasanopticalmaser),whoatonestageclaimedthattherubylaserwastheiridea,despiteSchawlowhavingdismisseditasunworkable.ABellteamdid,however,manage to get the first continuous laser running, based on a helium/neonmix(eventuallytobecomethetechnologyusedinthefirstsupermarketscanners)justbeforeChristmasofthesameyear.As for the patent battle on the underlying concept of the laser,Gould (who

neverdid receivehis securityclearance)managed toget aUKpatent in1964,buthehadtowaituntil the1980sbeforehispatentrightwasacknowledgedinthe United States. Bizarrely, the Nobel Prize awarded in 1964, whichconfusinglyandclumsilyreferredtothe‘maser-laserprinciple’,wenttoTownes,BasovandProchorov.DespitethepredilectionoftheNobelcommitteetoawardprizestotheoristsratherthanexperimenters,thisstillseemsanoddselectionofnamestochoose,asitisclearlyorientedtothemaserratherthanthemuchmoresignificantlaser.

Anewkindoflight

All the light that humans had experienced to date,whether from theSun or aheatedobject likeaflameor thefilament ina lightbulb,waschaotic.Photonsflewoffinanyolddirectionwiththeirphasesunconnected,andcameinawholemixof energies andhenceof colours.But thewaterfall triggeringof the laserandmaser meant that the photons were in step, a beam of near-monochromelightthatwashighlydirectional.Thebeamfromalaserissotightlyinstep,sodifficult to scatter, that laser beams have been bounced off the Moon andreturnedtoEarthstillinarelativelytightbundleofphotons.AsbecameobviousafterMaiman’spressconference,assoonas laserswere

publicised, thoughts turned tohow thisnew typeof lightcouldbeused. If thedevelopmentofthelaserwasaremarkableachievementinitself,thewaylaserswould be exploited took this light – the hallmark of the quantum age – intowholenewrealms.

CHAPTER7

Makinglightwork

Itwasinevitablethatatthepressconferencetoannouncethefirstlaser,thepresscorpswould think of that science fiction staple, the ray gun.Herewas a verypowerfulbeamof light thatsurelywouldsoonbemadeintoaweapon.Onlyaremarkably short period of time, just four years, elapsed between the firstexperimental laser being produced and JamesBond’s archetypal experience inGoldfingerwhere therubyredbeamcuts throughablockofgoldas if itwerebutter, heading inexorably towards splitting the immobilised Sean Connery intwo.As it happens, by far themajority of uses of laserswould not involve such

dramaticactivities,but there isnodoubt that laserscanpackapunch.Sohowdoesabeamofinsubstantiallightmanagetocutthroughgoldandsliceupspies?It’sworthtakingastepbacktosomethingmostofushaveplayedwithasachild–usingalensasaburningglass.

Heatfromlight

Weareusedtothefeelofsunlight’swarmthonourskins.Oneoftheforgottensenses, when we claim there are five, is the way that our skins can detectinfrared,lightthatisjustbelowthevisiblerangeinthespectrum.ButfocustheraysoftheSunwithaconvexlensandtheresultisfarfromapleasantwarmth–infactthereisenoughconcentratedenergytoscorchwoodandmakepaperburstintoflames. Some of the incident photons are re-emitted, enabling us to see theobjectinthesunlight,butwhenothersexciteelectrons,theresultantenergygoesintovibrationintheatoms:itheatsthemup.Byconcentratingthelightwiththelens there is simply too much energy pumped into too small a region. Thetemperature shoots up to the extent that the material starts to react with theoxygenintheairinthechemicalprocesswecallcombustion.The burning glass rarelymanagesmuchmore than setting a piece of paper

alight, although house fires are sometimes caused by glass vases focusing the

Sun’s rays and starting a blaze. But way back in Ancient Greek times, themathematician and engineer Archimedes proposed an optical defencemechanismforhiscity.IndangerofattackbyRomanvessels,hesuggestedthathuge curved mirrors be constructed on the harbour. These would be used toconcentratetheraysoftheSun,settingtheattackingshipsalight.Theprinciplemightwell haveworked had the ships been still enough, especially if tar hadbeenusedtosealgapsinthewood,thoughasfarasweawarethemirrorswereneverconstructed.Inprinciple,though,herewasthefirstdeathray.A laser cuts on exactly the same principle as a burning glass, butwith the

differencethatthelightisfarmoreconcentrated,evenataconsiderabledistance,and thewaves (or phase of the photons) are in step,making the impactmorelikely to transfer energy to the target efficiently.Whether it’smelting gold oroperatingonaneye,thelaserraisesthetemperatureinasmall,controlledregionextremely quickly, providing the desired cutting effect. The question thatinterestedthemilitary,ofcourse,waswhetherthisabilitytocausedamagecouldbeimposedatadistance,turningthelaserintoaformidableweapon.Theanswerwouldbeyes…andno.

Deathrays

AsGordonGouldsuggested,analmost inevitablemilitaryapplicationoflasersisintargetingweapons–eitherasavisualcue,whereaconventionalgunorrifleprojectsalaserdotonthetarget,orusedtoguidebombstoalocationpinpointedbyaninfraredlaser.Butwhenitcomestousinglasersdirectlyasweapons,weare still a fairway from the classic sci-fi ray gun. Probably the closest to thehandheld ray gun have been attempts to use lasers as a device to temporarilyblindanopponent.There isnodoubt thatdirect exposure to a laser cancausetemporaryblindness,but theborderline ineyedamagebetween temporaryandpermanent is a fine one and these weapons are widely considered unethical,resultinginthedevelopmentoftheUnitedNations‘ProtocolonBlindingLaserWeapons’whichhasbeenactivesince1998.The protocol, however, only prevents the use of weapons specifically

designedtocausepermanentblindness,a loopholethatmeansinpracticethereare still likely to be weapons deployed with the aim of causing a temporaryeffect,whetherornotthatmaysometimesresultinpermanentblindness.(Thereis something of a parallel with the use of tasers, which are deployed as non-lethalweaponsdespiteAmnestyInternational’sclaimthatover500peoplediedasaresultoftheirusebetween2001and2012.)

Tohavethekindofdestructivepowerweexpectfromagoodsciencefictionraygun,currentlasersneedtobefartoobigtobecarriedasasidearm.Mostlyasyetthesearestillunderdevelopment,butthereareexamplesthatareclosetobeing deployed like the US Navy’s ‘LaserWeapon System’, a ship-mounteddeviceintendedtouseaninfraredlasertocrippledronesandsmallboats.OtherUSconceptsincludeairbornelasersdesignedtomakesurgicalstrikesongroundtargets or destroy missiles. Such weapons would not work by outrightdisintegration but either by heating the outer skin of a target, causing stressfailure, or vaporising some of the surface sufficiently aggressively to causedamagingshockwaves.Atthetimeofwriting,mostattemptsatproducinglaserweaponshavebeencutasaresultofbudgetreductions.Thereisafairwaytogobeforealasercouldbeaneasilydeployedbattlefieldweapon.

Fusingwithlight

The idea of causing stress by vaporising the surface of a target may seemextreme, but it is an approach that is being used in one of themost dramaticapplicationsoflasers:nuclearfusionbyinertialconfinement.TakeavisittotheNational Ignition Facility at theLawrenceLivermoreLaboratory inCaliforniaandyouwillbefacedbyalaserset-upthatwouldmakeanyBondvillainproud.Intwovastten-storeyhalls,atinyinitialbeamundergoesatransformationthatwillmakeitintoamonster.A small triggering laser’s infrared output is split 48ways before each sub-

beamispassed throughanamplifying laser thatboosts thebeams’powerbyafactorof10billion.Eachof thosebeams is then split again,producinga final192beamsthatthenpassthroughthevastmainamplifiers,addinganotherfactorofamilliontobringtheoverallpoweruptoasizzling6megajoules.(Theflashis so powerful that for a few trillionths of a second it is as if the output of5,000,000,000,000 traditional light bulbs was concentrated into a tiny butimmenselypowerfulflareofcoherentlight.)These192beamsareup-convertedtoultraviolet,whichisbettersuitedtoits

finaltask.Inareactionchamberthebeamsconvergeonasmallfrozenpelletofdeuterium/tritiumfuel.Atthemomentofimpact,theoutersurfaceofthepelletis vaporised, producing an intense shockwave that compresses the remainingfueltosuchanextentthatnuclearfusioncantakeplace.Atthetimeofwriting,this was yet to achieve ‘ignition’ – where more energy is produced than isrequired to produce the fusion – though the National Ignition Facility hasmanaged toyieldmoreenergy than is applied to the target. (Lasersaren’t100

percentefficient,andmostoftheenergyconsumedbythedeviceiswastedintheamplifiersratherthanappliedtothepellet.)

Lasers,laserseverywhereWhilebothgasandcrystallasersarecommonenoughinindustrialandmilitaryapplications,andturnupinthefamiliarsupermarketcheckoutscanners,neitherof them features in the laserswe all are likely tohave around thehome– theones in laser printers, laser pointers and players for CDs,DVDs andBlu-ray.Theseareallsemiconductorlasers,inthesamefamilyastheLEDlightingthatistakingoverasalow-energystandard.Thefirstthoughtthatitmightbepossibleto produce laser light from a semiconductor goes all the way back to 1958,beforeanylasershadbeenconstructed,anditwasasearlyas1962thatavisiblelaserbasedonasemiconductorwasconstructed.Thiswasnotatruepredecessorof modern solid-state lasers as it could work only in pulses and at cryogenictemperatures, but by 1970, in parallel developments at Bell Labs and in theUSSR,themodernroom-temperaturesemiconductorlaserwasdeveloped.Thisnewformoflasermadeuseofastructureknownasa‘heterojunction’–

an interface between thin layers of semiconductor, which have unequal gapsbetween their valence and conductionbands. In the caseof the semiconductorlaser,thistooktheformofanarrowbandgapmaterialsandwichedbetweentwowidegaps.Therewouldbeawholefamilyofdifferentvariants,eacheffectivelya special version of a light-emitting diode, where the structure enables thespecial‘cascade’-likepropertiesofthelaser,producingcoherentlight,typicallyby surrounding the diodewith some kind of optical cavity.Not only are suchdevicesmuchcheaperthanthetraditionaltypesoflaser,theycanbemademuchsmaller to fit in a wide range of equipment. Semiconductor lasers outselltraditionallasersbyafactorofnearlyathousand,andarelikelytobepresentinmosthomesandworkplaces.

Trueview

Apartfromthreateningsecretagentswithbeingslicedanddiced,anotherearlyapplicationoflasersthathastakenlongerthanexpectedtobecomeusefulisthehologram,whichwas invented in principle before the laser even existed. Thehologram was dreamed up by the Hungarian-British scientist Dennis Gabor,soonaftertheSecondWorldWar.Gaborwasneveronetoconsiderpuretheory,alwayswantingapracticalapplicationforhisideas.Hehadbuiltado-it-yourselfX-raymachineinhisteens,butthoughhehadoriginallystudiedengineeringin

Berlin,theinfluenceofworld-classphysicistshaddrivenhimacrosstofocusonscience.Gaborwasthinkingofwaystoimproveontheelectronmicroscope,whichat

the time was in its infancy. He realised that a microscope’s image could beenhanced if it could somehow provide a wider viewing angle. This particularapplicationprovedimpractical,butitstartedGaboronthepathtodevisingawayto take a photograph, using light, that could be viewed just like a real object,seeing different perspectives from different directions, rather than just a flatpicture.The secret behind the hologram, Gabor’s idea of a truly three-dimensional

image that changedasyou lookedatdifferent angles,was thinkingabouthowlookingataflatphotographandlookingatarealscenediffers.Ifyouimaginehavingtwowindowssidebyside,onewithaperfectphotographofacityscapeinit,sohighinresolutionthatitisindistinguishablefromtherealthing,andthesecondanormalglasswindow,youcaneasilyseethedifference.Asyoumovearound, parallax caused by objects being at different distances behind thewindowmeansthattheyseemtomovewithrespecttoeachother.Whenyougoover to one side of thewindow, for instance, you can see an object that waspreviously behind another, because the light from the object is no longerscreenedbytheiteminbetween.Thisdoesn’thappenwiththeflatphotograph.Nowthinkofthesurfaceoftheglasswindow.Everysinglephotonfromthose

different objects, travelling at different angles, is arriving at the surfaceof theglass. All the photograph does is to sum up the intensity and colour at aparticularpoint.Butimaginethatinsteadyoucouldcaptureinformationoneverysingle photon, then take the window elsewhere. If you could stimulate thewindowtostartproducingthesamephotonsagain,thenyoushouldhaveatruethree-dimensionalviewcapturedontheflatsurfaceoftheglass.Thisishowahologramworks.Tosimplifythings, imaginethat insteadofa

cityscapewehaveasmallchild’stoy,anditisilluminatedwithamonochromelight (we’ll make it green, tomatch the early holograms). The light reflectedfromthetoyhitsourwindowglass.Eachphotonhasnotjustanintensityandadirection,ithasaphase–butonly

the intensity is captured ifwe use photographic film.But imagine insteadweshone a second beam of light onto the surface of the glass from the samedirection.Thelightfromthetoyandthedirectlightwouldinterfere,producingan interference pattern. If we could store that pattern, it would hold farmoreinformation than a normal photograph. And if we could use that pattern torecreate thestreamofphotonsemergingfromtheglass,wewouldrecreate thethree-dimensionalimage.

ThiswasallhypotheticalasfarasGaborwasconcerned,becausetomakeitwork, those photons would have to be very special, linking in phase and ofidenticalenergy,otherwisetheinterferenceeffectwouldnotproducethedesiredresult. Good filterswouldmake a relativelymonochrome light, but therewasnothingthatcouldbedoneaboutthephasesofthephotons–untilalasercamealonganddeliveredcoherentlightwithallthephotonstriggeredinstep.ItwasonlyfouryearsafterthefirstworkinglaserbecameavailablethatEmmettLeithand Juris Upatnieks at the University of Michigan produced the first truehologram, a bizarre still life of a model train and a pair of stuffed pigeons.Gabor’simpossiblevisionhadbeenmadepossible.Iremembervisitingoneofthefirstbigexhibitionsofholograms,called‘The

Light Fantastic’ and shown at theRoyalAcademy inLondon in 1977. In onesense itwasverymundane. Itwas like looking througha seriesof fuzzy littlewindowsonto objects illuminatedwith a sparklingly bright green light.But atthesametime,inasortofmentalquantumsuperposition,yourealisedtherewasnoobjectthere.Andthenthelaserwouldsnapoff,andyouwouldseejusthowmuch this apparently three-dimensional thing was an illusion. All you werelookingatwasapieceofglasswithameaninglesspatternofspecklesonit.

Ashatteredillusion

Thinkingofmyoriginal‘picturealongsidewindow’experimentgivesusanotherinsight into the remarkable nature of holograms and how they differ from atraditionalphotograph.ImagineIblankedoutallbutthetopleftsegmentofboth‘windows’,leavingjustasquaretwocentimetresoneachsidevisible.Asfarasthephotographisconcerned,Ihavelostalmostalltheinformationitheld.If,forinstance,thattopleftsquareshowedonlybluesky,thenallIwouldseewouldbesky.Icouldtellnothingaboutthecityscapefromthattinysegment.However,thingsarequitedifferentwiththerealglasswindow.Yes,standing

inthesamerelativepositiontothisasthecameramanwastothephotograph,Iwillseeonlyasquareofsky.ButifIgetupcloseandmovearound,Icanlookthrough the square of window at different angles and still see most of thecityscape.Iwon’thaveasagooda three-dimensionalviewbecauseIcanonlyseethroughthissmallsegmentofglass,butbylookingatanextremeangleIcantakeinprettywellallofthescene.Thesamemustbetrueofthehologram.IfIbreakoffjustthecornerofthehologram,unlikethetraditionalphotograph,Istillhaveinformationaboutthewholescene.There will be fewer photons hitting the small square of hologram, so the

image will be fainter and less clear (there are also limitations due to theresolutionlimitofthemediumusedtocapturetheimage),butthefactremainsthat unlike the traditional photograph, any piece of a hologram will containvastlymoreinformation,enablingtheviewertoreconstructthewholeimagethatwasvisible.

Onreflection

It’s fair to say that holograms were, even more than lasers, first seen as atechnology in search of an application. In practice, the most valuableapplications of holograms have gone in two very different directions – thesimplisticholographic stripsused for securityprotection, andholographicdatastorage.Themostcommonsecurityholograms,thesortofthingsyouseeoncreditand

debit cards, some fancy bank notes and on premium products, are usuallyreflectionholograms,wheretheinterferencepatternofaholographicimagehasbeen stamped on a metal foil. Even though this doesn’t produce a trueholographic3Dimage,itmakesuseofaclevertricksothattheviewerdoesnotjustseeaspeckledinterferencepattern.Areflectionhologramhastwoormorelayers.Ateachlayer,someofthelight

is reflected back, and it is the interference between these different layerreflectionsthatmakestheholographicimageemergefromwhatotherwisewouldjustbeamulti-colouredmess.Itissometimessaidthatthesesecuritytagsaren’thologramsatall.That’sabitunfair–theyare,buttheydon’tprovidethetrue3Dimagethatallowsyoutoseeanobjectfromdifferentdirections,asifyouwerelooking at the real thing. It is holographic technology, but not producing atraditionalvisualhologram.Some,however,areaspecialvariantofthetruehologramthatcanbeviewed

using white light, called a rainbow hologram, for the obvious reason that theimageappears tohaveanunnaturalrangeofrainbowcolours.Theholographicimageisproducedintheusualway,butthroughanarrowslit.Whenlightissentthrough the hologram, different parts of the image are seen depending on thewavelength of the light – so white light produces a whole image, but withdifferent strips of it in different colours. Like most holograms, rainbowhologramsneedtobelitfromtheback–tomakethemworkasasecuritytag,theyarebackedwith reflectivematerial so light from the frontpasses throughthehologram,thenbackoutthroughitafterhittingthereflector.

Three-dimensionalmemoryThere isnodoubt thatholographic security tagsarewidelyusedandvaluable,buttheyaren’tthesortofbreathtakingapplicationwewouldhopeforfromsucha dramatic piece of quantum technology. However, another way of usinghologramsholdsoutmorepromise:holographicdatastorage.Asourcomputersbecomemoreandmorepowerfulandever-present(remember,thephoneinyourpocket isa sophisticatedcomputer in itsown right),wealsoneedever-greaterquantitiesofstorage.Withcamerasalwaysready,manyofuswilltakehundredsorthousandsofphotographsayear,whereonce,usingfilm,wemighthavetakenafewdozen.At the same time, our storage requirements for music and video are going

through the roof. For a while, the laser provided one of our most commonstoragemechanisms,burningmarkings intowritableCDsandDVDsinopticalstorage, but these are now on the wane. Just as the once ubiquitous diskettedrives have disappeared from our PCs, so manufacturers are increasinglydropping optical drives as we make use of the internet to store most of ourmaterial in ‘theCloud’.But theCloud isonlyaconceptualentity.Behind thispleasantly fuzzy notion of our documents and pictures, videos and songsdisappearingintoawhitefluffymassisarealityofmassivedatacentres,wherethe information is stored on old-fashionedmagnetic disc drives.Although ourportabledevicesoftenholdinformationinsolid-statememory,thedatacentre’smagneticdiscs–muchcheaper tomakeona large scale–are theonlyviablesolutionforthevastquantitiesofstoragerequired.However, that storage requirement isalwayson the increase, takingupever

morespaceandpower.When,forinstance,aphotographicCloudsitelikeFlickrgives each of its users 1 TB – a million megabytes – of free storage, it isinevitablethatthosedatacentreswillendupcreakingattheseams.Hologramshave a potential answer, especially for the sort of information that is rarelychanged, likecopiesof songsandvideos.This isbecauseahologram is reallyjust a very compact way to store a lot of data. Usually this is the complexinformationrequiredtoreconstructthethree-dimensionalimage,butthereisnoreasonwhyitcan’tbeusedtopackawaytraditionaldigitaldata.The concept behind holographic storage, which is still more theory than

practice,isthatratherthanhavejustasingleplaneofinterferencepatterns,asweexperience in a visual hologram, the storewill be a three-dimensional crystal,storing information on thousands of planes within the material, effectivelystackinghologramsmanydeep.Ifsuchanapproachcanbemastered, therearethreehugeadvantagesoveratraditionaldiscdrive.Themostobviousisthatthe

crystal ismuchmore robust.A hard disc involves floating a head less than ahair’swidth,movingatthespeedofaBoeing747,overaverydelicateplatter.It’s no surprise that they are fragile.But a holographic crystal has nomovingpartswithinthestoragemechanism.The second benefit is sheer capacity. A magnetic disc is two-dimensional.

Although the drives are very slim these days, they can’t rival the way that aholographic crystal could stack thousands of layers into the same space as asinglediscdrive.Andthenthereisspeed.Adiscislimitedbythetimeittakestheheadtomovetotheappropriateposition,andthenbytheneedtorespondtomagnetic patterns on the disc’s platter in a linear fashion, bit by bit. In ahologram,thedatacanbeaccessedinparallel,pullinginfarmoreatatime,andcanbeswitchedaroundatthespeedatwhichabeamoflightcanbedisplaced.Thedownsideofholographicstorageisthatitismuchslowertowritetothan

is a magnetic disc. So once the significant technical issues in makingholographicstoragepractical, robustandcommercialareovercome, it is likelythatadatacentrewillbemixed-mode.Magneticdiscswillstillbeusedforthevolatile,quick-changingdata.Butinthebackground,datacanbeshiftedoff totheholographicstore,wheretheycanthenbeaccessedwithlightningspeed.Adocumentyoucreate,editacoupleoftimesoverafewhoursandthendeletewillprobablynevermake it to the crystal.But if I look atmydiscusage, thevastmajority of documents have not been edited in weeks, months or years; plusthere’sthemuchlargerspacetakenupbythelikesofphotos,musicandvideo.All these could be conveniently shifted off expensive, bulky magnetic drivesontoholographiccrystals,weretheyavailable.

Arealworldview

What we perhaps were expecting in the early days as just a matter oftechnologicaldevelopment–somethingthatwouldcomewithtime–weretrueholographicphotographs.Imagesinanewspaper,say,oronacomputerscreenorinthefamilyalbumthathadallthe3Dcrispnessofreality,withnoneofthemonochrome fuzz of the early holograms nor the cardboard cut-out oddity oftraditional 3D photography, a process that dates back to Victorian times andreliesonputtingaseparateimageinfrontofeacheye.Thereisathirddimensioninsuchphotographs (or in3Dmovies)but theycouldneverbemistakenforarealthree-dimensionalworld.Thechancesare,atleastunlessanduntilthereissomemassivebreakthrough

inholographictechnology, thatwearenotgoingtoseeanysuchdevelopment.

Lasersareinherentlymonochromaticandthehologram’sneedtohaveanimageilluminatedbylasermeansthatnocurrentgenerationhologramcouldbecapturedinnaturallightorcarrynaturalcoloration.Thereisnoobviousleaptobemadethatwouldtakehologramsintotheworldofrealisticportrayaloftheworldwesee – and though science fiction often representsmoving 3D images as beingsomeformofadvancedhologram(thinkofthePrincessLeiaimageprojectedbyR2-D2inthefirstStarWarsfilm),inrealitythechancesarethatanysuchfutureability toprojecta three-dimensional imageintoemptyspacewilldependonatotallydifferenttechnology.

AglitteringDiamond

Before leaving the strangeandwonderfulworldofquantum light, there isonelightsourcethatisverydifferentfromalaserandthatisdoingremarkableworkin the Oxfordshire countryside. Pretty well everyone has heard of the LargeHadronCollider(LHC)atCERNinGeneva.TheHarwell-basedDiamondLightSource resembles a miniature version of the LHC, but in terms of valuablecontributionstothequantumage, itmakesCERN’sworklooklikeyesterday’snews.AcceleratorsliketheLHCpushparticlestonearthespeedoflight.Thework

they undertake is usually described as particle physics rather than quantumphysics.Thedistinctionisabitlikethedifferencebetweenzoologyandbiology.Quantum physics, like biology, gives us the fundamentals of how quantumparticlesbehave,whileparticlephysicsgivesus thedetailsof theparticlezoo,justaszoologytellsusaboutspecificanimals.Butallthoseparticlesstudiedbyparticlephysicistsarequantumparticles.Acceleratorshavebeenbuiltsincethemiddleofthelastcenturytoundertake

a particularly brutal kind of science. They blast particles into each other atextremelyhighspeedsandseewhatcomesout.It’sabitlikeworkingouthowamechanicalclockworksbyhittingitwithasledgehammerandfilminghowallthepartsflyoutofitinslowmotion.Theresultistogiveusaccesstoparticlesthatwouldotherwiseneverbe seen.Andearlyon itwasdiscovered that thesevast machine laboratories had an unwanted side effect. When the earlyaccelerators known as synchrotrons were set up in the 1940s they werediscovered to pump out large quantities of electromagnetic radiation. Theyemittedlight.Synchrotronsuseaseriesofpowerfulelectricalfieldstoaccelerateparticles,

often electrons, which are steered around a loop by electromagnets. To get

aroundtheloop,theelectronshadtobeaccelerated.Thisisbecauseaccelerationisachangeinvelocity,andvelocitycomprisesbothspeedanddirection.Evenifsomething continues at the same speed, if it is pushed into a circle it is beingaccelerated.And,asNielsBohrdiscoveredwhentryingtouncoverthestructureoftheatom,whenanelectronisaccelerateditpumpsoutphotons.Ifitweren’tfortheconstantinputofthesynchrotronfields,theelectronswouldrapidlyloseenergy.Fortheearlyresearchersthis‘synchrotronradiation’wasawasteproduct,an

unwantedside-effect.Butjustoccasionallywasteproductscanbeuseful.Think,for instance,ofMarmite.Whenbeer ismade it leavesbehindagunky, sticky,dark residue.Rather than throw it away, thebrewers found that this substancecouldmakeaninterestingsavouryspread–andMarmite(alongwithVegemitein Australia) was born. What should have been a waste product becamesomethingvaluable(ifyoulikeMarmite)initsownright.Thesame thinghappenedwithsynchrotron radiation.The lightproducedby

theacceleratingelectronswasavailable inawidespectrumandwasextremelyintense, and eventually synchrotrons were built with the sole purpose ofgenerating these bursts of radiation. The first in Britain was at Daresbury inCheshire, replacedby themore sophisticatedDiamondatHarwell.Apart fromsize and flexibility, the biggest advance Diamond has over its predecessor iscontaining undulators and wigglers. These are series of alternating magnets,whichforcetheelectronsintoapatternofsinusoidalripples.Undulatorsproduceatight,narrowoscillationgeneratinganarrowbandofradiation,whilewigglersproduceawiderbandoffrequencies.Around themain storage ringatDiamondare rangedbeamlines, exitbeams

fortheradiationwhereworkstationsknownunromanticallyashutcheshousetheexperiments. In Diamond’s massive 45,000-square-metre floorspace (aroundeight times that of St Paul’s Cathedral) there are currently over twentybeamlines, with space for 40 in the final configuration. Depending on thepositionon the ring, thesecanproduce infrared,visible light,ultravioletorX-rays.

Insidetheblackbox

SomeoftheapplicationsofasynchrotronlightsourcelikeDiamondaresimply‘like the restbutbetter’.X-rays, for instance,havebeenused todetermine thestructureof crystals ever since theBritish father and son teamofWilliamandLawrenceBraggwontheNobelPrizein1915fordevisingthetechnique.It’sa

direct application of a quantum phenomenon to uncover a structure via adistinctlyindirectroute.Imagineyouhadafeaturelessboxaboutthesizeofashoebox,pepperedon

theoutsidewithlotsofholes.Insidetheboxisastructureyouwanttodiscover,butyoucan’topenit.Whatyoucoulddoisdropaballbearingthroughaholeandseewhereitcomesout.Repeattheprocesswiththeboxheldatallsortsofdifferentangles,andusingallthedifferentholes,andyoucouldstarttobuildupsomekindofpictureofwhat’sinside.X-raycrystallographyisabitlikethis.AbeamofX-raysisshoneintothecrystalfrommanydifferentangles.Inside,

theX-ray photonswill be absorbed by electrons on the atoms and re-emitted.Thesere-emittedphotonswill theninteractwithothersfromdifferentpositionsinthecrystallattice.Dependingonthespacingofthatlattice,thephasesofthephotons will either reinforce or cancel each other out, so when the X-rayseventually emerge they will form a pattern of dark and light markings. Bycombining thousands of these images taken from different directions it ispossibletodeducethestructureofthecrystal.ThiswasexactlythesametechniqueusedbyRosalindFranklintoproducethe

DNAdiffractionpatternsthatwouldbeusedbyCrickandWatsontodeducethestructureofDNA.Butasworkedinthelabitisaslowprocess.AgoodexampleofhowDiamondcanmakeadifferencecomesfromworkbyPierreRizkallah(acrystallographer) andDavidCole (a biologist) from theUniversity ofCardiff.TheCardiffpairwereworkingatDiamondin2013onT-cells.Thesearewhiteblood cells, themedical police of the bloodstream, responsible for destroyingbacteriaandotherinvaders.T-cellshaveauniqueabilitytolookinsideanothercellandseewhat’sgoing

oninside.TheydothisbyusingspeciallyshapedproteinsonthesurfaceoftheT-cellcalledT-cellreceptors.TheselatchontootherlargemoleculescallMHCs(majorhistocompatibilitycomplexes).MHCsprotrudefromtheoutsideofcellsin thebody,anddifferdependingonthe internalmake-upof thecell.AT-cellcandistinguishbetweenafriendlycellbelongingtothebodyandaninvaderbytheshapeoftheMHCandhowwellitfitsinoneoftheT-cellreceptors.Sometimes,though,thisprocessdoesn’tworkcorrectly.TheT-cellcanattack

friendlycellsitdoesn’trecognise,aswhensomeonehasatransplantedorgan.Oritcanfailtospotacellitcoulddestroybutdoesn’t,typicallyacancercell.TheMHCsonacancercellaretooclosetothoseofanormalcellforaT-celltospotthedifference.WhatRizkallahandColearedoingisusingX-raystostudytheshapeoftheT-cellreceptorsandhowtheyinterlockwiththeMHCs,sotheycanmodify thereceptors to latchon toacancercellbutnot theequivalenthealthycell.Ifthiscanbeachieved,alltheywillneedtodoisextractsomeofapatient’s

T-cells,modifythemandre-injectthematthecancersitetohavetargetedcancerkillers.Theproblemtheywouldtraditionallyfaceisthatinthelabitcantakearound

eighthourstotakeasingleexposure,becausethecomplexstructuresmakeforaveryfaintimage.Asittakesthousandsofexposurestobuildasinglestructureitcould takeup to threeyears toanalyseonemolecule.AtDiamond, theX-raysare100billiontimesmorepowerfulthananX-raytube.Theresultistoprovidesuchhighresolutionthatimageswereinitiallyproducedinminutes,andnowcanbetakeninafractionofasecond.Theteamcannowanalysethreestructuresadayandarehopingforevengreaterspeedinthefuture.

Zappingpoo

SomeoftheotherapplicationsofDiamondaremoreuniquelyduetoitsopticalpower, with output millions of times brighter than the Sun. For example, in2013, Mark Hodson from York University was using Diamond to studyearthworm poo. Specifically, he was studying tiny calcium carbonate spheresaround2millimetresacrossthataredepositedinwormcasts.Calciumcarbonateisaverycommonmineral–itformslimestone,marbleandchalk,forinstance–and is used in everything from cement to paper whitening. Usually calciumcarbonatehasacrystallinestructure,butinthesewormdroppingsitisalsoinanon-crystallineamorphousform.This is of real interest tomaterial scientists because the amorphous form is

usuallyunstable,collapsingintoacrystalwithinminutes.Butinthewormpooitcanlastforyears.Someimpurityisgivingitextrastability.Ifthisprocesscouldbe understood it could be useful in everything from reducing the build-up oflimescaleinpipestomodifyingthestrengthofbuildingmaterials.Inthelabit’spossible to grind up the granules and identify the composition, but it isimpossible to determine how the impurities are distributed through theamorphousmaterialtokeepitstable.ByblastingitwithpowerfulinfraredlightfromDiamondtheycanobtainenoughaccuracy(thebrighterthelight,thebettertheresolution)todeterminetheworms’secret.These are just two of many applications. Diamond runs 24 hours a day,

blastingoutlightintothehutches(somebrightyellowtowarnthattheyarethelead-lined home of intensely powerful X-rays), allowing many differentexperimentstogoonsimultaneously,eachmakinguseofquantumlighteffectsto gain better insights into the structures and nature of everything fromintegrated circuit designs and aircraft engine parts to the kind of natural

structureswe’vealreadyseen.We are never going to see synchrotron light sources around the house, but

lasers arehere to stay.Oneof theessentialsofmoving the laser frombeingaspecialistlaboratorytoollikeDiamondtosomethingthatwouldhavereadyuseintheworldarounduswastheabilitytouseitatroomtemperature,ratherthaninasuper-cooledcryogenicenvironment.Butinthenextchapterwewilltakeachilly plunge into a quantum phenomenon that was only discovered whenworkingattheextremesoflowtemperature.Itwas in the frigid region close to absolute zero that superconductorswere

firstdiscovered.

CHAPTER8

Resistanceisfutile

Ifyourememberanyphysics fromschooloverandaboveNewton’s laws,youmight recall a formula dealing with electricity: V=IR. The voltage (V) in anelectricalcircuitisequaltotheamountofcurrentflowing(I)timestheresistance(R).Resistanceistheelectricalversionoffriction.Frictionresiststhemovementoftheobjectasitrestsonasurface.Airresistancehasanequivalenteffectonaplane or a ball in flight. Similarly,when free electrons are flowing through apiece of metal they should just keep going for ever, but electrical resistanceprevents this from happening. However, just over 100 years ago, the DutchphysicistHeikeKamerlinghOnnesnoticedsomethingquiteremarkable.

DoctorCold

Kamerlingh Onnes was a professor at the University of Leiden, primarilyinterestedincryogenics,thestudyofmaterialsatverylowtemperatures.Hewasprobablytheworld’sleadingexpertonthemechanismsforproducingseriouslycoldsubstances.(AsIamgoingtomentionhimalotinthischapter,IamgoingtotakethelibertyofshorteninghissurnametoOnnes,althoughwereallyshoulduse thewhole ‘KamerlinghOnnes’.) In1908hemanaged toget liquidheliumdown to 1.5 K, the lowest temperature ever reached at that date. This is atemperatureontheKelvinscale,whichismoreconvenientdownatextremelowtemperatures.The Kelvin scale reflects the existence of absolute zero, the lowest

temperature. This limit exists because the temperature of a substance is ameasureoftheenergyofitsatomsormolecules,andatabsolutezeroeveryatomwouldbeinitslowestpossibleenergystate.(Inpracticeabsolutezerocan’tbereached, because atoms are quantum particles that can never be pinned downwithabsolutecertainty,aswe’veseen.)Whenabsolutezerowasfirstpredictedattheendofthe17thcenturynooneknewaboutatomsandmolecules,letalonequantum theory, but they observed that the pressure of a gas fell as itstemperature reduced. Absolute zero was the temperature at which it was

predicted that pressure would cease to exist. On the familiar Celsius scale,absolutezerois–273.15°C,butwhendealingwithtemperaturesneartoabsolutezero,theKelvinscaleconvenientlyhasthesamesizeunitsasCelsiusbutstartsfrom0atabsolutezero.Theunitsofthescalearekelvins,representedbyK(not°K),so1.5Kisthe

equivalentof–271.65°C.In1911,Onneswasexperimentingontheconductivityofmetalsattheseverylowtemperatures.Whenhegotapieceofmercurydownto 4.2 K (the familiar ‘liquid metal’ element is solid at these extremetemperatures,asitfreezesatabalmy234K),itsresistancedisappearedentirely.Itwas like taking amovingobject into a vacuum.With nothing to stop it, nofriction,noairresistance,theobjectwouldkeepmovingforever.Similarly,theelectronsinthemercuryactedasiftherewasnothingtostopthem.Therewasnoapparent resistance, they just flowed on indefinitely. If you set an electricalcurrentgoinginaringofsuch‘superconducting’materialitwouldsimplyflowuncheckedforaslongasthelowtemperaturewasmaintained.Having a suspicion that there was zero resistance was one thing –

demonstratingthatitwasexactlyandnotjustapproximatelytruewasanother.Inpractice it isn’tpossible todefinitivelyprove that theresistance is totallyzero;all that can be done is to establish that it is as nearly true as it is possible tomeasure.Even that is non-trivial.Oneway thatOnnes attempted to show thiswasbysettingacurrentgoinginaloopofsuperconductingwirewithinabathofliquidhelium.Hethentookmagneticfieldmeasurementsoutsidethevesselovertime and watched for a change – at its simplest, his experiment could beimagined as using a series of magnetic compasses, surrounding the vessel,watching for twitches in theneedles. If the current in the loopwasdecreasingover time due to electrical resistance, the magnetic field it generated shouldchange–butnothinghappened.Onneswasabletoperformthisexperimentonlyfor a few hours before his helium evaporated away, though by the 1950s asimilarexperimentwasrunforeighteenmonthswithoutanydetectedchangeinmagneticfieldduetoareductioninthecurrent.

Challengingauthority

Back in 1911, getting anything down to such low temperatures was achallengingexercise.(Itstillisn’tthesortofthingyoucandointhekitchenathome.) Take a look at photographs of Onnes’ laboratory and you will see asteampunkversionofCERNwithgreatbrassdevices, large, important-lookinggaugesandaspaghetti-tangleofmetalpiping.Inthislaboratoryequivalentofa

ship’s engine room,Onnes seems tohavewielded the samepower as a ship’scaptain. He had plenty of assistants, but you wouldn’t realise this from hispapers,wherehewasusually the soleauthor. (Admittedly thiswasacommonfailing among lead scientists then.)Evenby the standards of the time, hewasconsidered paternalistic and overbearing, an old-fashioned remnant of thetraditionalmethods andmodes of science that were being swept away by thenew scientific class, an approach driven by sheer intellectual capability ratherthansocialstandingandauthority.That’snottosaythatOnneswasanythingbutasuperbscientist,buthisattitudeswereplantedfirmlyinthepreviouscentury.Not surprisingly, the discovery of superconductivity was more than a little

startlingbothforOnnesand thephysicscommunity. Itwasfinefor theoreticalmodelstodealwithalackofresistance,buttherealworldwasgritty.Resistancewas a fact of life. Apart from anything else, superconductivity was the totalopposite of what was expected to happen. By 1911, physicists were quitecomfortablewith the conceptof the electron, aparticle that carried electricity,the charge of which had just been pinned down by the American physicistRobertMillikan.Itwasthoughtthatelectricitywascarriedbyaflowofelectronsthrough a wire, acting like a gas being pumped through a pipe. It seemedreasonable, therefore, that if the temperaturewas lowenough, the ‘gas’wouldfreeze,stoppingtheelectronsflowing.Manyexpectedtheelectricalresistancetoshootuptowardsinfinity,ratherthantodisappear.Onneshimselfhadadifferentviewthattheresistancewoulddropastemperaturefell,butthatitwouldnevermakeittozero,asabsolutezerowasimpossibletoachieve.Forhim,itwasn’tsomuchthefallinresistancethatwassurprising,butthesuddendroptozeroat1.5K.Electrical resistance,while highly useful in some electrical circuits, is often

thebaneofanelectricalengineer’slife.Think,forinstance,ofthewaythatwetransmitelectricityoverlargedistances.Thereasonweneedthosevastunsightlypylons,carryingextremelyhighvoltages,isthatthehigherthevoltage,thelessthetransmissionlossduetoresistance.Afairpercentageoftheelectricalenergyinaconductorisconvertedtoheatastheelectronsinteractwiththeatomsinthemetal (think of an electric fire element as an example of this). With asuperconductor there would be no transmission loss. A superconductingelectricitygridwouldrevolutionisethedistributionofpower.Butitdidn’t takeOnneslongtorealisethatthiswaslittlemorethanapipedream.Thetechnicaldifficultiesofkeepingaconductordownatsuchextremelylowtemperaturesfaroutweighanyadvantagegainedingettingaroundtransmissionloss.What’s more, while in principle superconductivity seemed to promise the

ability to produce unlimited currents (implying super-powerful magnets), in

practice the effect was self-defeating. As soon as any sizeablemagnetic fieldbuiltup,itseemedtointerferewiththeprocess,stoppingthesuperconductivityin its tracks.Similarly, foranyparticularwire therewasacurrent limit,abovewhich the superconducting effect was destroyed. With all these limitations itwouldbequiteawhilebeforepracticalapplicationsofsuperconductivity,forallits apparent magic, would emerge. But they certainly would do so. For themoment, though, thedisappearanceof resistancewas littlemore thananaturaloddity.Curious, but of little significance. It is notable that the citation for theNobel Prize that Onnes received in 1913 does not explicitly mentionsuperconductivity.

Magnetism-freezoneWewill come back to the implications of this lack of resistance, but firstweneed to skip forward to 1933,when theother keyquality of a superconductorwasdiscoveredbyWaltherMeissnerandhisgraduatestudentRobertOchsenfeldat the German national institute of natural and engineering sciences, thePhysikalisch-TechnischeBundesanstaltinBerlin.EversinceMichaelFaraday’swork,physicistshadbeenawareoftheconceptofamagneticfield,refinedandquantified by James Clerk Maxwell. It was the movement of an electricalconductorthroughamagneticfieldthatstartedacurrentflowing–thiswashowgenerators worked. Magnetic fields permeate space and pass through solids(though limited to some degree by permanent magnets). But Meissner andOchsenfelddiscoveredwhatwouldbecomeknownastheMeissnereffect.Attheexactsame temperatureaselectrical resistancedisappeared,aconductorwouldexpelanymagneticfieldthatpassedthroughit,causingthefieldtobendaroundtheobject.Just asOnnes’ original discoveryhadbeenunexpected, soMeissnerhadno

idea that this strange magnetic expulsion was going to happen. He andOchsenfeldweremerelystudyingthevariationinthemagneticfieldinametalcylinderas it transitionedtobeingasuperconductor.Therewasnoexpectationthat the cylinderwould totally expel themagnetic field.Meissner’s discoveryproved to be a significant boost for the attempts to explain superconductivity,which had been in the doldrums since Onnes’ discovery of the effect. Onephysicist, Felix Bloch, had commented: ‘The only theorem aboutsuperconductivitywhichcanbeprovedisthatanytheoryofsuperconductivityisrefutable.’ But the Meissner effect opened up a whole new avenue ofexploration.

Aquantumchill

Thesetwobehaviours,zeroresistanceandtheexpulsionofmagneticfields,arethe characteristic behaviours of superconductors and between them produce awhole host of practical applications for this eminently quantum phenomenon.However, observing them in action is one thing. Explainingwhy they shouldhappenisentirelyanother.Onneshadsuspectedthattherewassomethingofthenewquantummechanics involved in superconductivity, thoughat the time thetheory was very sketchy and incomplete. Later on it would be realised thatsuperconductivity could never be explainedwithout invoking quantum effects,farbeyondthesimpleobservationthatelectronsarequantumparticles.Itseemedapparentbythe1930sthattheelectronsinaconductorthatcarried

anelectricalcurrentwerestronglydelocalised throughout theconductor– theyweren’t particularly associatedwith one atom anymore – but the expectationwas that a combination of the impurities in the conductor and interactionsbetween the electrons and the jiggling atoms of the material, constantly inmotion even in a solid, would always provide a degree of resistance. Thisshould,asOnnesexpected,graduallyreduceasthetemperaturewentdown–butthere was nothing in theory to predict the sudden fall to zero observedwhensuperconductivitykickedin.Thefirsthintofanexplanationcamein1935whenFritzLondon,aGerman

scientist who with his brother Heinz had fled Nazi Germany (appropriatelyenough towork in London), suggested at ameeting of the Royal Society theradicalideathatthewholeofasuperconductingobjectcouldactasifitwereasinglegiantatomwiththeconductionelectronsinafuzzaroundit,shieldingitfromtheintrusionofamagneticfieldandcausingtheMeissnereffect.Atfirstsightthismightnotseemmuchofastepforward.Afterall,ithadlong

been established that the conduction electrons weren’t tightly associated withanyspecificatom.But the revolutionary, slightlyscaryaspectofwhatLondonwasproposingwasthatthiswasaquantummechanicalprocessoperatingonthescale of a visible, touchable object (at least when wearing thick gloves).Generallyspeaking,quantumphysics,withallitsweirdness,appliesonlyatthelevelof theverysmall–atoms,photons,electronsand the like.Typically, thebiggest thing that can still exhibitquantumeffects is about the sizeof a smallvirus. Anything bigger and there is simply too much interaction between theparticlesmaking it up, causing decoherence.ButLondonwas saying that in asuperconductor – which could in principle bemanymetres across – quantumbehaviourwasthenorm.

QuantumvibrationsFrom the point of view of understanding superconductivity, London’s keyobservationwas to suggest that the electrons behaved in a collectivemanner,sharingawavefunction,actingcoherently,ratherlikethephotonsinalaser(seepage 129) to produce an irresistible current. But how could electrons, whichrepeleachotherandarenotexactlysociable,acttogetherinthisway?Threekeyindividuals,JohnBardeen,LeonCooperandRobertSchrieffer,wouldcomeupwithananswer thathasbeenourbestunderstandingof thebasicprinciplesofsuperconductivityeversince.Butthiswasn’tayearortwoafterLondon’sRoyalSocietytalk.Awhole27yearshadpassedbeforethingsfinallycametogether.John Bardeen is something of a hero of this book, also involved in the

developmentof the transistor (seepage63).Hewouldreceive theNobelPrizeforbothpiecesofwork,puttinghimin theextremelyexclusiveclubofdoublewinners.EvenbeforetheSecondWorldWar,Bardeenhadbeenthinkingabouttherelationshipbetweentheconductionelectronsandtheatomsthatmadeupasuperconductor.Perhaps,hebelieved, thoseatomscould insomewaygive theelectronsaboost.All atoms vibrate, and in a solid, particularly one with a regular structure,

thesevibrationsproducewaves that travel through thematerial,vibrations thatare called phonons.As the similarity of the name to ‘photons’ suggests, thesewavesarequantised– theycan’tvibrateanyoldhow,butarerestrictedbythestructureof thematerial tovibrateonly incertainmodes.And though theyaretruewaves,notthe‘particleswithwave-likeprobabilityproperties’ofatypicalquantumparticle,thisquantisationmeansthattheyarestillsubjecttotherulesofquantummechanics.Bardeen felt that an interaction between phonons in the material and its

conduction electronswas significant to superconductivity, and by 1950 it hadbeenshownthattheatomscertainlyplayedapart–thiswasn’tapureeffectofthe electrons – because the critical temperature at which amaterial started tosuperconduct varied for different isotopes.An isotope is a variant of an atomwithdifferentnumbersofneutronsinanucleus,butthesamenumberofprotonsandelectrons.So,forinstance,uraniumfamouslyhastheisotopesU-235andU-238.Thesehave the samenumberofprotons andelectrons (so are chemicallyidentical),butU-238hasthreemoreneutronsinitsnucleusthanthe143inU-235.If superconductivity were purely dependent on electrons then you would

expect no difference in critical temperature between different isotopes of thesamematerial,butif,asprovedthecase,adifferencewasobserved,itsuggested

that theway the atom as awhole behavedwas a part of the superconductingprocess. What this special vibration seemed to do was to enable a verycounterintuitive effect. Electrons are all negatively charged and so we wouldexpectfromallourexperiencewithelectromagnetismthattheywouldrepeleachother. But in a superconductor, the electrons have to effectively attract eachothertoproducethekindofeffectsthatwerebeingseen.

Themattresseffect

Physicists have something of a fondness for bowling balls when it comes toprovidingavisualimagetosimplifyatheory.Whereattemptstoexplaingeneralrelativity and its explanation of gravity usually bring in themodel of bowlingballsdistortingarubbersheet,thosetryingtoexplainsuperconductivityputtheirbowling balls on a saggy mattress. A first ball is rolled along the mattress,leaving an indentation that is slow to restore, and which a second ball willinevitablyfollow.Theresultisthatthesecondballfeelsanattractiontothefirstviathemediumofthesaggymattress.Inthesuperconductor,thecooled,slow-reacting ions of the solid’s structure act like the limp springs of themattress,linkingtheelectronstogetherbygivingthemanaturalpathtofollow.Thefinalpushtodevelopatheorytoexplainsuperconductivitycamein1957

when John Bardeen, Leon Cooper and Robert Schrieffer came up with whatwould become imaginatively known as the BCS theory. Leon Cooper hadalready discovered that pairs of electrons (called Cooper pairs) can be linkedtogether in thismattress-like fashion.Cooper pairs are electronswith oppositespins,pushedtogetherbytheslow-respondinglatticeofthematerial,actingasiftheywerea singleentityas theypass throughaconductor.OnceelectronsaretravellingasCooperpairs,thepairwouldhavetobebrokenupiftheyweretobescattered enough by phonons to produce electrical resistance. Because of thefuzzy location of quantum particles, each electron pair overlaps with othersaroundthem,formingastatecalledacondensate,whereallthepairsinteractasiftheywereasingleentity.Whereitwouldbeeasyforasinglepairtobebrokenbyphonons,inacondensate,suchabreakwouldinfluencethewholecollectionof pairs, making it much less likely to happen. The pairs flow like a ghostlywholethroughthesuperconductor.Inparallelwithdevelopingtheoriesonwhysuperconductorsshouldexistand

how they avoided resistance, experimenters were managing to push up theoperational temperature of superconducting materials from the highlyimpractical1.5K toamoremanageablemid-20sK.Wearestill talkingabout

very cold materials – this stuff is hovering around –250°C – but suchtemperaturesweremorepracticallyachievableoutsideaspecialistlab.Anyhopethatthegradualincreaseinworkingtemperatureswouldcontinueseemedtobedashed with the development of the basic theory of the mechanism behindsuperconductivity,asthatseemedtosuggestthatitwouldneverworkathighertemperatures than had already been achieved. However, themid-1980s saw asuddenrevolutionthat(asrevolutionsoftendo)threwawaytheoldcertainties.Within months of 30 K being reported as the absolute limit for

superconductivity, Paul Chu, Maw Kuen Wu and their team announced at ameetinginNewYorkinMarch1987thattheyhadproducedsuperconductivityattherelativelybalmytemperatureof90K.Ratherthanametal,theywereusinganewceramicmaterialthatcombinedbarium,copperandyttriumwithoxygen.When the90Kbreakthroughwas revealed itproducedahugeupheaval in thefield.Ithadprettywellbeenassumedthatthe30Klimitwastheendoftheroad,and that the potentially very lucrative (as well as fascinating) possibility of aroom-temperature superconductorwas nothingmore than a pipe dream.Now,though,withthe30Klimitsmashed,theyhadmadeahugeadvancetowardsatemperaturethatwouldnotrequirecryogenicstomaintainthesuperconductivity.

Thenewalchemists

What followedhasbeen likened toalchemyrather thanconventionalmaterialsscience.Inthemiddleages,beforechemistryemergedasadiscipline,alchemistswouldrepeatedlytryheatingandcoolingdifferentmixtures.Theyhadnomodelfor what was happening to these elements and compounds as the substancesinteracted. Instead, they simply tried to reach their goals of transmutation ofmetalsandtheelixiroflifebyhaphazardlytryingoutcombinationsandseeingwhat happened. In modern physics, the more likely approach is to try tounderstand what is happening first, then to enhance the process using thatunderstanding–butafterthe1987meeting,allsortsofmixesofmaterials,oftenincluding rare earth elements like yttrium, were tried just to see how theyreacted.Rareearths,ratherparadoxically,areelementsthatarenotparticularlyscarce;

it is just that themineralsinwhichtheywerefirstdetectedarerare.Whiletheuse of rare earth elements in the mix had a certain logic – the firstsuperconducting ceramic had been based on barium, copper, lanthanum andoxygen, but this had only reached the 30 K limit, and switching in yttriumseemed to be the key to the sudden leap – themany attempts to try different

combinations, ina race tobothNobeland financial success, feltmore like theworkofthosealchemiststhananormalpieceofscientificwork.Therewas one guiding principle behind the frantic switching of component

elements, though. It had already been observed that putting a material underintense pressure would increase the critical temperature at whichsuperconductivity began.While thiswasn’t a practical tool in producinghigh-temperature superconductors, it suggested that finding away to get the atomscloser in the structure of the material would enhance the interaction betweenphononsandelectronsandcouldbenefit thesubstance’sability togo‘super’–and one way to do this would be to incorporate bigger atoms in what wasotherwiseasmallerlatticestructure.

Ditchingthehelium

Within a year, critical temperatures of around 125Kwere being reported formaterials that substituted thallium, strontium and bismuth for barium orlanthanumintheoriginalmix.Therewerealsooccasionalflurriesofexcitementwithtemperaturesreachingallthewayuptothe250sK–effectivelytoroom(oratleastfreezer)temperature.Theseinitialobservationswerereportedonce,butthenwouldalwaysfizzleoutastheyfailedtobereproduced.Withthe90to125Kmaterialsanewplateauhadbeenreached–butitwasaverysignificantone.Although these were still not superconductors that operated at room

temperature,andsowerenotlikelytoenabletheproductionofsuperconductingpowerlinesorsuperconductingapplicationsinthehome,theydidhaveonehugebenefitover the traditionalsuperconductors that required temperaturesof30Kand lower. Liquid helium, the essential medium to get down to thosetemperatures, is expensive toproduce.Bycomparison, liquidnitrogen ismorethan100timescheaper–cheapenoughtobewidelyavailableinGPs’surgeries,for instance. And liquid nitrogen boils at around 77 K. So these newsuperconductorsneedonlyliquidnitrogentofunction–withtheaccompanyingbenefitofsignificantlylessheavy-dutycryogenicstorage.

Anewmechanism

In parallel with the attempts to produce high-temperature superconductingmaterials,therewasalsoaracetocomeupwithatheorytoexplainwhatseemedto be a very different process to theCooper pair superconductivity of a basiclow-temperature superconducting metal: a race that is still under way. It was

realisedquiteearlyon that theclassicyttrium–barium–copper–oxygenmaterial(YBa2Cu3O7)hadaparticularlystrangestructure.Unliketheregularlatticeofametal,thisceramicconsistedofaseriesofstackedcopper/oxygenplanes,linkedby chains of copper and oxygen with barium and yttrium atoms interlacedbetween the two. This structure produced physical oddities, like a differentresistance to electricity at room temperature dependingonwhether the currentwas passed along the planes or in a perpendicular direction, passing throughthem.It ispossible thatsomesortof tunnellingmechanismbetween the layerscouldenhancetheprogressofCooperpairs.Asyet,though,thereisnowidelyacceptedexplanationastohowthesehigh-

temperaturesuperconductorswork(thephysicistsworkinginthisfieldconsider90to125Ktobepositivelyhigh-temperature).Itisstillamajorareaofresearchforthoseworkingtounderstandthecomplexstructuresofthesematerialsandtogetatheorythatwillmakeitclearwhythesuperconductivityismaintained.Thebestbetatthemomentisthattheroleplayedbyphononsinaconventionallow-temperaturesuperconductoristakenoverbyfluctuationsofthespin–ineffectamagneticmechanism.Butthisisalongwayfrombeingcertain.

Ambientsupers

Whilethetheoristsworkawayatanexplanation,possibleobservationsofroom-temperature superconductivity keep cropping up. It may be that these are thesuperconductingequivalentofthecoldfusionfuroreinthe1980s,whenStanleyPonsandMartinFleischmannclaimedtohaveproducednuclearfusionatroomtemperatures and pressures, only to find that their experiment could not bereproduced–thoughitisequallypossiblethattherewillbeabreakthrough,justasthe30Kbarrierwasbreached.Agoodexampleofthepossibilitiesbeingexploredistheworkreportedfrom

TokaiUniversityinJapanin2013.Theresearcherswereusingamaterialknownas HOPG – highly oriented pyrolitic graphite. This is like the conventionalgraphite form of carbon, butwith extra bonding between the sheets, giving itunusualbehaviour.Pyrolyticcarbonisoneofthefewmaterialsthatcanlevitateabove a permanent magnet the way magnets do above superconductors, it isextremely thermallyconductivealong theplaneof thegraphite sheets,and it’sthemostmagneticmaterialatroomtemperature.IntheTokaiexperiment,whentwoplatesofHOPGweredippedintoamixof

two organic compounds, heptane and octane, the resistance of the samplesdroppedbelowmeasurablelevels.Theexperimentersalsokeptacurrentrunning

withoutdecayinginaring-shapedcontainerholdingthecompoundsfor50days.As the researchers put it: ‘These results suggest that room temperaturesuperconductor [sic]may be obtained by bringing alkanes into contactwith agraphitesurface.’It isveryearlydays,butsuchexperimentsholdouthopeforthefuture.

Metamaterialmadness

Other scientists are coming at room-temperature superconductors from adifferent angle, hoping that the same special materials that make invisibilitycloaks possible can also transform the resistance-free world. These are‘metamaterials’,whereresearchersplayaroundwiththewaysubstancesinteractwithlightorsoundorelectromagnetism.Invisibilitymetamaterials are usually thosewith a negative refractive index.

Refraction is theway light bends as it travels from onemedium to another –causing effects like a pencil appearing to bendwhen put in a glass of water.Negative refractive index means that the light bends in the opposite way tousual,makingitcapableofbendingaroundsomethingandconcealingit–hencetheinvisibilitycloak.Butmetamaterialshaveothertricksuptheirsleeve.Vera Smolyaninova of Towson University and Igor Smolyaninov of the

University ofMaryland realised that somemetamaterials have a property thatmakes them very interesting from the point of view of a theory ofsuperconductorsfirstderivedbytheRussianphysicistDavidKirzhnitsin1973.Thislinkstheabilityofelectronstosupportsuperconductivitytoapropertyofamaterialknownasitsdielectricresponse.Thelowerthedielectricresponse,thebetter theelectrons interact. Inprinciple,ametamaterialcouldbemadewithanegligible or even a negative dielectric response, and this intrigued theSmolyaninovs.The hope is that by producing a special metamaterial that includes both a

metal that acts as a superconductor at low temperatures, likemercury or lead,and impuritiesofadielectricmaterial (a substance that is an insulatorbut thatcanbepolarisedtohavedifferentchargesonoppositeends–theyhaveinmindstrontium titanate), itmightbepossible toencourage the formationofelectronpairsatmuchhighertemperaturesthaniscurrentlypossible.Itmightnotreachroom temperature (it might not work at all – this is still only theory), but itwould be a significant step on the way to designing a material that broughtsuperconductivitytotheeverydayworld.Inthemeantime,though,wehavehadsuperconductorsforjustover100years

and it would be surprising if they had not been used at all, even with therestrictions on keeping them down at cryogenic temperatures. Thoughsuperconductorsmaynothavetheubiquityofelectronics,theyhavestillstartedtoplayasignificantroleinourlives.

CHAPTER9

Floatingtrainsandwell-chilledSQUIDs

On10September2008,theworldhelditsbreathastheLargeHadronCollider(LHC)atCERNwasswitchedon.Aftersomeprophetsofdoomhadpredictedthatitwouldcreateminiatureblackholesorstrangematterthatcoulddestroytheuniverseasweknowit–whileoptimistsseemedtothinkwewouldimmediatelybeledtothemuch-sought-afterHiggsboson–therealitywassomethingofananti-climax.Theworldstayedinonepiece,andtestrunswere theorderof theday.Butninedaysafterfirststart-up,thingswenthorriblywrong.

Quenchingcatastrophe

Anelectricalfaultledtothereleaseoftheliquidheliumusedtocoolthemassivesuperconducting magnets that keep the LHC’s protons, travelling at near thespeed of light, on track. Suddenly the magnets dropped out of thesuperconducting phase. This shift, known as a ‘quench’, produced an intenseburst of heat as the immense currents were suddenly exposed to resistance,blastingtheremainingheliumgasoutwithexplosiveforce.Theresultwasthat50magnetsweredamagedandoverayear’sworkwouldbe required to fix it.Thiswasanaccidentwithsuperconductivityatitsheart.ThevastmagnetsoftheLHCarethebiggestsingleapplicationevermadeof

superconductors, but they represent only a small fraction of the ways thatsuperconductorshavecometobeused–someofthemwithmuchmorepotentialforanimpactoneverydaylives.It’s true that the early promise was probably overstated.When Onnes first

discoveredsuperconductors,hiscontemporarieshadvisionsofsuperconductinggrids that would carry vast currents around a countrywith no losses. But thecombinationoftheneedtokeepthecablesatverylowtemperaturesandtheway

that superconductivity is self-defeating when currents reach a certain level,producing strong enoughmagnetic fields to destroy the superconductivity, hasmeant that the applications, while important, have remained rather less thaneveryday.Unliketransistorsorlasers,wedon’thavesuperconductorsaroundthehouse(yet)–theyarerestrictedtospecialistapplications.Yetthoseapplicationsareveryimpressive.In order to produce the kind of magnets needed by the LHC, or for other

applications like the magnetic levitation trains we will see shortly, it wasnecessary to get around the way that the early superconductors lost theirsuperconductivity in the face of a high-strength magnetic field. Some alloysturnedouttohaveadifferentkindofsuperconductingbehaviour.These‘typeIIsuperconductors’have regions (knownasbundles)where themagnetic field isallowedtopenetratethematerial,andintheseregionsthematerialceasestobeasuperconductor – but the bundles are surrounded by matter in which thesuperconductivitypersists.Thebundlesarepinnedinplacebyimpuritiesinthematerial,preventingthemfrommoving,whichwouldcauseakindofelectricalresistance.Theresultisasuperconductorthatcancopewiththekindofcurrentrequiredformassiveindustrial-strengthmagnets.

Thequantummagneticbottle

It might seem that the LHC, as the biggest machine ever made, would alsoprovide themostdramaticchallenge forengineerswhowanted tomakeuseofsuperconductingmagnets,buttheLHC’sproblemspaleintoinsignificancewhenset alongside the requirements to build a tokomak. ‘Tokomak’ is a Russianacronymforanamethatroughlymeans‘toroidalchamberwithmagneticcoils’(there issomeargumentoverexactlywhat theoriginalphrasefor theacronymwas). It is a magnetic confinement reaction vessel for nuclear fusion. Inprinciple,nuclearfusion,thepowersourceoftheSun,wouldbeasuperbwaytogenerate energy, far better than any of our existing sources, but it’s not fornothingthatfusionpowerstationshavebeenpromisedforover60yearsandarestillagood40yearsintothefuture.It’sasupremelydifficultbusinesstocontainasmallparceloftheSunontheEarth.Thegoodnewsaboutfusionwhencomparedwithconventionalnuclearfission

powerstationsisthatitusesfuelthatismuchmoreeasilyobtainedandthatdoesnotproduceanyhigh-levelnuclearwaste.Buttheproblemisthat togetfusionoperatingyouhavetocontainaplasma–acollectionofions–attemperaturesof around 150million°C. This is ten times the hottest temperature in the Sun

itself,butthefusionreactorhasn’tgottheSun’simmensegravitationalpressuretohelptheprocessalong–itneedsallthatenergysimplytomakefusionoccur.The intenselyhotplasmacan’tbeallowedtocomeintocontactwith themetalvessel that contains it, or theplasma temperaturewould instantly collapse andthemetalwallswouldbeseriouslydamaged,sothechargedionshavetobeheldin place by a series of powerfulmagnets that surround the reaction chamber,which is usually in the formof a ring doughnut (a torus) that has aD-shapedcrosssection.Early tokomaksused conventional electromagnets tokeep a relatively small

amountofplasmainplace,buttogetthesortofmagneticfieldstrengthrequiredfor a production tokomak generator requires a whole set of superconductingmagnets.ITER,thenext-generationtokomakunderconstructionat themomentatCadaracheinthesouthofFrance,isnotfullscale,butisthelastgenerationoftest reactors before a production version is built, and it will havesuperconductingmagnets, aswill its successors. Superconductors are a centralcomponentofthishugelyimportantstepintheproductionoflow-carbonenergy.JustthinkofthechallengesthatwillbefacedbytheITERengineers.Notonly

havetheytocreateandmanageamonstrouslyhotmassofions,butitwillseemto have a life of its own. A plasma twists and turns as if it were alive in anattempt to escape themagnetic field. Simply keeping the reactor running is amajorchallenge.Butaswellashavingtoproduceanimmensetemperatureandkeep the plasma in check, the engineers have tomanage thiswrithing infernorightnext to superconductingmagnets that are cooledwithin a fewdegreesofabsolutezero.Asifitisn’thardenoughtomakefusionwork,theneedtokeepthosemagnets cool in the vicinity of such vast temperatures adds yet anotherchallengetothedesign.Andyetsuperconductingmagnetswillbeused,iffornootherreasonthanbecauseitisimpossibletoenvisagehowafullscalegeneratingreactorcoulddevelopastrongenoughmagneticfieldwithoutthem.Theyhavebecomeasimpleessentialinthefutureofelectricpowerstations.

Scanningwithachill

ITERisstill in the future,but inoneapplicationwitha triedand testedvalue,MRI scanners, superconductors are already in common use. More accuratelyknownasnuclearmagneticresonance,butrenamedmagneticresonanceimagingbecause of the negative associations of ‘nuclear’,MRI is a powerful medicalscanning technique that works doubly at the quantum level, relying on aquantumeffecttoproducepowerfulmagneticfieldsandusingasecondquantum

phenomenon to produce its images. The scanner requires extremely strongmagnetic fields, most often produced using superconducting magnets, like asmall-scaleversionofthemagneticconfinementinITER.The scanner works by manipulating the quantum spin of protons in the

nucleusof thehydrogen inwatermolecules,present throughout thebody.Thesubject tobescanned ispassed throughamagneticcoilwitha rapidlyvaryingmagneticfieldthathasafrequencytunedtoflip thespinof theprotons.Whenthis field is turned off, the protons flip back, producing radio frequencyelectromagnetic radiation from within the body – in effect, water moleculesbecometinytransmitters,detectedbythereceivercoils.Extra,varyingmagneticfieldsareused topinpoint the signal in threedimensionsandproduceacross-sectionalimageofthebodyasitpassesthroughthescanner.MRIscansareidealtodistinguishbetweennormalandabnormaltissue,detectingtumours.The main component of an MRI scanner is its magnet, usually cooled by

liquidhelium to4K(–269°C),producingasuperconductingeffect.Secondaryelectromagnets known as gradient coils vary themagnetic field by position toenablea3Dimagetobebuiltup.Thechangesinthefieldgradientresultinrapidexpansionsandcontractionsinthesecoils,producingloudhammering.Withoutappropriate soundproofing, the coils are as noisy as a jet takingoff, at around120decibels.AnMRIscanneristheapplicationofsuperconductivitythatisclosesttobeing

everyday.Stillperhapssomethingofararity,butcommonenoughformostofusto be aware of them.Up to now, the applications of superconductivity, whileimportant,havebeenlimitedtoresearchfacilities.Butthefeelingisthatnowisthe time when superconducting applications will really take off around theworld.Perhaps thebest known such application really involves takingoff – ifonlybya tinydistance– in themechanism that allowsamaglev train to floatabovetheground.

Thelevitationtrain

There is no doubt that railways have the potential to be the best transportmechanism available to us. Unlike flight, rail can make use of low-carbonelectricityasasourceofpower,itisverysafe,andisfarmoreefficientandlowinpollutionthanusingcarsorbuses.Becauseofitsseparateenvironmentitcanalsoruna lotquicker thananyotherformofgroundtransport.High-speedrailnowregularlyoperatesataround250kph(155mph),makingitcomparableontimingtoairtravelforend-to-endshorthauljourneys–becauseatraincantake

you straight to your destination with far fewer delays than with airportbureaucracy.Butconventionalrailisreachingthepracticallimitstowhichitcanbepushed–andthisiswheremaglevcomesin.Maglev is short formagnetic levitation.We’veall playedwithmagnets and

feltthealmostmagicalrepulsionwhenthesamepoles–northtonorth,orsouthtosouth–arebroughttogether.Balancethemagnetonasuitablestructureandthisrepulsionenablesittofloataboveasurface.Nowaddsomemechanismforpropulsion–typicallyalsobasedonmagnetism–andyouhaveadifferentkindof train,one thathasnofrictionfromitscontactwith therails,meaning itcanreach higher speeds, and that is far quieter than a traditional railed train.However,keepingmanytonnesoftrainoffthegroundisbeyondthecapabilityofanyconventionalmagnet–and this iswhere superconductorscome in (andone of the reasonswhy the search for high-temperature superconductors is soimportant).Therehavebeenanumberofexperimentalmaglevtrains,andat thetimeof

writing there are two short-run lines in operation, though as yet none isprovidingafullscalerailservice.Alreadyamaglevtrainhassmashedtheworldrail speed record: the Japanese experimentalMLX–01, using liquid helium toproduce superconducting magnets, reached 581 kph (361 mph). The firstcommercial maglev is now planned in Japan – the Chou Shinkansen, linkingTokyo,NagoyaandOsaka.It’snotanovernightdevelopment–itcouldwellbe2045beforeit’soperational–butwecanexpectspeedsofaround500kph(319mph).Thetrainisexpectedtofloat10centimetresabovethetrack,usinglargehelium-based superconducting magnets on board, which both levitate thecarriagesandprovideitspropulsion.

Gettingmaglevright

Althoughmaglevtakesawaythedisadvantagesoffriction, thetrainstillhas tofaceuptoairresistance,whichbecomesamajorissueover400kilometresperhour,wastingaround83percentof theenergyat this speed.And then there’snoise.Althoughthetracknoiseofaconventionaltrainismissing,atthisspeedthesheernoiseofforcingyourwaythroughtheairhasreachedanunacceptablelimitof90dB–theequivalentofadieseltruckaround10metresaway.Ifevenfasterspeedsaretobeachieved(andintheorymaglevtrainscouldreach1,000kph),thetrainwouldhavetorunthroughatunnelthathasatleastsomeoftheair removed. This might seem an extreme solution, but it has already beensuggestedasapossibleapproachforaSwissmetrosystem–undergroundlines

lendthemselvestothis‘vacuumtrain’approach.Wearen’t going to seemaglev trains takingoverour rail networks anytime

soon.ThenoisypoliticalwranglingintheUKintheearly2010soverbuildingaconventional high-speed line (HS2) shows how difficult it can be in somecountries to make a major change to infrastructure. But the Japanese havealreadydemonstratedthattheyarepreparedtobuilddedicatedhigh-speedlinesand will no doubt have a similar success with maglev, which will inevitablybecomemorewidely accepted over time as air travel becomes less acceptabledue toglobalwarming,particularly if room-temperaturesuperconductorscouldeverbediscovered.Althoughmaglevispotentiallyagreenmodeoftransport(dependingonhow

the electricity is generated and the coolant produced), it has resulted in oneenvironmentalconcern,particularlyinChina,wherethereareworriesaboutthedangers from ‘radiation’. There is often confusion about this, as most peopleassociate‘radiation’withthepotentiallydangerousoutputofnuclearreactions–high-energy particles and gamma rays. But electromagnetic radiation frompower lines and phone masts – the sources of most concern in our presentenvironment–isjustanotherformoflight,intheradiofrequencyratherthanintheformofdestructivelypowerfulgammarays.Whileit istruethatverycloseexposure toextremelystrongmagnetic fieldscanhavean impacton thebrain,passengers and passers-by do not experience anything close to this with amaglevtrainandthereseemsnoreasonforconcern,butitdoesreflectthatsuchabigchangeintechnologycansometimesbeahardsell.Although trains have seen the biggest investment in the application of

superconductivity to transport, there has been some work done already on asuperconductingelectricmotorforships.Current,diesel-basedshipenginesarehighlypollutingandsignificantcontributorstoCO2levels(theydoalsocounterglobalwarmingbecausethedirtyparticulatestheyemitblocksunlight,butthispollutionishardlydesirable),butexistingelectricmotorssimplycan’tbescaleduptothesizerequiredtopowerafull-sizedocean-goingship.Prototypemotorsusingmagnetsbasedonhigh-temperature(liquidnitrogen)superconductorshavealreadybeentestedandcouldbeincommercialusewithintenyears.

Josephson’squantumgeniusIt’snaturaltothinkofthesekindsofapplications,becausethemostfamiliarusesof superconductors like the LHC’s giant magnets and MRI scanners are bigmachines,undertaking their superconductingonagrandscale–butoneof themostflexibleusesofthisquantumphenomenoncomesinattheverysmallend

ofthescale in theformofaSQUID.This isnot themarineinvertebrate,butaSuperconducting Quantum Interference Device. To understand these, we firstneedtomeettheJosephsonjunction,aquantumdevicethatwasdevisedin1962by the then graduate student Brian Josephson, who would later win a NobelPrizeforhiswork.Josephsonisoneofthemoreunusualcharactersofthephysicsworld.Hehas

been reviled in later life bymany of his contemporaries, because he seems toexhibitawide-eyedacceptanceofmanyphenomena–likethememoryofwater,and telepathy – that other scientists regard as time-wasting fruitloopery. Yetthere isnodoubt that inhis twentiesJosephsonwasoneof thesharpestmindsworking in physics. A few years ago I went to meet him at CambridgeUniversity, where he had an honorary position at the Department of AppliedMathematics and Theoretical Physics that enabled him to work on his mind–matterunificationproject.While he came across as having some of the characteristics that we

traditionally associatewith amad scientist, or at least an eccentric one in theEinstein vein – wild hair, once he took off his cycle helmet, and a certainvaguenessofconversation–itwasinterestingthatwhenIlaterattendedalecturebyquantumentanglementexpertAntonZeilingerandsatnexttoJosephson,hewasstilltreatedwithgreatrespectbytheotherphysicistspresent.Back in the 1960s, though, Josephsonwas amuchmore intense and driven

character.Hewaswell known forpickingupon lecturers should theymake aslip.Asoneof them,PhilipAnderson,commented: ‘[HavingJosephson takeacourse]wasadisconcertingexperienceforalecturer,Icanassureyou,becauseeverythinghadtoberightorhewouldcomeupandexplainittomeafterclass.’Josephsonwasonly22whenhemade the initialdiscovery thatwouldbecometheJosephsonjunction,andhereceivedtheNobelPrizeat33.WhatimpressedallwhowitnessedJosephsonatworkinthoseearlydayswasthecompletenessofhisvision–thewaythathehadtakenexistingtheoryandbuiltonittogiveatotaldescriptionoftheimplicationsofconstructingaJosephsonjunction.AnexpertpatentlawyertoldAndersonthat‘inhisopinionJosephson’spaper

was so complete that no one else was ever going to be very successful inpatentinganysubstantialaspectoftheJosephsoneffect’.Andthiswassomeonewhowas still two years away from completing his PhD. From the physicists’viewpoint, Josephson’s work was probably most important because it gave afundamental insight into the nature of superconductivity, and in particular itclarifiedtheroleofthephaseoftheelectronpairsinasuperconductor.Butfromthe outside this gave new possibilities for practical applications ofsuperconductors.

So what is this effect that brings SQUIDs to life? A Josephson junctionconsistsofapairofsuperconductorswithabarrierbetweenthem,whichcanbeaninsulatororaconductor that isn’t inasuperconductingstate. Itwasalreadywidely understood how quantum particles could tunnel through a barrier (seepage 34) – but Josephson predicted that Cooper pairs of electrons could alsotunnelthroughthebarrierinsomecircumstances.Josephsonalsonotedthatwithan AC current, where the phase varies with time, the junction will act as anincredibly sensitive voltage measurement device, as the frequency will bedirectlylinkedtovoltage,andfrequencyismucheasiertomeasurethanvoltage.Josephson junctions turn up in a number of applications, from a range of

quantum computing mechanisms (see page 230) to very wide-spectrumequivalentsofthecharge-coupleddevicesusedindigitalcameras,whichmakesthem ideal for astronomy applications.But thewidest application of this purequantumeffecttodateistheSQUID,thesuperconductingquantuminterferencedevice.ThismakesuseofaJosephsonjunctiontodetectverysmallchangesinthemagnetic fieldaround theSQUID,asevena tiny inducedcurrent fromthechangingfieldwillhaveadetectableinfluenceonthejunction.At the moment SQUIDs are a little like lasers in the early days of their

developments. Itwasobvious fromearlyon that lasersought tobeuseful,buttherewasabiggapbetweenthespeculationandthereality–ittookawhileforthem to settledownand forus to really appreciatewhat they coulddo forus.ThesameistruetosomeextentforSQUIDs,thoughtheyarealreadystartingtomake a mark in areas like detecting neural activity from the tiny shifts inmagnetic fieldproducedby thebrain,and insome typesofMRIscanner.Onesurprisingareaofdevelopmentisthedetectionofunexplodedordnance,knownin the tradeasUXO(or ifyouarebeing formal, ‘munitionsandexplosivesofconcern’).Frighteningly,between10and15percentofbombsandshellsfailtoexplodeandareleftinthefieldasalong-termhazard.Just how long-term the problem is can be understood from the fact that

thousandsofUXOsfromtheSecondWorldWararestilldiscoveredinEuropeevery year.Back then, the level of unexplodedmaterielwaswell over 25 percent. Obviously there are also large residues on recent battlefields and ondisusedmilitary training grounds. In theUS alone it is estimated that there isover40,000squarekilometresoflandcontaminatedbyUXOs.Various methods have been used to detect these unwanted leftovers, from

basic metal detectors to complex magnetic field monitors, but nothing cancomparewiththesensitivityofdetectingchangesinmagneticfieldprovidedbyaSQUIDinnewdevicesthatmeasuretheexactgradientoftheEarth’smagneticfieldbelowthem,allowingthemtopickupthelocationandshapeofanomalous

objectswithunparalleledclarity.Becauseoftheextremesensitivity,thedetectordoesnotneedtobeasclosetotheUXOaswithaconventionalmagnetometer,so it is better for coping with heavy undergrowth and water. Using high-temperaturesuperconductors,theequipmentcanbemadesufficientlyportabletoscan for UXOs both under the ground and under water. At the moment thistechnology is just being tested, but it could soon be a common sight on oldbattlefieldsandtestinggrounds.It’s likely that we’ve only scratched the surface of the possible uses of

SQUIDs,andlikeallsuperconductingdevices, theyarelikelytobecomemuchmorecommonshouldweeverachievearoom-temperaturesuperconductor.Butone final application that is worth mentioning is the scanning SQUIDmicroscope. This systematicallymoves a SQUID over an area to be scanned,usingthevariationinthemagneticfielditdetectstobuildupapicture.Thiscanbeused,forexample,toscananintegratedcircuittocheckforshortcircuitsandto ensure that the circuit is acting as expected. The SQUID is not in directcontact with the item being scanned, so the sample can be kept at roomtemperatureandinair,ratherthanthecryogenictemperaturesandlowpressurerequiredfortheSQUIDitself,makingtheprocessnon-destructive.

Superconductingsewage

Findingsuperconductorsinpowerfulelectronicdevicesandscannersmaynotbetoo much of a surprise, but the last example of an application ofsuperconductivityisamillionmilesawayfromthedelicacyofSQUIDs.Itisinsewagetreatment.Weliveinaparadoxicalworldthatisawashwithwater–italmostdefinesourplanet–andyetatthesametimewherethereisashortageofclean drinking water. It shouldn’t be that way. The world contains around200,000,000,000litresofwaterforeverylivingperson.Ifyouthinkofthatintermsofconsumption,assumingatypical5litresaday,

thewaterout there should lastover100millionyears.And thatwouldbe if itwere all used up, whereas we know in practice that most of the water weconsumeisreleasedbackinto theenvironment inshortorder.Ofcourse that5litresrepresentsonlyourdirectconsumption.AtypicalWesternwater-userwillbe responsible for up to 10,000 litres a day. In part this is due to washing,wateringthegardenandflushingthetoilet,butalsobecauseoftheindirectuseintheproductionof thegoodswebuyand the foodsweeat. Justonehamburgertakesaround3,000 litres,whilea1kg jarofcoffee requiresamassive20,000litres. (Though once again, most of this water will be recycled – it doesn’t

remainintheproduct.)Theproblem,ofcourse,comesnotfrompooravailabilityofwaterperse,but

the lack of clean drinking water in the right place for those who need it.Arguably this makes any water shortage more of an energy problem thananything else – that’s the energy required to clean up thewater,whether it isdesalination or removing dirt and sewage, and to get thewater towhere it isneeded. And superconductivity can play its part in overcoming this. Mostexistingwastewatertreatment–whethercleaningupsewageorcleaningwaterfromarivertouseinanindustrialplant–isexpensivetobuildandhastobeonalargescaletobecost-effective.Therearemanycircumstanceswhereasmaller,distributed system would work better. Surprisingly, superconductors offer asolution to cleaning water that is bothmore cost-effective andmore compactthanaconventionaltreatmentplant.What’smore,itworksmorequicklytoo.Theprocessmakesuseofapowerfulsuperconductingmagnettoseparateoff

thesuspendedmaterialinthewater.Thisisobviouslyfineformagneticmetals,butitseemsanunlikelysolutionfortherest–thetypicalgunkthatweassociatewith sewage and polluted water. But by adding a substance known as aferromagneticadsorbenttothewater,thismessbecomesaccessibletomagneticfields. The suspended particles stick to the adsorbent material, which is thendraggedoutofthewaterbythemagnets, leavingcleanwaterbehind.Theonlywaytogetasufficientlystrongmagneticfieldistousesuperconductors.

Moretocome

Theseexamplesareonlythebeginningof thewaysthatsuperconductorscouldbe used in the future. A considerable amount of work has been done onsuperconductingcables.Aswehaveseen,all thewayback toOnnes therehasbeenarealisationthatoneofthebiggestlimitationsonanelectricitygridisitstransmission loss– theenergy that is lost to resistance in thecables.Althoughdesigns exist for superconducting cables than can carry ahigh currentwithoutthe magnetic field generated disrupting the superconductivity, they are veryexpensivecomparedtoaconventionalcableandreallyrequiresomethingclosertoaroom-temperaturesuperconductortobeviable.Still,itremainsasignificantareaofresearch,asdoessuperconductingenergy

storage. Clean energy production from wind or solar, for example, has thedisadvantage of not necessarily generating the power when it is needed. Butstoring large amounts of energy for any time until it is required is tricky.Probablythebestsolutionwehaveatthemomentistousetheenergytopump

waterupahilltoahigh-levelreservoir,thentousethatwatertogeneratehydro-electricity when the demand kicks in. But work is under way, particularly inJapan andKorea, to produce energy storage in the form of a superconductingcoil that retains the energy as a magnetic field until required. These aresignificantly more efficient than water storage, and very fast to store anddischarge,butasyetworkonlyonarelativelysmallscale.

Computingwithquanta

Superconductivitymaynotcropupinyourkitchenyet–thatwouldhavetowaitfor true room-temperature superconductors – but it is already playing asignificantpartinourlives.However,thereisanotherdecidedlycoolapplicationof quantum physics that could transform an everyday, essential piece oftechnology.Whetherit’susingasmartphonetosurfthenetorgetdirectionsinastrange town,or sittingatmydesktopwriting thisbook, computersplayabigpart inmy life, as they do formost of us. Even thosewhowouldn’t touch acomputer use technology from washing machines to cars and personal videorecorders that have computers built in.And being electronic, those computersare quantum devices. But waiting in the wings is the possibility of usingcomputersinawaythatputsquantumphysicsattheveryheartofthewaytheywork.Beforemeetingthecomputers,though,wehavetoestablishjustwhatputEinsteinintoatangle.

CHAPTER10

Spookyentanglement

Perhaps the most worrying aspect of quantum theory is that Albert Einsteinhated it. It’s not that Einstein couldn’tmakemistakes – he could and he did.However,whensomeonewithEinstein’svisionfundamentallydetestsatheoryitisnotanaversionweshouldtreatlightly.

Goddoesn’tplaydiceJohnBell,theNorthernIrishphysicistwhoplayedaleadingroleintheaspectofquantumphysicsweareabouttocover,oncecommented:‘IfeltthatEinstein’sintellectual superiority over Bohr, in this instance, was enormous; a vast gulfbetweenthemanwhosawclearlywhatwasneeded,andtheobscurantist.’Bell,who would make it possible to test whether Einstein was right, came downfirmlyonEinstein’sside.Aswehaveseen,Einsteinhadnoproblemwiththequantumnatureoflight–

hewas largely responsible for its discovery.But he refused to accept that thequantum events that underlie all of reality are based on randomness andprobability. He was sure that if you looked deeply enough you would find‘hidden variables’ that gave real, fixed values to the properties of quantumparticles, rather than the fuzzy probabilistic view of Bohr, Heisenberg,Schrödingerandfriends.ThisledEinsteintowritein1926tohisfriendMaxBorn,themanbehindthe

probability interpretation: ‘Quantum mechanics is certainly imposing. But aninnervoice tellsme that it is notyet the real thing.The theory says a lot, butdoesnotreallybringusanyclosertothesecretofthe“oldone”.I,atanyrate,amconvincedthatHeisnotplayingatdice.’ToseewhatEinsteinwasgettingat,imaginethatyoutossacoin,itlandson

thebackofyourhandandyoucoveritwithoutlookingatit–anormalcointoss.Atthispoint,assumingit’safaircoin,thereisa50percentchanceofgettingaheadand50percentatail.Butweknow,inreality,thatthecoinisshowingaparticularface.Wedon’thappentoknowwhichone,buttheinformationisthere

in the system, in a ‘hiddenvariable’– in this case, the coin.According to thequantum brigade, a quantum particle is totally different. Before looking, itgenuinelyisinasuperpositionofstates.Itdoesn’thaveeithervalue,‘heads’or‘tails’ (or whatever the possible values are), it just has two probabilities thatdeterminethelikelihoodofanyparticularoutcome.

EinsteinboresBohr

Although Einstein moaned in writing to Born – and rightly, given Born’sresponsibility for interpreting Schrödinger’s equation as representingprobabilities–hismainattackwasfocusedonNielsBohr,the‘ringleader’oftheincreasingly mainstream view of quantum theory (the generally acceptedinterpretationofthetheorywasevencalledthe‘Copenhageninterpretation’afterBohr’sDanishcentreofoperations).Einsteinsoonfoundawaytopresshiscase.Since1911,thegreatandthegoodofphysicshadmetupinconferencesknownas the Solvay Congresses. These were originally set up by the BelgianindustrialistErnestSolvaywiththehopeofhavinganintelligentaudienceforhisownideas,butSolvaywasquietlysidelined,leavingthebiggunstogetonwithasuperbscientificconference.Atboththe1927and1930Congresses,Einsteinbuttonholed Bohr and presented to him a series of thought experiments thatEinsteinhopeddemonstratedthefailingsofquantumtheory.Some of the challenges Bohr was able to dismiss straight away. He

commented on one: ‘I feelmyself in a very difficult position because I don’tunderstandwhatpreciselyisthepointwhichEinsteinwantsto[make].Nodoubtitismyfault.’Othershehadtoworkonthroughtheday,orinonedisquietingcaseovernight,whenhewasable topointoutoverbreakfast,withasatisfyingsense of irony, that Einstein was wrong because he had failed to take intoaccounttheinfluencethatgeneralrelativitypredictedgravitywouldhaveontheexperiment, causing time to run slowly, cancelling out the effect thatEinsteinhopedwouldchallengequantumtheory.

TheEPRparadox

Forfiveyearsafterthe1930Congress,Einsteinwasquietonthetopic,andBohrmight have hoped that the attacks had finished. But then in 1935 Einsteinpublished a paper that he believed set the cat among the quantum pigeons,finallyanddefinitivelydemonstratingaflawinquantumtheory.Theironywasthat,unknowntoEinstein,notonlywouldtheeffecthedescribedhelpvindicate

quantumphysics,itwouldprovetobeacentralpillarofmodernquantumtheoryandonethatwouldprovetohavedramaticpracticalapplications.The paper was clumsily titled ‘Can Quantum-Mechanical Description of

PhysicalRealityBeConsideredComplete?’,butitwouldbeuniversallyreferredto by the initials of its authors. Einsteinwas joined by two young physicists,BorisPodolskyandNathanRosen,andsothepaperbecameknownasEPR.EPRdescribes a thought experimentwhere a particle breaks into two equal

parts,whichflyoffinoppositedirections.Afterawhile,accordingtoquantumtheory, the particles don’t have definitive values for, say, theirmomentum orposition. Insteadeachmerelyhasa setofprobabilities,whichcollapse intoanactual value only when a measurement is taken. The thought experimentimaginesmakingameasurementononeparticlewhentheyarefarapart.Saywemeasureitsmomentum.Thenconservationofmomentumtellsusthattheotherparticlemust have exactly the samemomentum in the opposite direction.Yetuntil themeasurement wasmade, these values weren’t fixed. So how did thedistantparticlefindout,instantly,whatvalueitsmomentumshouldhave?Thepapersuggestedasimilarargumentcouldbemadeforposition.It’srather

unfortunatethatEPRusedboththesemeasurements,asitledtosomeconfusionthat this was a challenge to the Uncertainty Principle. In fact, the paperconsidered each measurement separately (and later versions of the thoughtexperimentusedadifferent,ofteneasiertohandleproperty,quantumspin).Butthewordingwasalittlemisleading.EinsteinwasnotverygoodatEnglishatthistime and relied on his collaborators for the wording.When questioned aboutwhythepaperusedthetwoconfusingproperties,EinsteintoldSchrödinger:‘IstmirWurst’,whichliterallymeans‘It’ssausagetome’,anidiomatictermfor‘Icouldn’tcareless’.

Nomorelocality

What the authors of EPR concluded was that either quantum theory wasincomplete – that therewere hidden values that simplyweren’t known about,rather than true probabilities – or that it wasn’t possible to assume that theuniversekeptthingslocalandreal.Ifquantumtheorywasright,therehadtobewhatEinsteincalled‘spookyactionatadistance’,theabilityfordistantparticlestosomehowinstantlycommunicatewitheachotherinapparentcontradictionofEinstein’srelativityanditsassumptionthatnothingcantravelfasterthanlight.The linkage between these particles is quantum entanglement. It is a

phenomenon that we will meet a number of times in the quantum world. In

Einstein’s eyes this prediction was a counter to the quantum theorists, butSchrödingerturnedthingsontheirheadwhenhecoinedthetermentanglement.Hesaid:‘Iwouldnotcall[entanglement]onebutratherthecharacteristictraitofquantummechanics,theonethatenforcesitsentiredeparturefromclassicallinesofthought.Bytheinteractionthetworepresentatives[thequantumstates]havebecomeentangled.’InitiallyEPRwasseenaslittlemorethananinterestingchallengetoquantum

orthodoxy, but in the 1960s John Bell proposed an indirect mechanism thatcoulddistinguishbetween trueentanglementand theworkofhiddenvariables,and by the 1980s, experimenters like the French physicist Alain Aspect hadshown thatEinsteinwaswrong.Entanglementwas real.And itwouldproveavaluabletoolinthenextgenerationofquantumdevicesthatarejuststartingtobeconceivableinthe21stcentury.

Theinstantaneouscommunicator

Thereissomethingenticingaboutentanglement.Assoonasanyonehearsaboutit,theycanimmediatelyseehowitcouldbeusefullyapplied.Makeachangetoone particle and it is instantly reflected in another at any distance.What youhave here is instantaneous communication. We already face problems fromcommunications delays. Phone calls over satellite links can be plagued withunnervinggapsintheconversation.Andthingsaregoingtogetfarworseaswetakeonlong-distancespacetravel.AradiosignalfromEarthtoMarscantake20minuteseachway,whileshouldweevermakeittothestars,ourneareststellarneighbour,ProximaCentauri,isagoodfouryearsawaybyradio.Admittedlythisisn’tsomethingthatislikelytobeanissueforquiteawhile,

buteventhosesmalllocaldelaysforcommunicationsaroundtheEarthcancausedifficultiesforelectronicsystemsandvoicecommunicationsalike.Andthereisalso the intriguing proposition that, technically speaking, an instantaneouscommunicationalsomakesitpossibletosendamessagebackwardsintime.Bycombiningtheabilitytosendamessageinstantlywithareceiverwheretimehasrunslowly–whichaccording tospecial relativityonly requires the receiver tomove at very high speeds – it should be possible to transfer amessage to thepast.However, shortly after the realisation that instant communication is a

possibility comes the let-down.Although entanglement genuinely does enableinformationtogetfromAtoBinstantly,thatinformationisrandomandoutsideour control. Say, for instance,we use the spin property of a pair of particles,

which in a particular circumstancemight have a 50:50 chance of being up ordownwhenmeasured.WemakeameasurementonparticleAanditturnsouttobedown.Instantly,particleBhasspinup.Butwehadnocontroloverwhichofthesuperposedvaluescameupwhenthemeasurementwasmade.ThefactthatBwasspinup tells thepeopleat thatdistant location thatA isspindown,butthat can’t carry any useful information because it merely describes a natural,randomoccurrence,notamessagewewantedtoconvey.Despitethisabsolutelimitation,theappealoftheinstantcommunicationisso

strong thatphysicists (usuallyyoungphysicistswith something toprove)haveoften attempted to find away around it.And somehave appeared to get veryclose.Back in the1980s,AmericanphysicistNickHerbertwasconvinced thathehadcrackedtheproblemwithadesignthatevenRichardFeynmancouldnotfind fault with. Herbert tried to make use of the property of photons calledpolarisation–thepropertythatisusedbyLCDdisplays(seepage72).

Apolarisedmessage

Polarisationcomesintwoforms,linearandcircular.Inthemorefamiliarlinearform,thepolarisationofthedifferentphotonsislinedupinthesame(oratleastsimilar) directions, while in the circular variety, the polarisation rotates withtime,soasthephotonstravelalong,thepolarisationdirectioncorkscrewsaroundthe direction ofmotion. Herbert’s idea was to start with an entangled pair ofphotons,thensendone,the‘local’photon,throughapolarisingfilter,choosingtomake it either linearlypolarisedor circularlypolarised.These two formsofpolarisation would correspond to 0 or 1, allowing the system to instantlycommunicateinbinary,asthepolarisationofthesecond,‘distant’photonwouldinstantlyreflectthelocalone,howeverfarawayitwas.Theproblemwiththisapproachisthat,givenasinglephoton,youcan’task,

‘Isitlinearlyorcircularlypolarised?’Itispossible,forinstance,tocheckifitispolarisedinaparticulardirectionandgeta‘yes’or‘no’answer,butnottomakethe required distinction. So Herbert planned that before the polarisation isappliedtothenearphoton,thedistantphotonwouldbesentthroughalasergaintube,adevicethatproducesmultiplecopiesofthephoton.Thenthedistantbeamof photons would be split in two, half going through a linear polarisationdetectorandhalf throughacircularpolarisationdetector.Herbertreasonedthatthiswouldmakeitpossibletodetecttheoutcome.Unfortunately there was a fault in his logic. Laser gain tubes don’t make

perfect copies of photons.Theyproducemultiple photons that are similar (for

instance in energy) to the original, but they aren’t identical in terms of theirquantumproperties. In fact theso-called ‘nocloning theorem’proves that it isnotpossibletocreateasecondphotonthatisabsolutelyidenticaltoanother.Thiscomplexbutthoroughquantumphysicstheoremshowsthattheclosestthatcanbeachievedistomakeaperfectcopywhiledestroyingtheoriginalinaprocesswewillsoonmeetinmoredetail,calledquantumteleportation.Butthisdoesn’tmakeitpossibleto‘breed’multiplecopies.Thelasergaintubelosesthewholepointoftheexerciseandthebeamsproduceddonotmakeitpossibletodeducewhathadhappenedtothesingle,nearphoton.

Quantumsecrecy

No one since Herbert has even come close to dreaming up ameans of usingentanglementforcommunication,anditishighlyunlikelythatanyoneeverwill.However,therandomnessthatgetsinthewayofsendinganinstantmessagehasproved a strength for a fruitful potential use of entanglement – in encryptingdata.Keepingmessagessecretisachallengethathasbeenfacedeversincewebegan to communicate. In the early days, concealmentwas themost commonapproach.Amessagemight bewritten on a tablet thatwas then coveredwithwax,onwhichaninnocentcovermessagewaswritten.Therewereevensecretnoteswrittenonmessengers’shavedheadsthatwerethenconcealedasthehairgrew–notexactlyidealforourmodern,high-speedworld.AsearlyasRomantimes,itbecameobviousthatsomethingmoreflexiblewas

needed, a way of communicating openly in which eavesdroppers could notunderstand themessage. Themost obviouswaywas simply to use a differentlanguage,whichcouldbeeffectiveaslongasthatlanguagewasunknowntoanyeavesdroppers. As recently as the Second World War, Navajo ‘code talkers’wereusedtosendUSmilitarymessagesinthePacifictheatreonthereasonableassumption that the Japanese enemy had little chance of interpreting thelanguage. Failing that, an artificial language could be constructed, and this iseffectively what a code is, where a word (which can be nonsense or plainEnglish)standsinforadifferentmeaning.Thankstothistotallackofconnectionwiththemeaning,codesprovideavery

strongwaytoconcealamessage,buttheyarealsoinflexible–ifyoudon’thaveacodeforaparticularwordormessage,youcan’tsendit.Codesalsodependonhavingacodebookateachendofthecommunicationline,whichisinevitablysusceptible tobeingcopied,allowing themessages tobe readbya thirdparty.For these reasons, the vast majority of coded messages are actually ciphers,

mechanisms that provide a rule to systematically change the letter values in amessage.Thesimplestcipheristheso-called‘Caesarcipher’,usedsinceRomantimes,

where the letters to be sent are simply shifted along the alphabet by a fixedquantity. So, for instance, if that quantity is 3 (apparently popular with theRomans),AbecomesD,BbecomesE,andsoon.Thisway,amessagelike‘starta bombardment’ becomes ‘vwduw d erpedugphqw’, or even better, ‘vwduwderpe dugph qw’,with the characters split into regular blocks so there are nocluestobegainedfromwordlength.Asimplecipherlikethisisrelativelyeasilybroken,especiallyonceyouknow

thatsomelettersappearmorefrequentlythanothersinanyparticularlanguage.With a frequency table it is possible to guess which substitutions have beenmadeandgraduallydecipherthemessage.Todayitwouldbeveryunusualtouseanythingother thanakey-basedcipher.Here, rather thanhavinga single shiftvalue,eachletterinthemessageisdisplacedthroughthealphabetbyadifferentamount.Asimplekeymightbejustawordthatisrepeatedly‘added’tothetext(adding on the position value of each letter of the key in turn). Others usecomplexmechanismstoproducethatkey.

Theunbreakablemessage

In principle, most key-based ciphers can be broken, as happened with thosegenerated by the German Enigma machines during the Second World War,becauseitisusuallypossible(ifverydifficult)toduplicatethemechanismusedtoprovidethekey.Butonetypeofcipherthathasbeenaroundsincetheearlyyearsofthe20thcenturyistotallyunbreakable.Thisiscalleda‘onetimepad’.Theideaisthatyouhaveanentirelyrandomkey–arandomsetofnumbersthatareaddedtothetexttoencryptit,thentakenawaytodecrypt.Thekeyisaslongasthemessage,soafterencryptionyouhavearandomsetofletterswithnowaytobeabletoguesshowtheencryptionwasdone.Evenifyoumanagedtobreakasmallpartofthemessage,itgivesnoclueonhowtobreaktherest.Thereisadownsidetotheapproach,though,whichissimilartotheproblem

ofsafelydeployingcodes.Touseaonetimepad,thoserandomvalueshavetobeavailabletobothsenderandreceiverofthemessage.Andhoweverthevaluesaretransmitted–whetherbyradioormoresecurelyasaphysicalobject,printedon paper or stored on a memory stick – they are vulnerable, liable to beintercepted. And once an eavesdropper has a copy of the random values, anymessageusingthiskeyiswideopentobeingread.

This is where quantum physics, and entanglement in particular, come in.Because there is true randomness at the heart of the behaviour of quantumparticles, it iseasyenough touse this togenerate theone timepad justbeforeuse–but there isstill the issueofgetting the informationfromoneend to theother, so the receiver can decrypt what the sender has encrypted. Quantumentanglementhasawayaroundthis.By basing the key on measurements made of entangled values, the key

literallydoesnotexistateitherendofthecommunicationuntilthemomentitisused.Itisneverstored,nevertransmitted.Imagine,forinstance,weareusingthequantumpropertyofspin,whichwillbeeitherupordownwhenmeasured,andallocate, say, ‘up’ as 1 and ‘down’ as 0.We now have a source for a key inbinaryform.With the rightentangledparticles, thespinhasa50:50chanceofbeing up or down when measured. So up until the sender makes thatmeasurementofasequenceofparticles,thekeydoesnotexist,butthemomentshedoes,thereceiverhastheappropriatevaluetodecryptthemessage.Theonlypotentialflawtothisapproachisthatathirdpartycouldinterceptthe

entangled particles immediately before the key is used, producing the key,noting it and passing it on.However, this can be detected, as it is possible tocheck if a particle is still entangled or not, and interception would break theentanglement. If this check is undertaken every few particles, the link can bekeptsecurewithoutariskofinterception.

BeammeupAnother impressiveuseofquantumentanglement is tobring to lifeoneof themost dramatic aspects of the Star Trek franchise – the transporter. Thissupposedlyscanstheparticles insomethingorsomeonetobetransmittedfromthe starshipEnterprise to the surface of a planet and reproduces them at thedestination.ItwasdoneintheTVshowprimarilytosavemoneyonexpensivemodelworkshowingshuttleslanding,butitmakesforaremarkableconcept–atleastasfarassciencefictiongoes.Therealityisfullofholes.One problemwas identified, in rather an unsubtleway, in the 1950smovie

The Fly, originally starring Vincent Price and remade in 1986 with JeffGoldblum in the role of the unfortunate scientist who invents a mattertransmitter.Aflygetsintothetransmissionchamberalongwiththetestsubjectand the result isahorriblehuman/flyhybrid.While thespecificsof themovieareveryunlikely–therewouldbefarmore(iflessgrotesque)problemswithalltheairmoleculesinthechamber–theunderlyingconceptthatitwouldbeverydifficulttosafelyscanandreproducealltheatomsinapersonismorethantrue.

In part this is because there are so many atoms in a human – around 7,000trillion trillion – that itwould take thousands of years to scan themall at anyconceivablerate.

Sendintheclones

Therewasoneother impossibility for thematter transmitter to face.Aswe’vealreadyseen, the ‘nocloning’ rulemeanswecan’tmakeacopyofaquantumparticle. And make no mistake, such a transporter is not really movingsomething from A to B: it is making a copy of the original from A at B.However, quantum entanglement stops this frombeing a problem.By using apairofentangledparticleswecantransferaparticle’squantumstatetoanotherparticle.Thisgetsaround thenocloning rulebecause the information isneverrevealed.Itgetsfromoneparticletotheotherwithoutbeingmeasured.Effectively what happens is that an entangled particle at the transmitter is

interactedwiththeparticletobetransmitted.Dependingontheoutcomeofthatinteraction, some information is sent conventionally to the receiver and thatinformation tells the receiver what to do to the second entangled particle toproduceafinalparticleinthesamestateastheoriginal.Aspartoftheprocessthe original particle has its quantum state scrambled – and anythingmade oftheseparticleswouldbedisintegrated.Thatwouldgive any scientistwho invented amatter transmitter like that in

TheFlypauseforthought.Whattheyhavereallyisnotamattertransmitterbutamatter duplicator that produces an exact copy at the remote location whiledestroying the original. If you passed through such a transmitter, as far aseveryonewasconcernedthepersonattheotherendwouldbeyou.Itwouldhaveyourmemories,yourthoughts,beperfectineveryphysicaldetail.Butthe‘you’thatsteppedintothetransmitterwouldbedust.Inpractice,becauseofthenear-impossibility(quantumphysicistslearnnever

tosay‘never’)ofscanningawholepersonandthenrecreatingthemattheotherend,thequantumteleportationprocessislimitedtoindividualquantumparticles.Butthishasprovedaboonforaspecialtypeofdevicethatneedstheabilitytotransferquantuminformationfromoneplacetoanotherwithouteverfindingoutwhat it is–anessentialbecausetakingameasurementwillchangetheparticleirretrievably. These are devices thatmake use of the quantum peculiarities ofsuperpositionandentanglementtodofarmorethananynormalcomputercouldachieveinalifetime.Thenextbigstepofthequantumagewillbetheintroductionofthequantum

computer.

CHAPTER11

Frombittoqubit

Computershavebecomecentraltooureverydaylives.Weareeducatedbythem,work on them, entertain ourselves with them and use smartphones tocommunicatewitheachother.InVictoriantimes‘computers’alreadyexisted–butwerenothingmorethanpeoplewhocomputed,individualswhoworkedwithnumbers.Thefirstpossibilitythatthetermcouldmeansomethingdifferentcamewith the work of Charles Babbage. In 1821, the 30-year-old Babbage waslaboriouslycheckinganewsetofastronomicaltables.

Computingbysteam

Babbage was a rich man who certainly didn’t need such mind-numbingemploymenttoearnmoney–hisworkonthetableswasafavourforhisfriend,John Herschel (son of the astronomer William Herschel, the discoverer ofUranus). Babbagewas clearly not impressed by themind-numbing job and issaidtohavecriedout:‘MyGod,Herschel!HowIwishthesecalculationscouldbeexecutedbysteam.’Whetherornot,aslegendhasit,thiswastheinspirationfor his masterwork, soon afterwards Babbage set out to design a mechanicalcalculator thatcouldreplacesuch tediousrepeatedcalculationseffortlesslyandwithouterror.Itwouldbeacomputerextraordinaire.HisDifferenceEnginewasagearedmechanismwheretheinputvalueswere

first set up on a series of dials and then a handle was repeatedly turned toliterallycrankouttheresults.BabbageonlyeverbuiltasmallpartofhisEngine(though a full-size working version has been constructed by the ScienceMuseum inLondon), as hismindwas already on greater things. The problemwith the Difference Engine, compared with a human computer, is that themachinewas limited toundertaking the simplearithmeticoperations thatwerefixedbyitsgearing.Brainsenabledhumancomputerstobemuchmoreflexible,and Babbage wanted to avoid having the instructions hard-coded in themechanism.HeturnedawayfromtheDifferenceEngine.Thegovernmentwasnot amused. They had given him £17,000 – the equivalent of between £1.2

millionand£13milliontoday,basedonsimplecashvalueorthelabourvalueofthe money respectively. Babbage’s political masters expected something inreturn,butheonlyevergavethemapart-finishedmachine.The notion that distractedBabbage fromwhat no doubtwould have been a

veryusefulmachinewas the ideaofusinga seriesof flexible instructions, fedinto hismechanical computer using cardswith patterns of holes punched intothem.Suchcardswerealready inuse in theFrenchJacquard loom,whichhadrevolutionised the silk weaving industry by specifying patterns as a series ofholesinachainoffabric-linkedcards.InBabbage’smind,thesamemechanismthatcontrolledtheloomcouldalsocontrolacomputingengine,providingboththedataandtheinstructionsonwhattodowiththatdata.Thismachinecould,inprinciple,undertakepracticallyanycalculation.The Analytical Engine, Babbage’s great vision for a true mechanical

computer,wasneverbuilt. Itwasprobably impossible todo so facedwith theengineering limitations of the day; but Babbage did think through therequirements,developing, for instance, the ideaofhavingseparatepartsof themachine to act as a memory and for processing the data. He could have gotfurther,atleastonpaper,ifhehadtakenmorenoticeoftheworkofthewomanwhoisoftendescribedasthefirstcomputerprogrammer,aslightlyoddportrayalgiventhatsheneversawacomputer.ThewomaninquestionwasAugustaAdaKing,CountessofLovelace.Born

AdaByron,daughterof theRomanticpoet, shehadknownBabbage sinceherteens, and might well have married him had her mother not set her heart onmarryingoffherdaughtertonobility.AdaKing(orAdaLovelaceasshetendstobe known) translated a paper on the Analytical Engine written originally inFrench by the Italian scientist Luigi Menabrea. Rather than just work on thedocument,sheappendedlongnotesonhowtheEnginemightbeused.Itwouldbe exaggerating to say, as is often claimed, that shewrote programs, but shecertainlygavethoughttohowshewouldgoaboutit,andprobablywouldhavedonesohadnotBabbagecoldlyturneddownheroffersofhelp.Asitwas, theAnalytical Engine never saw the light of day, and nor did the opportunity toprogramit.

Thetruefatherofcomputing

Babbage is often, if inaccurately, called the father of computing, which isarguablythefaultofWinstonChurchill.ThankstoChurchill’sparanoiawehaveonly relatively recently become aware of just howmuch theWest owed to a

youngmancalledAlanTuring.Babbagecouldatbestbecalledthegrandfatherofcomputing,asTuringwasindubitablyitsfather.Babbage’sworkwasadeadendintheevolutionofcomputing,withnorealtechnicallinktothecomputingindustry that would eventually develop, but Turing set the entire direction itwould take. Thanks to Churchill’s insistence that Turing’s work on code-breakingatBletchleyParkduringtheSecondWorldWarbeheavilysuppressed–primarily in thehope that theUK’spost-warenemieswouldnot realisehowmuchcode-breakingwasachieved–ithastakenalongtimeforallthepiecestobeputinplacethatshowjusthowmuchweowetoTuring.Alan Turing has become something of a mythical character. He certainly

proved brilliant as a code cracker and again in his analysis of the nature ofcomputing,layingthefoundationforallmoderndigitalcomputersinawaythatstill isn’t always recognised. Part of the reason for his mythical status is theoften-repeatedsuggestion,startedathis inquest, thathisdeathat theageof41wassuicide,asaresultofthepersecutionhereceivedasahomosexualwhenthiswasstillillegalintheUK.ItiscertainlytruethatTuringsufferedacruelandunnecessarypunishmentfor

his ‘crime’ by undergoing a session of ‘chemical castration’ – treatmentwithfemale hormones, which was offered as an alternative to prison. (Turing waspardoned by the UK government in 2013.) It has been suggested that thistreatment so devastated Turing that he committed suicide by eating an applelacedwith cyanide. In reality, all the evidencewas that Turing had recoveredfullyfromthetreatmentandwashappywhenhedied.Hehadalaboratorynextto his bedroom in which he had been running a chemical experiment that isthoughttohaveproducedthecyanidefumesthatkilledhim–toallappearancesanunfortunateaccident, rather thansuicide.Buthoweverhedied, therecanbenodoubtinghislegacy.

Theultimatelimit

Turingshowedthepowerofcomputers inhisworkatBletchleyandexplainedtheirworkingswith his development of a hypothetical ‘universal computer’ –but hewas also the first to realise their limitations.We are used to computermanufacturers bringing out more powerful, faster computers every year. Thegrowthincomputingpowerofaprocessorhasbeendescribedformanyyearsby‘Moore’s Law’, an observationmade by one of the founders of Intel,GordonMoore,backin1965thatthenumberoftransistorsonachip(aroughmeasureofthecomputer’spower)seemedtodoubleeveryyear.Thiswaslatermodifiedto

doubling every two years. And ever since then,Moore’s prediction has comeclosetoreality.Butthisuncheckedgrowthcan’tgoonforever.At some point we will hit physical limitations, where the increasing

miniaturisationof thecircuitsonachipmeanthat it isreducedtodealingwithindividualquantumparticlesandthereisnofurthertogo.What’smore,downatthat level, quantum effects like tunnelling can cause serious problems for theoperation of the chip. But even more significantly, Alan Turing showed thatthereisafundamentallimittothecapabilityofsoftware,whateverthehardwarehappenstobe–hencehisuniversalcomputerdesign.Turingshowedthattherearesomeproblemsthatcanneverbesolvedbyacomputerandmanymorethatwould take longer than the lifetime of the universe to crack. For example, itproved impossible towrite aprogram thatwoulddecide if anyotherprogram,fed into itas input,wouldcometoastoporkeeprunningforever.AfamiliarexampleofaproblemthatwouldtakefartoolongtocalculateisworkingoutthebestroutefromAtoBontheroadnetwork.Whenasatnavcomputerdoesthis,ithastouseapproximation,unabletoreachadefiniteconclusion.Turing’s imaginary universal computer consisted of a paper tape on which

zeroes and ones could bewritten, read or erased, but it is easier to think of aconventionalelectroniccomputerasaseriesofswitches,eachwithtwopossiblepositions, represented by 0 and 1. All that any computer does, from Turing’sconceptualversiontoyourlaptoporsmartphone,isspenditstimeflippingtheseswitchesbetween1and0,under thecontrolofaseriesof instructions thatarethemselvesstoredasonesandzeroes.Onacomputerchipholdingmemory,eachtinycomponentpartofthecircuitistheequivalentofapairofelectronicdevices– a capacitor and a transistor. The capacitor is the memory itself, with anelectricalchargerepresentingthebit,andthetransistoractsasaswitchtoenablethestateofthecapacitortobereadorchanged.

Quantatotherescue

Although, as we have seen, any type of electronics is a quantum device, thelimitations envisaged by Turing face a new challenge in the form of a trulyquantumcomputer,whichtakesthingstothenextlevel.Insteadofbeingbasedonthe0or1valueofanindividualbit,storedasawholecollectionofelectronsmakingup an electrical charge, aquantumcomputeruses the stateof a singlequantum particle, known as a qubit (pronounced cue-bit), to hold its value.Because this is a quantum particle, it can be in a superposition of states.Typically, for instance, youmight use the spin property of a particle to store

information.As this can be in a superposed statewith probabilities of havingeithervaluewhenmeasured,insteadofhavingabitthatcanbeeither0or1,wegetanexpansionofthecapabilityofthecomputer.At its simplest this becomesobvious as you add extra bits to the computer.

Imagineathree-bittraditionalmemory.Thiscanstoreasinglenumberbetween0 and 7. But three qubits can store all eight numbers simultaneously insuperposition. What’s more, in a sense a qubit can store an infinitely longdecimal. This is because a superposition doesn’t have to be 50:50. If youimaginetheprobabilitiesofbeingupanddowncorrespondingtothedirectionofan arrow between up (100% up) and down (100% down), thenwith the rightrelationshipofprobabilities,thedirectionofthatarrowcouldbeusedtospecifyanynumberbetween0and1.Aqubitis,ineffect,analogueratherthandigital,holdingasmoothlyvaryingquantityratherthananincrementalvalue.Itgivesacomputergreatlyexpandedcapabilities.Atleastintheory.Thatprovisocomesinbecausedealingwithqubitsisnoteasy.Superpositions

tendtocollapse,anditisdifficulttomakeuseofthevalues,becauseobservingaquantum particle changes its state. Butmanage to get it right and a quantumcomputer promises to be able to perform operations that would take aconventional computer the lifetime of the universe to undertake. And, as weshall see, we even have some of the programs that will perform otherwiseimpossibletasksalreadywritten,atleastinconcept.It’s easy to see why working with quantum computers is so fiddly if you

comparewhathappensifyoucheckthevalueofatraditionalbitwithreadingaqubit.Atraditionalbit,dependingonthechargeitholds,willeithercomeup0or 1. And unless you change its value it will continue to do so indefinitely.Simpleandstraightforward.But imagineaqubit that is storedas the spinofaquantumparticle.Remember,thisisapropertyoftheparticlethatwon’thaveanactual value until measured, just a collection of probabilities. The quantumcomputermighttreatthevalueas0.4,becausethequbithasa40percentchanceofbeingspinupanda60percentchanceofbeingspindown.Butifwe,outsidethecomputer,takeameasurementofthespin,wewillstillonlygeteitherup(0,say)ordown(1).Wewillget0fortytimesoutofahundredand1sixtytimesoutofahundred.Butwecan’ttellthatbysimplytakingasinglemeasurement.The qubit works in analogue but reads out in digital, which can be hugelyfrustrating.

Vapourwarealgorithms

There is a striking parallel between those thinking about the ways we couldprogramaquantumcomputerandAdaKingponderingonhowtouseBabbage’snon-existentAnalytical Engine. Just asKingwas able to consider the kind ofprocesses theEnginewould use, so some scientists have thought about howaquantumprogramwould run.Even thoughwedon’t yet have large and stableenough quantum computers to do much more than was achieved in abreakthrough in 2001,when a quantum computer calculated the factors of 15correctly as 5 and 3, there are already algorithms – essentially logicalrepresentationsofprograms–that,givenaworkingcomputer,wouldenableustocracksomeotherwiseinsolubleproblems.Theapproachthatwasusedtoworkoutthefactorsof15isbasedonahugely

powerful quantum computing algorithm produced by Peter Shor at AT&T.Datingbackallthewayto1994,itisawayforaquantumcomputertoworkoutthefactors,thenumbersthataremultipliedtogethertoproducealargernumber.It can do this with numbers that would take the best conventional computersthousands of years towork out. Strangely enough, although this is potentiallyvery useful and is one of the simpler quantum computing algorithms, it issomething that most computer scientists had hoped would never be madepossible.Theexpertsareconcernedbecausethedifficultyofworkingoutthesefactors

liesbehind theencryptionused tokeepour transactionson the internetsecure.Wheneveryouseealittlepadlocksymbolinyourwebbrowser–typically,forinstance,when you are entering credit card details – it is using an encryptionmechanismcalledRSA.Thisisaso-calledpublickey/privatekeysystem.Itisamethodofencodingdatawheretherearetwokeys–thevaluesusedconcealthedata.One,thepublickey,isusedtoencryptthemessage.Thiscanbegivenouttoanyone.Butwithout theprivatekeyit is impossible todecrypt themessage.Andthisiskeptby,say,thebank.Thismeansanyonecanencryptthemessagetheysendtothebank,butonlythebankcanreadtheresult.This mechanism was devised by three computer scientists at MIT in 1977

(RonaldRivest,AdiShamirandLeonardAdleman,henceRSA).Theyweren’tthe first. The approach was first developed by the British scientist CliffordCocks in 1974, but Cocks was working at the UK government’s intelligencecommunications centre GCHQ, and his discovery was kept secret until afterRSAwasreleased.(ShadesofChurchill’sdeadhand.)Theprivatekeyisbasedontwoverylargeprimenumbers,whicharemultipliedtogethertoproducetheevenvasternumberthatformspartofthepublickey.Withoutknowingthetwoprime numbers it is impossible to decode themessage. Finding out the primefactors of such a huge number would take far too long using a conventional

computer, so the mechanism remains secure. But Shor’s algorithm breaksthrough this problem and makes it possible to produce the factors with arelatively modest quantum computer – hence the concern of the computingcommunity.

InsearchofaquantumneedleBreakingsecurewebsitesisn’t,ofcourse, theonlyuseofShor’salgorithm–itcanbeappliedtoawiderangeofcomputingproblems–butthatnaggingabilitymakes it of serious concern. Another powerful way to program quantumcomputershasalreadybeenestablished inGrover’s searchalgorithm.This toohas been around awhile, datingback to 1996.WhatLovGrover realisedwasthataquantumcomputerhadahugeadvantageoveraconventionalonewhenitcametosiftingthroughadatabasewithoutanindex–somuchsothatthepaperGrovercontributedtothestaidjournalPhysicsReviewLettersonthesubjectwasentitled‘QuantumMechanicsHelpsinSearchingforaNeedleinaHaystack’.Although computers are very fast at finding information, they use clever

techniques to make this happen. The simplest of these is an index. So, forinstance, if you imagine a database of customers for a company, it will useindexes tomake it possible toget to the informationquicklyby, say,nameorcustomer number. But it may well not have an index of their driving licencenumbers (assuming your database included these) – so if therewere amillionpeopleinthedatabase,youmighthavetolookthrough999,999ofthemtofindthecorrectentry.Thatwouldbeunlucky–butonaverageyou’dexpecttohaveto lookathalfamillionrecords.Withaquantumcomputer, theGroversearchalgorithmcouldguaranteefindingtheentryusingjust1,000searchesbecauseitsquantumalgorithmworkswiththesquarerootofthenumberofentries.Itmightseemthatthere’snoneedforthis,asGoogleandothersearchengines

seemcapableofsearchingvastquantitiesofdataintheblinkofaneye–buttheymanage this by constantly building indexes, a process that consumesincreasinglyvastamountsofenergyandstorage.Withsuchahugequantityofinformation,a truequantumsearchenginecouldtransformthesearchindustry.What’s more, because it works on probabilities, rather than looking at eachindividual item, theGrover algorithm can domore human-like searchingwithvague,fuzzycriteria.Grover gives the example of finding someone’s phone number.Maybe it’s

someoneyoumettheotherday.YoucanrememberthathisfirstnameisJohn,andthathehasacommonsurname,butnotexactlywhatthatsurnamewas.Sayyouthinkhe’saSmithwitha50percentprobability,Joneswith30percentand

Millerwith20percent.YoualsorememberthathesaidhecouldseetheTowerofLondonfromhisflat,andthatthelastthreedigitsofhisphonenumberarethesameas the lastdigitsofyourdoctor’snumber.With just these sortsof fuzzyinformation, a typical starting point when attacking unstructured real-worldrequirements,Grover’snewalgorithmenablesaquantumcomputer tohomeinon the right result vastly quicker than anything possible with a conventionalsearch.

Firstcatchyourcomputer

Thesetwoapplications–fastsearch,andfactoringlargenumbers–arebetweenthem the current bighopes for quantumcomputing, though theremaywell bemany other uses for it that have yet to be devised.According toLovGrover:‘Not everyone agrees with this, but I believe there are many more quantumalgorithms waiting to be discovered.’ As yet, the development of thesealgorithmsremainssignificantlymoreadvanced than thecomputers theycouldrunon.Although,aswewilldiscoverlater,thereisacommercialcomputernowavailablethatiscalledaquantumdevice,itisn’tcapableofmakinguseofthesefull-scalequantumcomputeralgorithms.Therearedozensofteamsaroundtheworldworkingonquantumcomputers,

andsomehavegotahandfulofqubitsbrieflyoperating,butthesearestillverymuchbreadboarduniversity rigs,often involvingcryogeniccooling–certainlynotthekindofthingyoucanbuydownatPCWorld.JustasBabbagestruggledto overcome the basic engineering principles that held back his computingengines, because his designs pushed tolerances to the limits, so real-worldquantumcomputerscomeupagainstthetighttolerancesofquantummechanics,alwaysindangerofcollapseofsuperpositionandlossofentanglement.

Insidetheengine

To understand the difficulties quantum computer engineers face, we need todiscover a little more about how computers actually work. We are used tointeracting with a pretty graphical interface, but way underneath that, theprocessorischurningaway,manipulatingwholeseriesofzeroesandones.Thereareonlyreallythreeprocessesthatmakeeverythingfromawordprocessortoasuper-realisticvideogamepossible.Thevalueinabitmightberead,itmightbecopied or it might be changed. Those processes are undertaken using devicescalledgates,whichcanchoosewhetherornottochangeabitdependentonthe

valueofanotherbit.Gates are simple devices that are the physical representation of a rule. In

principle there are all sorts ofways they can embody a rule. Typically in thecomputersweusetheyareelectronic,withtransistorsprovidingthecontrol,buttheycouldbemechanical– Ihadaworkingmodelofacomputerwhen Iwasyoungthatworkedonasetofphysicalgates–orthegatescouldberepresentedbyasetofactionsonTuring’spapertape.Wehavealreadycomeacrossgatesinelectronicformwhenlookingatthedevelopmentofthetransistor(seepage65).Quantum computers also have gates, but they tend to be significantlymore

complexthantheirtraditionalequivalents.Inpartthisisbecauseofthatfactoflife in the quantum world, the ‘no cloning theorem’. The good news is thatquantum entanglement gives a partial workaround. The process of quantumteleportation(seepage222)meansthatitispossibletogenerateanexactcopyofaquantumparticleifthestateoftheoriginalparticleislostintheprocess.Entanglement makes copying possible by transferring the properties to the

newparticlewithouteverfindingoutwhattheyare.Butevenso,quantumgateshaveamuchtrickierjobthanconventionalones.So,forinstance,thenearestthatthequantumworldhastoaNOTgateisanXgate.TheNOTgatewemetbeforesimplyturns0into1and1into0.TheXgatetakesaqubitwithaprobabilityAofhavingthevalue0andaprobabilityBofhavingthevalue1andproducesaqubitwithaprobabilityBofhavingthevalue0andaprobabilityAofhavingthevalue1.Afterpassingthroughthegate,theprobabilitiesareswapped.

Jugglingquanta

Muchof the logicrequired tooperatequantumcomputers,and thestructureoftheunderlyinggates,hasbeenestablishedsincethelastcentury–thedifficultbitis getting the computer to work. The problem is not in the software but thehardware, and specificallywith handling quantum particles. Say, for instance,youwanted to use photons as your qubits. It’s easy enough tomake photons.Turn on a traditional light bulb and you are pumping out around 100 billionbillionphotonsasecond–plentytobegoingonwith.Butthatisn’tparticularlyhelpfulifyouwanttoworkonasingleparticle.It isn’t impossible to perform computationswith awhole batch of quantum

particles–infactthisishowthefirstactualdemonstrationoftheShoralgorithmworked. This particular experiment, undertaken in 2001 at IBM’s AlmadenResearchCenter by IsaacChuang andhis team,madeuseofmolecules ratherthan photons. Rather than attempt to handle a single molecule, they took a

billion billion of their bespoke seven-atom fluorine/carbonmolecules, each ofwhicheffectivelyactedasa7-qubitdevice.Theyusedradiofrequencypulsestoinfluence the nuclear spins of the atoms and an NMR (nuclear magneticresonance)device,likeanMRIscanner,todetecttheoutcome.Thecleverpartwas that thesewerespeciallydesignedmolecules,structured

so that the atoms within them would influence neighbouring atoms. So, forinstance,oneatommightundergoaflipofspindirectiononlyif thenextatomalongwasalready in the samestate,making themact likegates.Having largequantitiesoftheatomsmeantboththatitwaspossibletodetecttheresultswiththe NMR device and that the inevitable errors would be drowned out by themajority of correct values. The experimental computer, as we have seen,managedtodeducethatthefactorsof15were3and5–notexactlychallenging,butdemonstratingShor’salgorithminpractice.However,thiskindofapproach,usingtheatomswithinamoleculeasqubits

andaveragingacrossacloudofmolecules,worksonlywithasmallnumberofqubits,soitisstilllikelythataseriousworkingquantumcomputerwouldhaveto resort to handling individual quantum particles, andmost of thework nowgoing on in this field is based on this assumption. There are mechanisms toproducesinglephotons,whileatoms(orat least thechargedversionsthathavegained or lost electrons, ions) have been manipulated individually since the1980s.

Pinningdowntheparticles

Photons have some significant advantages as qubits because they are easy tomakeandarestable,andthismeansthatthegatesrequiredtosetupaquantumcomputerareoftenassimpleasacombinationofbeamsplitters(seepage115).At theendof thecalculation itshouldbeeasy toreadoff theresultsaswell–photon detection technology is highly advanced. But the downside of usingphotonsasyourqubitsisthattheydon’teasilyinteractwitheachotherandtheywon’tkeepstill,althoughtheycanbetrappedinasmallspaceusingamirroredcontainer,orsloweddownusingmaterialslikeBose–Einsteincondensates.Atoms,ormore likely thechargedversion, ions,makemore tractablequbits

thatstayinplaceandinteracteasily,thoughthegatemechanismsbecomemorecomplex than they are for photons. Ion traps are among the most popularapproachesusedbyquantumcomputingexperimenters.OthersarelookingatthepossibilityofcomputingwithSQUIDs(seepage200).Perhapsthemostobviousapproach,though,isusingquantumdots,solid-statetrapsthatcanholdasingle

electron. This approach is less messy than many experimental quantumcomputingrigs–moreliketheeverydayenvironmentofacommercialcomputer–butlikeeveryotherquantumcomputer,adotbaseddevicefacestheproblemofdecoherence.

Isolatedfromtheworld

To make a quantum computer work, the qubits have to be isolated from theworldaroundthem,interactingonlywithgatesandotherqubits.Inpracticeitisalmostimpossibletoisolateaquantumparticleentirelyanditsooninteractswiththematter around it, producing decoherence, the process bywhich it loses itssuperposedstateandceasestohaveanyvalueinthecomputer.Thisisdoublyaproblemwithentangledparticles.AsXiasongMa,workingintheentanglementcapital of the world, the University of Vienna, commented: ‘Entanglement ishardtoprepare,hardtomaintainandhardtomanipulate.’Stretchingthetimebeforedecoherenceoccurs–inearlyquantumcomputers

thiswasmeasuredinmillionthsofasecond–isanessentialifthedevicesaretobeused forany real-world task.Quantumcomputersmaybe fast,but theyareliable to be employed on heavy-duty tasks that take more than a fewmilliseconds tocompute.Onepossibleapproach toovercomingdecoherence isthe ‘hotpotato’mechanism,whereanyparticularqubit isusedonly foraveryshort space of time before its properties are passed on to a different quantumparticleusingteleportation.Butthemoreacomputerisscaledup,thebiggerthedecoherenceproblemsbecome.Keepingthosequbitsfreshseemstobesomethingthatwearegettingbetterat,

though.In2013,ateamworkingatSimonFraserUniversityinCanadamanagedto keep a collection of qubits in a superposed state for around three hours attemperaturesnear to absolute zeroor35minutes at room temperature (thoughthe process still had to be startedwith the qubits cooled to cryogenic levels).Thisisaround100timeslongerthanhadeverbeenachievedbefore.Thequbitswereformedfromthespinsofphosphorusionsheldinachipofextremelypuresilicon.However,beforegetting tooexcitedabout thisdevelopment it isworthnoting that it involvedaround10billion ions,all in thesamespinstate,whichwasn’tmodified–whichmakesituselessaspartofarealquantumcomputer.Itisn’tatrivialsteptogofromthistohavingtheionsindifferentstatesandabletointeractwithoutdecoherence.

Alongroad

There is something to be said for the thoughts of Richard Hughes of the USNationalLaboratoryatLosAlamos,whodescribestheeffortstodateas‘stillinthevacuumtubeeraofquantumcomputing’.Aswehaveseen,intheearlydaysof traditional electronic computing, computerswere built using these tubes orvalves. It simplywould not have been possible to have scaled up the 18,000-valveENIACtohavethesamecapabilitiesasthemulti-million-transistorchipsweusetoday.Theanswerwasn’ttoconstructabettervalve,itwastodevelopawholenewtechnologythatdoesthesamejobbutinaverydifferentway,andthesamemaybethecaseintheracetoscaleupthequantumcomputertoastable,workablenumberofqubits.Onepositiveexampleofthekindofsidewaysstepnecessarytobringquantum

computersintothepracticalworldwasmadein2013.Theentanglementprocess,whichenablesthequbitstointeractandprovidethescalingthatmakesquantumcomputing so powerful, was shifted from complex experimental rigs usingdifficult-to-handle quantum particles to a solid-state chip. A team at theUniversity of Queensland were the first to entangle two locations on a chip.Ratherthanatoms,electronsorphotons,theexperimentused0.2mmaluminiumstructures – massive in quantum terms – as artificial atoms that could act asqubits and that were entangled, linked via a superconducting microwavewaveguide, a metal tube that acts on microwaves like a fibre-optic cable forvisiblelight.

Theharmonyofdiscord

Most researchers believe that it is only a matter of time before the bugs areironedout,butavoidingdecoherenceandkeepingqubitsentangledpresentssuchdifficultiesthatafewhavesuggesteditwon’teverbepossibletocreateausefulquantumcomputerthisway.However,hopehasarrivedfromastrangedirection.Itseemsthatitmaybepossibletomakeuseofthemessinessofacollectionofless-than-pristinequbitsandstilldocomputation,employingsomethingknowninthetradeasdiscord.Thepossibilitiesfirstemergedin2001whenitturnedoutthat theapparentlysuccessfulquantumcomputer thatdiscovered the factorsof15usingShor’squantumalgorithmwasactually flawed. Itcouldn’thavebeensafe from decoherence. The quantum computer was operating at roomtemperature,and theentanglementbetween thecomputer’ssevenqubitswouldhavecollapsedlongbeforearesultcouldbeobtained.Yetitworked.In a traditional quantum computer, a quantumgatemight take two ormore

pristine, entangledqubits as inputand the resultwouldbe readoff afterwards.

Butitwasdiscoveredthatputtingthroughthegateonetraditional,cleanlysetupqubit,carefullyprotectedfrominteractionwithitsenvironment,andonequbitinamore‘normal’messystate thathasbeensubject tomeasurement,wouldalsoenableacomputationtotakeplace.Thequbitscouldn’tbeentangled,butthereseemed to be enough of an interaction to allow the quantum calculation toproceed. ‘Discord’ is a measure of how much a system is influenced byobserving it.A traditional classical system has zero discord, but any quantumsystem in a superposedor entangled state has a positive discord, and it seemsthatdiscordcangiveadegreeofcorrelation,akindofpseudoentanglementthatisn’tsosusceptibletocollapse,linkingtogetheramixofpurequbitsandmessyones.Theoutputofadiscordantcomputerisalittleunnervingtothoseusedtothe

precision of traditional computing. Because of the messiness involved, theresultsarenotexactbuthavetobeaveragedacrossanumberofruns–butifthisisdone,theresultseemstobereliable.Whatwehaveinadiscordantcomputerisstillaquantumdevice.Itdoesrequireatleastonepurequbitthatisprotectedfrom decoherence, and though the rest of the qubits are in normal classicalstates,discorditselfisaquantumeffect.Thewholeprocesscollapsesandfailstoworkifthatonepurequbitisallowedtogomessy.Butupuntilthen,itisalmostasiftheadditionofnoiseanddisorderinthemessinessoftherestofthequbitsmakes for a better, more stable quantum computer than one that is carefullyprotected from its environment – a decidedly hopeful thought for anyoneattempting to build a commercial model and struggling with the menace ofdecoherence.Atthemomenttheapproachisoflimiteduse,becauseweonlyhavethemaths

tobeabletomakeuseofverysimpleset-upswithdiscordlinkages.Asyet,theexperimentalphysicistsarewaitingforthetheoreticianstocatchup.Butthereisa lotofpromise there,anddiscord isbeing takenseriously.2012saw the firstdiscordconferenceinSingapore,withover70researchersattending.Thishybridapproach to quantum computing certainly has legs.Which is probably a goodpointtointroduceD-Wave.

D-WavingbutnotdrowningBackin2007,theCanadiancompanyD-WaveSystemsunveiledwhatitclaimedtobeaquantumcomputer.Noteveryoneagrees.Thismightseemcrazy–eithersomethingisorisn’tusingquantumeffects.Inreality,thereisnodoubtthattheD-Wavecomputer isquantumtechnology,butwhat isuncertain iswhether theparticular approach taken, which is very different from conventional quantum

computing, will give the kind of scaling benefits that make all the effortworthwhile.D-Waveisan‘adiabaticquantumcomputer’.Thismeansthatratherthan have the sort of quantum logic gates we have discussed, you have acollectionofqubitssetuptouseaprocesscalledquantumannealing.Quantumannealingmeansthatthequbitswilltrytoreachtheirlowestenergy

state,and it requires thecalculation tobesetup insuchaway that the lowestenergystateshoulddelivertheanswerrequired.It’sabitlikesearchingforthelowest point in a landscape by first taking an overview that finds the lowestcollection of points and then homes in on the lowest point within that area.Quantumtunnellingallowsakindofrandomwalkthroughpeaksofenergythatwouldactasbarrierstofindlowerandlowerenergystatesuntiltheminimumisdiscovered.Thereisadangerwithanyalgorithmforfindingaminimuminthiswaythatitcouldsettleonapointthatisn’ttheactualabsoluteminimum,whichissomethingthosewritingalgorithmsforadiabaticquantumcomputershavetobeawareof,butinprincipleitdoesprovideamechanismforfindingasolution.Anexperimentalversionofsuchacomputerachievedanearlytriumphback

in 2006 by smashing the ‘factoring 15’ trick by providing a solution to thefactors of 143, using only four qubits. This isn’t using the speedy Shoralgorithm,butratheraspecial‘adiabatic’algorithmthatencouragesthesystemtonaturallyproducethefactorsofthenumber.AndwhileitisclearthatShorismuch,muchfasterthananyconventionalmeansofproducingfactors,ithasnotbeen demonstrated that the adiabatic algorithm is any quicker. The claim hasbeenmadethataD-Wavecomputercansolvesomeproblems‘asmuchas3,600times faster than particular software packages running on digital computers’.Thisistrue,butitinvolvedcomparingaspeciallytunedalgorithmdesignedforaspecificpurpose runningona$10millionD-Wavewithoff-the-shelf softwarerunningonaPC.Thisisn’tclearproofofanadvantage.ThelatestversionofD-Wavewith503qubits,boughtbyGoogleandinstalled

atNASA’sAmesResearchCenter,iscertainlyalotmoresophisticatedthantheearlytestmachine–anditlooksmorelikeacommercialcomputer(thoughthinkmoreofasupercomputerthanalaptop)–buttherearestillsimilardoubtsaboutwhethertheadiabaticprocesswilldeliverrealquantumcomputingbenefits.Todate, the best success from the D-Wave approach has been in producing analgorithmtorecogniseimages.ThisisobviouslyvaluabletoasearchenginelikeGoogle,butagainthereisnotyetgoodevidencethatD-Wavecanachievethisfasterthananequivalentconventionalmachine.Thejuryisout,butonethingiscertain:D-Wave isnotageneral-purposequantumcomputer,but ratheraveryspecialistdevicethatcanrunonlycertainlimitedalgorithms.

It’sgoodtotalkGettingquantumcomputerstoworkisonething–achievingaquantumversionof the internet is another. Being able to establish entanglement between twodistant points is already of value, because it would make it easy to useentanglement-basedquantumencryption(seepage220)betweenthoselocations.Forthisreason,aconsiderableamountofworkhasbeendoneonachievinganentanglement link across the equivalent distance of a link between an Earthstation and a satellite, initially beaming entangled photons from building tobuilding inVienna. (Thiswasharder than it sounds: the owners of oneof theofficeblockshad tobekindenough toagree to temporarily replace theirglassafteritwasdiscoveredthatthespecialheat-reflectingwindowstotallyruinedtheentangledsignal.)AftersomepreliminarytestworkusinganexperimentontheInternational Space Station, the first experimental satellite that will beamentangledparticles to twowell-separated locationson theEarth– theessentialforentangledquantumkeydistributionthisway–waslaunchedbytheJapanesein2014.There is also the challenge of achieving a similar distribution of entangled

particles through optical cables, either for encryption or so that quantumcomputerscanbelinkedatthequbitlevelandsharecomputations.Aparticularlysparklingideaforawaytomakethishappenwaspublishedin2013fromworkatDelftUniversityofTechnologyintheNetherlands.Thenovelconceptwastousediamonds.Intheexperiment,entangledqubitswereheldintwodiamonds3metresapart

fromeachother,thoughobviouslythiswasintendedasaproofofconceptforalarger-scaleseparation.Thereasonthisisofinterest,whenremoteentanglementhasalreadybeendemonstratedinmoreconventionalqubitsliketrappedions,isthat it is thought that connectingmanyqubits in diamondsmaywell bemucheasierthantakingothersystemsupinscale,becausethestructureofthediamondcrystalwasdiscoveredtohaveaparticularlyvaluablebehaviour.The qubits in theDutch diamonds are based on impurities in the crystal.A

pure diamond is a perfect lattice of carbon atoms, but by combining nitrogenatomimpuritieswithgapsinthelattice,aqubitcanbeformedbasedonthespinstateofelectronsheld inagap.Asyet this isaninefficientprocess,producingonlyoneentanglementin10millionattempts,butitisexpectedthatthiscanbemadesignificantlymoreefficient.Thebigadvantage,though,thatdiamondhasover ion traps is that the qubits are relatively stable at room temperature. Bycontrast, ion trapshave tobeheavily cooled.The entangleddiamonds cangetawaywiththehighertemperaturebecausethecrystallatticeshieldsthequbitsin

its midst from potential sources of decoherence. What’s more, it has alreadybeen demonstrated that qubits, which inevitably decay, can be passed aroundnearby gaps in a diamond, keeping them stable for seconds rather than themicroseconds typical of a qubit – and diamond has the potential to be morescaleable than many other qubit technologies. It’s early days, but diamond isdefinitelyonetowatch,anditmayendupbeingaquantumcomputerscientist’sbestfriend.Althoughitisn’tessential,mostquantumcomputingatthemomentrelieson

the use of cryogenic environments to keep the qubits relatively stable. Beingsupercool isn’t just about electrical and magnetic effects, though. It cantransformthepropertiesoffluidstoo.

CHAPTER12

It’salive!

Imaginehavingaringofliquidsittingonatable.Yougiveitaquickstirtogetitrotating.Anditgoeson.Andon.Forever.Justascurrentswillflowindefinitelyinasuperconductor,asuperfluidexperiencesnoviscosityorfriction.Ithasnointernalstickiness;thereisnothingtostopitmoving.Fillacupwithsuperfluidliquidheliumandthisbizarresubstancewillclimbupthewallsofthecupofitsownaccord,rollovertheedgeanddripout.

Aspecialliquid

JustasHeikeKamerlinghOnneswasthefirsttoobservesuperconductivity,hisachievements in thesupercoolalsomeant thathefirstobservedasuperfluid inaction–thoughhedidnotdiscoverthestrangebehavioursmentionedabove.Asheliumwasbeingcooled towards thecritical temperature, the tinyobservationwindowsinhisreactionvesselenabledOnnestoseetheliquidboilingaway.Infact the cooling processmade use of this boiling, as it involved removing thehelium vapour from immediately above the liquid, taking away the fastest-movingatomsandleavingtheslower,cooleronesbehind.As the temperature fell below 2.17 K, the appearance of the liquid was

transformed. The seething of the boiling process abruptly stopped, leaving acalm, level surface. Clearly some kind of phase change had occurred. Whathappened(thoughOnnesdidnotrealisethis)wasthatsomeoftheliquidheliumhadbecomeasuperfluid,withnoviscosityandno thermal resistance.Bubblesform in a boiling liquid because it is not entirely uniform. There are hotspotswheremoreof the liquid turns tovapour,formingabubble.But thesuperfluidwassuchagoodconductorthatittransferredanybuild-upofheatawaybeforeabubblecouldform,leavingacalm,untroubledsurface.Onnesnotedthiswithouttakingitanyfurther,whileanunderstandingofwhat

washappeninghad towait until the 1930s,whenPyotrKapitsa inRussia and

John Allen with Don Misener in Cambridge discovered the full reality ofsuperfluidity. Kapitsa had begun his lowtemperature work at the MondLaboratoryinCambridge,wherehedevelopedanewmechanismforproducinglargequantitiesofliquidheliumshortlybeforereturningtoRussia(withsomeofhis equipment) in 1934 to set up the Institute for Physical Problems.Here, in1937,hewoulddiscoversuperfluidityincollaborationwiththeotherpairbackattheMond.

Escapologistbosons

Superfluidsareescapeartists.Giventheslightestaperture,theirlackofviscositymeansthatthenaturalmovementofmoleculesisenoughtoenablethemtoworktheirwayoutovertime.Ineffect,despitebeingliquids,theyactasiftheyweregases.Moredramatically,asuperfluidheldinanopencontainerlikeacupwillformathinfilmupthesurface,overthelipanddowntheoutside,meaningthatover time itwilldrainoutof thecup,apparentlydefyinggravity.Theeffect iscausedbyaverysimilarphenomenontosuperconductivity.Inasuperconductor,electronspairupandproduceoverlappingwavefunctions through thematerial.In a superfluid, larger bosons get a linked wavefunction to form a type ofsubstanceknownasaBose–Einsteincondensate.It’s worth making a brief detour to explain this ‘boson’ word, as it has

frequentlybeenmisusedinthemedia(typicallywhenreferringtoanotherboson,theHiggsboson).AbosonisatypeofparticlenamedaftertheIndianphysicistSatyendraBose, so it is pronouncedboze-on, notbosun, asmanynewsreadersmangledit.Allquantumparticlesareeitherbosonsorfermions,referringtothekind of mathematics that describes their behaviour. Bosons follow Bose–Einstein statistics while fermions behave according to Fermi–Dirac statistics.The two types of particles have different spin values – half-integer values forfermionsandintegervaluesforbosons.Inbehaviour,bosonsarehappytocollecttogetherinlargequantitiesinexactlythesamestate,whilenotwofermionscanbe in the same quantum state in the same system, with a combinedwavefunction.This isknownas thePauliexclusionprincipleand isoneof thefundamentalrulesofquantummechanics.For basic particles, ‘matter’ particles tend to be fermions and the force-

carryingparticles(likephotons)arebosons.Butcombinedparticles,likeatoms,can be either, depending how the spin values add up. Superconductivity andsuperfluidityaredependentontheparticlesbehavingasbosons,whichiswhyitisnecessaryfortheelectronsinasuperconductortopairup–theirhalf-integer

spinscombinetomakeanintegerspin:thepairbecomesaboson.Wecanseethesignificanceofthiswiththetwotypesofhelium.‘Standard’ helium – helium-4 – has two protons and two neutrons in its

nucleus.Thisevennumbermakesitaboson,soitiscapableofgettingintothespecialstatewhere itbecomesasuperfluidrelativelyeasily.But the isotopeofheliumwithjustoneneutron–helium-3–isafermionwithanoddnumberofparticles.Itisonlywhenhelium-3atomspairuptoformabosonthatitbecomesasuperfluid,whichoccursatasignificantlylowertemperature.

Morethananoddity

Foralongtimeitseemedthatsuperfluidswerenothingmorethananodditythatmade for impressive lab demonstrations, but recently they have been used inspecialist gyroscopes to measure very precise variations in gravity and as a‘quantumsolvent’ thatmakesothermatterclumptogetherof itsownaccord,aphenomenonthatisveryusefulinspectroscopy.Spectroscopymakesuseoftheinteractionbetweenmatterand light todiscover thecompositionofamaterial,and is often performed on a gas.The peculiar physical nature of the quantumsolvent means that dissolved particles act as if they were gaseous, sospectroscopicanalysiscanbeperformedonmaterialsthatarehardtostudyinagaseousform.Thewaythatasuperfluidactsasifitwereagascanalsobeusedtoprovidea

kindofsuper-refrigerator.Fridgesrelyonaphysicaleffectwherealiquidforcedthroughanarrowaperturenaturallycausescooling,becauseitflash-evaporates,taking energy out of the liquid and reducing its temperature. Superfluids areparticularlyeffectiveatthisprocessandwereused,forexample,intheInfraredAstronomical Satellite, an infrared space telescope launched in 1983, whichemployed supercooled helium in such a refrigerator-like system to keep itssystemsextremelycold,avoidingvibration.Butoneveryspecialapplicationofasuperfluidstandsoutaboveallothers–

because this is a substance that can capture the most elusive naturalphenomenon.Somesuperfluidscantraplight.

Slowglass

Imagineaspecialwindow,whichittooklightayeartotravelthrough.Setitupin front of a beautiful view for a year – then you could place it in a houseanywhere and you would have that view for the next twelve months. You

wouldn’t be looking at aTVpictureofwhatwasout there, but the real view,simplyseeingthelightasitcamethroughthewindowtwelvemonthslater.It’sscience fiction (BobShawwrote an excellent book calledOtherTimes,OtherEyesbasedonthisconceptof‘slowglass’)–butitisalsostrangelyclosetothetruthofaspecialkindofsuperfluid,awaytouseaBose–Einsteincondensatetoinfluencethespeedoflightitself.We are used to the speed of light being referred to as a universal constant.

Nothing,wearetold,cantravelfasterthanlight.Andthisistrue–butitisalsoan unacknowledged shorthand.Whatwe really should say is that nothing cantravelfasterthanthespeedoflightinavacuum.Thisis299,792,458metrespersecond(exactlyanddefinitivelybecausethemetreisdefinedas1/299792458thof the distance light travels in a second), and it is indeed a universal limit.(Forgetting the oddity of quantum entanglement.) But send light through amedium like air or glass and it slows down. It’s easiest to see why at thequantumlevelofindividualphotons.We tend to think of a transparent substance letting light pass through

unaffected, but in fact those photons are irresistibly drawn to interact withelectrons,boostingtheelectronenergyanddisappearing.Soonafter,theelectronwillre-emitaphotonanditcontinuesinthisconstantdanceofQEDthroughthematerial. But that process inevitably slows the photon down. In glass, forexample,lighttravelsatabouttwothirdsofitsspeedinavacuum.Thisslowing,incidentally, leads to the strange blue glow that comes out of the liquid thatsurroundssomekindsofnuclearreactor.Thereactorisspittingoutelectronsthatzapthroughthewateratahigherspeedthanthespeedoflightinwater,andtheresultisakindofopticalsonicboom,calledČerenkovradiation.Soinprinciplewecoulduseapieceofglasstomakethatspecialwindow.The

onlyproblemisthattwothirdsofthespeedoflightinavacuumisstillveryfast.A piece of glass holding a year’s worth of light would be 5,000,000,000,000kilometres thick.But a superfluid candoawhole lotbetter.At the endof the1990s, a Danish scientist working in the Rowland Institute for Science atHarvardUniversity,LeneVerstergaadHau,usedaBose–Einsteincondensatetoslowlighttoawalkingpace.

Insidethecondensate

IntheHarvardexperiment,sodiumatomswerecooleduntiltheyformedaBose–Einstein condensate. Usually a condensate would be opaque, but Hau’s teamused a laser to blast a path through the material. This laser modified the

condensatesothatphotonsinasecondlaserbeam,followinginitspath,becameentangledwiththeparticlesofthecondensate,resultinginthesephotonspassingthroughthematerialatamuch-reducedspeed.Theearlyresultsgotthespeedoflightdowntoaround17metrespersecond,butwithfurtherworkitwasreducedtowellunderametrepersecond.However,thiswasnotthefinaltrickthatthissuperfluidhadupitssleeve.In 2001, Hau’s team were experimenting with the effect of reducing the

intensityoftheinitialbeamthatproducesthepath,calledthecouplinglaser.Ifthe beam strength is gradually reduced to nothing, something remarkablehappens to thesecondbeam. Itneveremerges. It’snot that ithas simplybeenabsorbed,though,likelightenteringablackcavity.Becausewhenthecouplinglaser is restarted, the second beam flows out of the condensate. Thematerialmanages to trap light inside it, producing a kind of mix of matter and lightknownasa‘darkstate’.Thedarkstatewouldlaterbedevelopedinawaythatcouldbeveryvaluable

in the future of quantum computers. In 2006, Hau’s team managed to get aphoton-basedqubit to be absorbed into a condensate, then releasedwhen laterrequired, leaving the qubit unaltered.What seems to be happening is that thequbitistransferredtoawaveinthecondensate,whichthenrecreatestheopticalqubit. Because the condensate has a single wavefunction, the sodium atomsbehave coherently and don’t lose the quantum information. In the experiment,the qubitwas passed to a second condensate a very short distance away (160micrometres) andwas recovered from that second, totally separate condensateafterthequbitpassedfromonetotheotherintheformofaphysicalwaveinthematter.

Ajumbleofphotons

Thisisallveryremarkable,thoughitisn’ttrulyslowglass.Theproblemisthatlight travelling indifferentdirectionswould takedifferent times toget throughtheglass.Infactthisisalwaysthecase,butusuallytheeffectissosmallthatitcanbeignored.Imagine,though,youhadapieceofglassacentimetrethickthatlighttookayeartogothrough.Adiagonalpaththroughtheglassmighttake1.4yearstogetthrough.Sotheimageseenontheothersideofthewindowwouldbe a jumbleofphotons that didn’tmake any sense.Whatwouldbeneeded tomaketrueslowglasswouldbeacombinationoftheslowtransittimeandsomekind of holographic technique that would allow thewhole image at the outersurfacetopassthroughasasingleunit.

However, thislimitationwouldnotbeaproblemifyouonlywantedlighttopass through, rather than an image. Imagine, for instance, youhang aplate ofslowglasswitha‘timedepth’ofabouttwelvehoursafewmetresabovearoad.Duringtheday,thelightwouldgraduallypassthroughtheglass.Asitgotdark,that light would start to pour out, illuminating everything below, not withartificial light but with actual sunlight, just a little delayed in arriving. Itwouldn’tmatterthattheviewwasscrambled,becauseyouaren’ttryingtolookthroughawindow, justmakinguseof the light it emits. Itwouldbe aperfectartificiallightthatdidn’tevenneedelectricalpowertokeepitgoing.In practice this is still science fiction. Hau’s Bose–Einstein condensate

required expensive cooling near to absolute zero – far more expensive thanrunninganystreetlamp–andworkedonlywithlaserlight.Buttheprincipleofaslow glass that it demonstrates is still a fascinating opportunity to think ofpossible applications for the future, should such a room-temperature materialeverbemade.

Feeltheforce

In 2013, the interaction between a Bose–Einstein condensate and light onceagain entered the realms of science fiction, but this time a remarkable newphysicalbehaviourwaslikenedtoalightsaber.Thepresshadafieldday:‘StarWars lightsabers finally invented’; ‘Scientists Finally Invent Real, WorkingLightsabers’; ‘MIT, Harvard scientists accidentally create real-life lightsaber’.Tobefair,oneofthescientistsbehindthediscovery,ProfessorMikhailLukinofHarvard,fuelledthefirebycommenting,‘It’snotaninaptanalogytocomparethis to light sabers’–andHarvardnodoubtappreciatedall thepublicity–buttherealitywasverydifferentfromtheheadlinefrenzy.In some ways a better comparison, had the media been as familiar with

quantumphysicsasyounoware,wouldhavebeentosaythatthiswasanopticalequivalent of a Cooper pair (see page 183), the linked electrons that makesuperconductivitywork,becauseLukinandMITprofessorVladanVuletichadproduced light ‘molecules’, pairs of photons that were linked together. Not alightsaberinsight,thoughIadmititdoesn’tmakesuchexcitingPR.Thepairingofphotonsisabigdeal,notbecauseyoucoulduseit tomakea

lightsaber (youcouldn’tevencomeclose),butbecausegenerallyspeakingoneof themost frustratingaspectsof trying touse light incomputing,particularlyquantum computers, is that photons are solitary creatures. They ignore eachother,passing throughotherphotonsas if theyweren’t there. Ineveryday life,

this is agood thing. Just think for amoment aboutwhat’sgoingon in the airrightinfrontofyournose.Itisthreadedwithbillionsofphotonsheadinginalldirections.There’sthevisiblelightthatisenablingyoutosee(andtoreadthesewords). Radio frequency light being used by radio, TV, phones, Wi-Fi,Bluetooth andmore. Infrared fromyour radiators. It’s a sea of inter-threadinglight.Now imagine that these photons did interact. All hell would break loose,

optically speaking. In the massive collection of collisions between all thephotons youwould lose the ability to see, and all our radio-based technologywouldnolongerwork.Notagreatpicture.Butinacontrolledway,inaquantumcomputer,orindeedanycomputerusingphotonsratherthanelectrons,itwouldbeveryvaluableifwecouldgetphotonstointeractwitheachother,becausewedon’t want qubits to be totally isolated. There has to be some interaction toenablethecomputertowork.

Switchinglight

There has already been one step made in this direction. Also in 2013, anMIT/Harvard/ViennaUniversityofTechnologycollaborationwithmanyof thesameparticipantsas the ‘lightsaber’experimentproduceda ‘light transistor’,aswitch that allows a single photon to decide whether light is transmitted orreflectedbythedevice.Itconsistedofapairofmirrorswithagasofsupercoolcaesiumatomsbetweenthem.Themirrorswerecarefullypositionedtocreatearesonant cavity that produced a quantum effectmeaning that light could passstraightthroughthemirrors;butfireasinglephotonintothecaesiumgasandthequantumstatechangedsufficientlytopreventalmostallofthelightfrompassingthrough.Thisisexcitingforthoseworkingonquantumcomputersbecauseitisjusta

single photon,with all the quantum abilities of superposition of states, that iscausing the effect. What we have here is a kind of real, small-scaleSchrödinger’scatwherethequantumpeculiaritiesofthephotonaretransferredto the light beam it is controlling.Although the initial experimentwas only aproof of concept – a real computerwould need a solid-state equivalent of thesupercooled gas – it is a valuable contribution to the concept of opticalcomputing.Butprobablynotassignificantastheopticalmolecule.

ManningtheRydbergblockades

So what was going on in that ‘lightsaber’ experiment? When one or moreelectronsinanatomhavebeenpusheduptoveryhighenergylevels,theexcitedatom isknownas aRydbergatom, and it influencesnearbyatoms,preventingthemfrombecomingexcitedtothesamestate.Iftwophotonsarepumpedintothe medium used in the experiment, the first sets up one of these ‘Rydbergblockades’ and the second is held up until the first photon hasmoved furtherthrough the medium – the two photons become linked together, pushing andpullingateachotherastheysequentiallyinteractwithaseriesofexcitedatoms.The experiment is at the very early stages of practicality, but the

experimenters suggest that it may lead to the ability to create more complexstructuresoutofphotons–perhapsevencrystalsoflight–andthatitshouldbepossibletousethesestructurestocreatethesortoflogicgatesnecessarytomakecomputerswork,butemployingphotonsrather thanelectrons.Funnilyenough,it’shardtoimagineanythingfurtherawayfromtheblistering,fixed-lengthlightbeams of a lightsaber than these delicate photonic molecules – yet theexcitement the press release generated is perhaps not so unrealistic, as thisgenuinelyhasrealpotentialtoenableustodomuchmoreatthequantumlevelwithlight.Whenwedealwithsuchopticalcomputing,orstrangestatesofmatterlikea

Bose–Einsteincondensate,orthefascinatingphenomenaofsuperconductivity,itseems thatweareworkingat the extremes, seeing somethingwewouldneverfindoutsidethelab.Butthat’saveryparochialview.Aswehavealreadyseen,quantum physics is constantly escaping from the world of science into oureverydayexistence.

CHAPTER13

Aquantumuniverse

It’s almost pointless to try to imagine a world without quantum technology.Taken at the base level, everything depends on atoms and light and theirquantum dance of interaction. But even if you limit your view to thosetechnologies like electronics that are designed around quantum physics ratherthannaturallymakinguseofit,ourmodernliveswouldfallapartwithoutit.

Quantumtech

AsI sit typingonmy iMac,adevice filledwithquantum technology,viewingthistextonaquantumLCDscreen,Icanseephones(mobileandwired),abankPIN device, a laser printer, a CD drive, even a lightsaber (though I have toadmit, that’s a toy). I simply couldn’t do my job anymore without quantumsupport.Andallthetimethosequantumdevelopmentsthatarecurrentlyontheleading edge are moving outwards towards being practical, everydayapplications.Takequantumencryption.Thisisnotjustamatterforivory-towerscientists–

oreventheacademicswhoventureoutintotheworld,likeAntonZeilingerwithhisexperimentallink-upsinVienna.Thefactthatawiderangeofcompaniesareworking on quantum key systems, from big, established computer businesseslikeIBMandToshibatospecialistsliketheAmericanMagiQandtheSwissIDQuantique, shows that there is a realpotential formakingpracticaluseof thispowerfulquantumeffect.

Entanglementinspace

The essential to transform the distribution of quantum keys from a laboratoryone-offtoawidelyavailableservicethatcanworkreliablyforeverydayuseistoset up a kind of quantum version of the internet – or at least a very singularprivateinternet.Tomakethispracticalwouldrequireanetworkofstationsthat

coulddistributequantumkeystoremotelocationssothat,forinstance,abankintheUKcouldsafelycommunicatewithabankintheUS.Theideallocationforthese quantum stations is in space, as a satellite would have a wide reach ofpossible targets. This is why Anton Zeilinger’s team in Vienna spent timesendingentangledphotonsfrombuildingtobuildingoverdistancescomparableto that toasatellite.Thesamechannelscouldalsobeused tosetupakindofdistributed quantum computer, linking qubits in one location with qubits inanotherusingentanglement.Keepingentanglementliveacrossadistanceofmanykilometresisn’taneasy

process.Usuallyaradioorlasersignalsentoveralongdistanceiscomposedofhugenumbersofphotons.Manyoftheindividualphotonswillbelostalongtheway, scattered by air molecules or otherwise interacting with matter. But aquantumsignalworksat the levelof individualphotons.Lossesalongthewaycanseriouslydisrupt theprocess. It’snotpossible tohavemultiplecopiesofasingle bit of the key, or someone could intercept part of the stream withoutnecessarily disrupting the entanglement. This makes satellites as switchingstationsdoublyattractive,asafairamountofthedistancethephotontravelswillbeinspacewherethereislittlematterwithwhichthephotonscaninteract.There are a number of attempts under way to get an experimental satellite

quantum station in operation, fromAntonZeilinger’sworkwith theEuropeanSpaceAgencytosetupanexperimentontheInternationalSpaceStation(whichhas the advantage while still experimenting of having astronauts on hand totweak the process) to a collaboration between the Canadian Institute forQuantumComputingandtheCanadianSpaceAgencywiththeJapanesesatellitementionedearlier (seepage248).Althoughit isawayoff, itseemslikely thatthere will eventually be a commercially available, satellite-based ‘quantumoverlay’totheinternetthatwillalloweasy,totallyunbreakablecommunicationbetweentwolocationsonEarththatcouldbeuptohalfaglobeapart(dependingonhowhighthesatelliteorbits).

Amoralvacuum

Like many new technologies, this possibility is both useful and worrying. Itmeansthat inprinciplefinancial transactionscouldbemadetotallysecure,andthisprocessmayarrivejustintimetopreventthecurrentpublickey/privatekeysystem from being devastated by the use of Shor’s algorithm in quantumcomputerstofactorthoselargeprimes.Oureverydayonlinesecuretransactions– the ones with the little padlocks that appear in the web browser – could

becomeprotectedbyquantumkeydistributionfromthenextgenerationofthesesatellites. But at the same time, this truly unbreakable communication wouldpotentially be a boon to terrorists and spies,who could communicate securelywithout fear of government interception.A system likeRSA can be set up insuch away that security services cangain access to encrypted files.Thismayraisecivillibertyhackles,butsometimesitmaybejustified.Withgoodquantumkeydistributionthereisnobackdoor.Secureissecure,whateverthemessage.Like any new technology, the applications of quantum physics are morally

neutral.Theycanprovidecertaincapabilities,anditisuptoustodecidewhetherwewanttomakeuseofthosecapabilities.Butthegenieofknowledgeisoutofthebottleandisnotgoingback.Insomewaysquantumphysicsanditsapplicationshasaparallel intheway

thatelectricitywastreatedbyscientistsandengineersinanearlierage.Theyhadno idea what electricity was, but they could make use of it to deliver theincreasing benefits of the electrical age.We understand the kind of electricaleffectsthatdelightedtheVictoriansfarbetternow,butwehavehitupagainstafar bigger barrier when trying to comprehend the quantumworld because wedon’t have a mechanism for understanding. In one sense, physics can neverdelivertheabsolutetruthofthenatureofrealityatthequantumlevel.Allitcaneverdo isprovideuswithmodels. Justconsidercommentsonwhat thingsarelike at the quantum level from two great Nobel Prize-winning physicists,RichardFeynmanandSteveWeinberg.

Whatarequanta?Feynman,aswehavealreadyseen,said:‘Iwanttoemphasizethatlightcomesinthis form–particles.’Feynmanhappened tohavebeen talkingabout light,buthewouldhavesaidthesameforotherquantumparticles.Weinberg,ontheotherhand,haswritten:‘[T]heinhabitantsoftheuniverse[are]conceivedtobeasetof fields – an electron field, a proton field, an electromagnetic field – andparticles[are]reducedtomereepiphenomena.’It’seasytothinkthattheolderFeynmanmadehiscommentsearlier,andhis

ideas were replaced by Weinberg’s new thinking, but Feynman’s particledefinitionappearedinabookpublishedin1985,nearlytenyearsafterWeinbergmadehiscomments.Itseemsperhapsmorenaturalfornon-physiciststogowithFeynman’sparticleview,becauseon thewhole, stuffasweexperience itdoescomeinparticles,notintheformofafield.Fieldsseemacerebralconceptthatclearlycan’tbereal.Itseemswastefullyexcessivetorequireafieldthatfillstheentire universe to define the location of a single electron, rather than have an

apparently simple tiny object. The majority of modern physicists, however,wouldsaythatWeinbergwascorrect.Inpracticetheyarebothwrong–orbothright,dependingonyourpointsofview.Thefactisthatbothwaysofdescribingquantumphenomenafittheobserved

results.Andwe have no otherway to distinguish between them.We have nowaytolookatanelectron,say,andseewhatit‘really’is.Allwecandoistestthemodelsagainsttheobservationswecanmake–andtherearelotsofthese–andseehowwellthosemodelsholdup,howwelltheirpredictionsmatchwhatisobserved. Both the particle and the field approach fit equally well. The fieldapproachhappens toworkmore easilywith themaths that is currently inuse,whichiswhymodernphysicistspreferit–butWeinbergwasentirelywrongtorefer to particles as ‘mere epiphenomena’. They are just as real (and just asimaginary)asfields.

Guessinginthedark

Bruce Gregory in his book Inventing Reality points out that the job of thescientist,andthephysicistinparticular,isabitlikelookingatacomplexclockthatistotallysealedupandtryingtodecidewhatmechanisminsideisproducingthe resultswe observe on the outside.We can comeupwith various differenttheories as towhy it displays various indications on its dials at any particulartime.Therecouldbeaclockworkmechanisminside,therecouldbearadiolink,pickingupanexternallybroadcast timesignal–ormanyotherpossibilities. Itcouldbe,forinstance(thoughthisisprobablyunlikelyanywhereotherthanthefictionalDiscworld), that there isan imp inside, turningahandle tomatch theregular beat of its own pulse. Similarly, theway that information reaches thedialscouldworkthroughawholehostofdifferentmechanisms.Allwecando,bearinginmindthattheclockcanneverbeunsealed,istosee

howwell our theoriesdo at predictingwhatwill turnupon thedials.Wecanmatchour theoriesagainstobservation.Andthose theories thatmatchbestandthatareusefulatpredictingwhatwillhappen in the futureare the theorieswecangowith.Sometimestherewillbeseveraltheories,allofwhichproducethesameresultsandthatitmaybepossibletoshowmathematicallyareequivalent.Ifthatisthecase,itistotallyarbitrarywhichofthetheoriesweuse.Wemightprefer one over another because themathematics is easier – or because it sitsbetterwithournaturalinclination–butitisonlyamatterofpersonalpreferenceaboutwhatlanguageweusetodescribewhatishappening.Most importantly, we need to get away from the idea that our models are

reality.Whenwe describe how an atom behaves or what happenswhen lightinteractswithmatter, we are not describing reality. Insteadwe talk about ourmodel thathappens to fit theobservedresults ratherwell.Thinkof that sealedclockonceagain. It isentirelypossible (thoughIhopeunlikely) that theclockworksbecausetherereallyisaverysmallimpishpersoninsideitpullinglevers.Thatcouldbereality.Butprovidedthatpersonactsinsuchaconsistentwaythatit could be clockwork inside, and such thatmymaths for clockwork predictswhatthehandswilldo,itdoesn’treallymatter,exceptataphilosophicallevel.Scienceismorepragmaticthanphilosophy.Ithastobe.Andthat’sjustaswell.Ifourscientistshadspentalltheirtimetryingtogetto

the heart of reality we would never have been able to make use of quantumphysics.Itwouldbeasteriledisciplinewithnosignificancetotheoutsideworld.Butinfactphysicistshavegivenusmodelsthatfitreality1wellenoughthatwecanmakeuseofallthestrangenessofquantumbehaviour.

Buildingwithquanta

Even at the most fundamental level we are just beginning to open up thepossibilitiesofquantumphysics. It ispossible touseourquantumexpertise toproduce new materials that behave quite unlike anything known to nature.Scientists wince when bad science fiction comes upwith ‘some new elementpreviously unknown to Man’ that is inevitably superstrong or otherwisemiraculous,becauseweknowa lotabout theelementsandwhatpossiblegapsthere are in the periodic table – but the field is much more open for newmaterials.We are used to the three basic forms of matter – gas, liquid and solid.

Physicistsusuallyrecognisefive,addingplasma–matterthatisheatedsomuchthatitlosesorgainselectronsandbecomesacollectionofions–andtheBose–Einsteincondensate.ButasMalteGrosche,headoftheQuantumMatterGroupat the Cavendish Laboratory in Cambridge points out, there’s an interestingparallelwithchemistry.Chemistshaveonlyaround100elementstoplaywith.Butwhenitcomesto

compounds – different ways to combine the elements – the possibilities areendless, from simple two-atom forms like sodium chloride to themagnificentstructureofthevastDNAmoleculesthatformourchromosomes.Similarly,byusingquantummethodology,itispossibletomakenewstatesofmatterinwhichthewaytheelectronsself-organisecantransformthenatureofamaterial.It is early days and very difficult to predict how the exotic-sounding list of

examplesGroscheproduceswillchangethings.Hespeaksof‘unusualparticle–holecondensates,suchasspinorchargePomeranchukorder’,andof‘skyrmionlattices in chiral magnets, magnetic monopoles in spin ice materials, andtopologicalinsulators’.Theymaysoundlikeitemsintheequipmentcupboardofa badly written science fiction starship, but they are real, and they offer newbuildingblocksforquantumdevicesandmaterialsweareyettoimagine.

Seeingquantumeffects

We also see increasing examples of quantum effects cropping up in ‘macro’objects– thingswecan seeand interactwith.Thebest-knownarephenomenasuch as superconductivity thatwe have alreadymet, but perhaps the strangestexampleofallemergedinanexperimentundertakenatParisDiderotUniversityin2005.YvesCouderandEmmanuelForthadputabathofoilontoaplatformthatproducedverticalvibrationsinthemassoftheoil.Theythendrippedsmalldropsofoil(butbigenoughtobevisiblewiththenakedeye)ontothesurfaceofthebath.Itmightsoundlikethesortofexperimentthatstudentsdreamupafteranightonthetown,buttheresultswerestartling.Theobviousexpectationwas that thedropswoulddisperse into thebodyof

theoil,helpedon theirwayby thevibration,but in fact they remainedwhole,bouncingupanddownonthesurfaceandsendingwavesripplingoutacrosstheoil.Astheexperimentersturnedupthepower,thedropletsstartedtoskipacrossthewaves, bouncing back and changing direction just before they reached theedge of the bath.Couder andFort called the animated drops ‘walkers’ – theirbehaviourwasalmostasiftheywerealive–butalsonotedadistinctsimilaritywith the quantumworld. The bouncing droplets and thewaves that propelledthemhadakindofunity.Itwasasortofvisiblewave/particleduality.Notsurprisingly,thepairbegantoexperimentfurtherwiththeir‘walkers’and

found that theseweren’t their only similaritieswith the quantumworld. TheymanagedtoduplicateaversionofthequantumYoung’sslitsexperimentwhereasetofsingleparticleswouldbuildupaninterferencepatternasthedroppassingthroughoneslit interactedwithwaves thathadpassed throughboth.And theyobservedadropletversionofthekindofquantisedorbitsthathadsetNielsBohroffonthepathtoquantumtheoryinthefirstplace.In particular, the walkers reflect the early ideas of Louis de Broglie, who

thought that wave/particle duality emerged from actual particles that wereaccompaniedby a ‘pilotwave’ that brought in thewave-likebehaviour that isobserved.Inmanyways,theoildropintheParisexperimentsissteeredbythe

pilotwavesonthesurfaceoftheoilbath.Afewphysicistsbelievethatthereisadirect connectionwith quantum physics – that this is a real quantum effect –while most see it as an elegant and entertaining parallel to the processes webelieveunderliequantumeffects,notdirectlybasedonthesamephenomenabutprovidinganexcellentanaloguemodelforwhathappens.There are certainly some clear differences between this type of quantum

phenomenon and the usual version. Quantum particles don’t ‘run down’ –withoutaninteractiontheycarryonforever,whereasthewavesintheoilbathhave tobeconstantly fedwithenergy from thevibrator.And thewaves in theParisexperimentarelimitedtotwodimensions,whereaseachquantumparticlehasprobabilitywavesinitsownsetofthreedimensions(sotwoparticlesrequiresixdimensions,andsoon).Thisindependentsetofdimensionsseemsessentialfor entanglement to function, so the walkers are unlikely ever to exhibit thatmostquintessentialofquantumphenomena.

FallinginlovewithyourmodelThe peculiarities of quantum physics, and the distinction betweenmodels andrealities,iseasystufftomisunderstand,evenfortheprofessionals.Infactalotof scientists find it difficult to remember that their models aren’t real. In hisbook The Grand Design, co-authored with Leonard Mlodinow, StephenHawkingproclaimed thatphilosophy isnowdeadbecause sciencecanaddressthe questions that philosophywas once required for.Yet this portrays a naiveidea of the nature of science. In fact, scientists would have a much betterunderstandingofwhat theyactuallydo if theywereforced tostudya touchofphilosophyofsciencealongtheway.Take, for instance, Brian Cox, who we previously met allowing quantum

particles to be in more than one place at a time. Cox does a great job ofpopularisingscience:Iusehimonlyasanexampletoshowhoweasyitistogetitwrongwhen talking aboutmodels andquantum theory. In one of his books(writtenwithJeffForshaw),Coxwrites:‘Quantumtheoryprovidesadescriptionof Nature that, as we shall discover, has immense predictive and exploratorypowers…’Aswehaveseen,quantumtheoryiscertainlyimmenselyvaluableinhelpingusunderstandwhatNaturewilldo,and toenableus toharness that ineverything from electronics to lasers. But quantum theory doesn’t describeNature.Itcan’t.AllitcaneverdoispredicttheobservationsthatwewillmakeofNature,andthatisaverydifferentthing.

Avoidingquantumwoo

Just as we have to be aware that quantum theory is all about models, not‘reality’, we also have to be wary of the siren call of an opposing effect.Influencedinpartbythevagariesofpostmodernism,therehasbeenatendencyto extend quantum observations to the ‘macro’ world: to think that theUncertaintyPrinciple,forinstance,meansthat‘everythingisuncertain’,andthatthe mysterious nature of quantum theory means that anything that soundsmysterious,andthathasafewquantumtermsthrownin,hasasgoodachanceofbeingtrueasanythingelse.Butthattotallymisunderstandsthesituation.Yes, we are only dealing with models. Yes, it’s true that quantum physics

does not describe nature, and is only our bestway of predictingwhatwill beobserved given the current data. But those who play free and loose with theterminology forget that quantum physics is very good at that role. It makespredictions thatmatchrealitysoaccurately that it is theequivalentofguessingthe distance from London to New York to the width of a hair. Even whenquantum theory says thatwecanonlymakepredictionsbasedonprobabilitiesbecause there is no underlying reality, those probabilities are crisp, predictivemathematicalvalues,notvaguehand-wavinguseofterminology.Itisn’tthecasethatalltheoriesareequalintheirabilityandworth.Norisit

thecasethatbythrowinginwordslike‘quantum’it’spossibletoturnfictionalwoointoanythingthatevencomesclosetomodellingreality.Itdoesn’t takealotofsearchingonlinebeforeyoufind‘quantum’devicestomagicallytransformwater to ‘restore a special balance needed to hydrate’ or that bring in special‘energy fields’ or other legitimate physical terms and sprinkle them throughadvertisingtogiveaproductasenseofbeingscientificallybased.

Thenewreality

Simplyusingtheterminologyofscience,andspecificallyquantumphysics,doesnotmake something valid or useful.But in away it’s not surprising that it isquantum terminology that ismost used in this fashion– for the simple reasonthat quantum physics is so important to our everyday lives. Just as somesocieties have in the past attempted to copy the outward appearance oftechnological societies in so-called cargo cults, buildingmock versions of thereal things, so this misapplication of quantum terminology is what RichardFeynmancalledcargocultscience.Itisrubbishinitself–butitemphasisesthesignificancequantumphysicshascometohaveinallourlives.WelcometotheQuantumAge.

Footnote

1.Whateverrealityis–andwearenevergoingtobeabletoopenthemetaphoricalclock.

Index

absolutezero174,177accelerators166adenine36adiabaticalgorithms246Adleman,Leonard234Allen,John252amber45ammonia130–1amplituhedron111–12AnalyticalEngine226–7,232Anderson,Philip201Archimedes152ARPA138–40Asawa,Charlie145Aspect,Alain214atomicbomb94–5atoms5,15–16structure17–19,52

Babbage,Charles225–8,237Ball,David127–8Bardeen,John59,181–3bases36Basov,Nikolai128–30,149BaywatchPrincipleseeleasttime,principleofBCStheory183beamsplitters115–16,240beamlines167–8Bell,John209,213BellLabs130,133–6,141,144–5,148–9,156

BletchleyPark228–9Bloch,Felix179Bohr,Niels15,16–17,52,108,166–7,272andEinstein210–11

Boole,George64Born,Max21–2,24,209–10Bose,Satyendra253Bose–Einsteincondensates253,255,256–7,259,270Bose–Einsteinstatistics253bosons253Bragg,Lawrence168Bragg,William168Brattain,Walter59Brown,Robert11Brownianmotion11,15bundles192

Caesarcypher218calciumcarbonate170–1cameras75–7cancercells169–70capacitors128,145–6,230carbondioxide39cargocultscience275catalysts35cathoderaytubes69–71cathoderays52–4,69CCDs76–7Čerenkovradiation256chlorophyll40ChouShinkansen197–8Chu,Paul184Chuang,Isaac239Churchill,Winston227–8ciphers218–19key-based219

circuitboards60clock,sealed268–9Cloud,the162,163

CMOSsensors76–7Cocks,Clifford234codes217–18coherence40Cole,David169–70Colossus58colour83–4,104–5communication,instant214complexnumbers21compounds270computers58–60,64–71,207seealsoquantumcomputers;universalcomputer

computing,history225–30condensates183–4conductors61consciousness42Cooper,Leon181,183Cooperpairs183,187,202,259corpuscles82,84,86,114,116seealsophotons

Couder,Yves271–2Cox,Brian23,273Crookes,William53cytosine36

D-Wave245–7Dalton,John15darkstate257Darwin,Charles31,32–3datastorage162–4Davy,Humphry47,48deBroglie,Louis19,272deSaussure,Theodore39deathrays152,153–4decoherence38,43,180,241,243–5Descartes,René81–2DiamondLightSource165,167–72diamonds248–9dielectricresponse190

DifferenceEngine225–6Dirac,Paul20,92,93discord244–5DNA36–7,169doping62

e-readers123–4Edison,Thomas113–14Einstein,Albert11–15,92,129,209–14andBohr210–11andprobability24

electricalconduction54–6electricity2–3,45–56,77electromagneticwaves55,89,91electromagnetism49–52electronics3,56–77electrons13,15–16,176charge176discovery53–4flowinelectricity54–6

encryption217–20,233–4,247–8,263–6energystorage206–7ENIAC58,242Enigmamachines219entanglementseequantumentanglementenzymes35EPRparadox211–14ether87–91Euclid80evolution31,33eye118–19,123

Fabrikant,Valentin130Faraday,Michael46–50,52,87–9,108–9Fermi–Diracstatistics253fermions253ferromagneticadsorbents206Feynman,Arline95Feynman,Melville93

Feynman,Richard6–7,93–9,103,105–8,215,266–7,275Feynmandiagrams93–4,96–7,109–10,111–12fields98–9,108,267flashmemory67–9Fleischmann,Martin188TheFly221,222Forbes,James51Forshaw,Jeff273Fort,Emmanuel271–2Franklin,Rosalind169freeradicals41friction173

Gabor,Dennis157–9GalileoGalilei11,82,90gates64–6,237–8,240–1,244generalrelativity1glass114–16‘slow’255–9thickness116–17

gluons110–12Goldblum,Jeff221Goldmuntz,Lawrence138,140Gordon,Jim131Gould,Gordon132–3,136–43,146,149,153gradientcoils196Grassmannian111Gregory,Bruce268Grosche,Malte270Grover,Lov234–6guanine36gyroscopes254

Hameroff,Stuart42Hau,LeneVerstergaad256–7,259Haven,Emmanuel43Hawking,Stephen273heat,fromlight151–3Heavyside,Oliver52

Heisenberg,Werner19–20,24–5helium33–4liquid173,175,186,251–2types253–4

Herbert,Nick215–17Herschel,John225Hertz,Heinrich52,53heterojunctions156Hodson,Mark170holes62–3holograms156–60applications160–5

holographicphotographs164–5holographicstorage162–4Hooke,Robert84HOPG188–9Hornberger,Klaus26‘hotpotato’mechanism241Hughes,Richard242HughesCorporation141,146,148hutches167,171Huygens,Christiaan9–10,85hydrogen15–16,18,33–4hydrogenbonds36–7

imaginarynumbers20–1indexes236InfraredAstronomicalSatellite,255insulators61integratedcircuits60–1,63–4ions,asqubits240–1iridescence117–18isotopes182ITER194–5

Josephson,Brian200–2Josephsonjunctions200,201–2

KamerlinghOnnes,Heike173–80,192,251–2

Kapitsa,Pyotr252Kelvin,Lord(WilliamThomson)7–8,31–3Kelvinscale173–4Khrennikov,Andrei43King,Ada(CountessofLovelace)227,232Kirzhnits,David189–90

LargeHadronCollider(LHC)165–6,191–2,193lasergaintubes216–17

lasersapplications151–72coupling257development132–50schoolproject127–8

Lawrence,D.H.7LCDs72–4,124leastaction,principleof102leasttime,principleof102–4,118LEDscreens74–5Leith,Emmett159Lenard,Philipp14lenses81,118–19

lightcrystalsof262earlyviewsofnature79–82asfielddisturbance98heatfrom151–3interactionwithmatterseeQEDparticlenature82,84,86,91,97–9,266–8speedof82,255–7straightlinetravel104wavenature8–11,84–91,97–9,267–8

lightbulb113–14‘lighttransistor’260–1lightsabers259–60,261–2liquidcrystals72logicgates64–6,237–8,240–1,244London,Fritz180–1Lovelace,AdaKing,Countessof227,232Lukin,Mikhail259

Ma,Xiasong,241macroscopicity27maglevtrains196–200magneticfields178–9expulsion178–9

magnetism47–9Maiman,Theodore141–8,150ManhattanProject95masers129,130–2,134matrixmechanics20

matterformsof270–1interactionwithlightseeQED

mattresseffect182–3Maxwell,JamesClerk50–2,88–9,178Meissner,Walther178–9Meissnereffect178–9,180memory66–9Menabrea,Luigi227mercury74,90,174metals54–6metamaterials189–90MHCs169–70Michelson,Albert89–91Millikan,Robert176mirrors80–1,99–105Misener,Don252Mlodinow,Leonard273MLX–01197models,versusrealities273–4Moore,Gordon229Moore’sLaw229Morley,Edward89–91MOSFET63MRIscanners195–6,203,239

Napier,James88NationalIgnitionFacility154–5neutrons110Newton,Isaac82–4,114–16Nimmrichter,Stefan26nitrogen,liquid186–7‘nocloningtheorem’216–17,221–2,238nuclearfusion2,33–5,154–5,188,193

Occam’srazor80,105–6Ochsenfeld,Robert178–9oildrops271–3OLEDs75

onetimepads219Onnes,HeikeKamerlingh173–80,192,251–2opticalpumping133,136,141opticalstorage67orbitals55orchestratedobjectivereduction42oxygen39

Parke,Stephen111Pauliexclusionprinciple253Peel,Robert49Penrose,Roger42,111phase97philosophy273phlogistontheory39phonons181–2,183,188phosphors70,74photoelectriceffect12,13–14,120photons14absorption17,99emission17,99interference23,25,106pairsof259–60,261–2asqubits238–9,240scattering117,123virtual108

photosynthesis38–41,119photovoltaiccells120–1piezoelectricity57pigeons41–2pixels73–4,124Planck,Max8–9,11,13,108plasma193–4,270plasmascreens74plenum81–2Podolsky,Boris212polarisation72–3,87,215–16Pons,Stanley188postmodernism274

Price,Vincent221Priestley,Joseph38–9prism83probability22–3Prochorov,Alexander128–32,149protons18,33–4,110publickey/privatekeysystem233–4

QCD(quantumchromodynamics)110QED(quantumelectrodynamics)7,92,96–7,256quanta9,11,13,266–8quantumalgorithms233–6quantumannealing245–6quantumchromodynamics(QCD)110quantumcomputers230–49,257–8,261adiabatic245–7

quantumdots240–1quantumelectrodynamics(QED)7,92,96–7,256quantumentanglement213–23,238,241,243,247,264quantumkeys248,253–6quantumsolvents254–5quantumstations264–5quantumteleportation217,221–2,238,241quantumtunnelling34,35–6,69,246quantumvibrations181–4quarks110qubits43,231–2,237,239–49,257–8quench191

radiation199radios58–9rainbowhologram161RAND140rareearthelements185Reagan,Ronald139reflection80–1,99–105partial114–15

reflectionhologram161refraction85,102,189

refractiongrating105refractiveindex,negative189relativity1,11,90general1special1,11

resistance173–7zero175,177,179

resistors56Rivest,Ronald234Rizkallah,Pierre169–70Rosen,Nathan212RoyalInstitution47–9,87–8RSA233–4,266rubies135,141,142–5,146–7Rutherford,Ernest16Rydbergatoms261

Sandberg,Herbert142scattering117,123,144scatteringamplitude111–12Schawlow,Arthur131–5,138,142–4,146,149Schrieffer,Robert181,183Schrödinger,Erwin20,213Schrödinger’scat25–7,261Schwinger,Julian96searchalgorithms234–5searchengines65selenium62semiconductorlasers155–6semiconductors61–3Senebier,Jean39sewagetreatment204–6Shamir,Adi234Shaw,Bob255Shelburne,Earlof38shipengines199–200Shockley,William59Shor,Peter233Shoralgorithm233,239,243,246,265

sight79–80silicon63,120silicondioxide63‘slowglass’255–9Smolyaninov,Igor189–90Smolyaninova,Vera189–90solarcells120–1solarsystem17,18Solvay,Ernest211SolvayCongresses211sound85spacetravel214specialrelativity1,11spectroscopy254spectrum91–2spin41,253spotlights115squareroots20–1SQUIDs200,202–4,240StarTrek221staticelectricity45steamage2Stoney,George53–4storage67–9sumoverpaths106Sun1–2,30,122,136,193superconductors174–90,252–3applications191–207materials184–7operationaltemperature184–90typeII192

superfluids251–9superposition25–7,261Swan,Joseph113–14synchrotrons166–72

T-cells169–70tasers153Taylor,Tommy111

TechnicalResearchGroup(TRG)137–40,142Tesla,Nikola114thermionicvalves56–9,61Thomson,J.J.15–16,52–4Thomson,William(LordKelvin)7–8,31–3thymine36tokomaks193–5Tomonaga,Sin’Itiro96Toshiba68Townes,Charles130–2,133–8,149trains196–200transistors59,61,63–4,134,135,230floating-gate68–9

transmissionloss177,206transporters221–2triodevalve56–7Turing,Alan228–30twistors111two-slitexperiment19,105–6

UncertaintyPrinciple24–5undulators167universalcomputer229–30Upatnieks,Juris159uranium182UXOs203–4

valencebands61–2valves56–9,61voting43Vuletic,Vladan259

walkers271–3water204–6Watson,ThomasJ.Snr7–8,58wavemechanics20waves,transverse86weather30Weber,Heinrich12

Weinberg,Steve266–8Westinghouse114Wheatstone,Charles87–8wigglers167WilliamofOckham105–6Wollaston,William48worldlines93–4wormcasts170–1writing121–3Wu,MawKuen184

X-raycrystallography168–9

Young,Thomas9–10,86–7Young’sslits9–10,23–6,86,272yttrium185

Zeilinger,Anton201,263–5zincoxide120