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Association canadiennedes physiciens et

physiciennes

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Serving the Canadian physics community

since 1945 /Au service de la commu-

naut canadienne dephysique depuis 1945

Volume 66 No. 2April - June 2010avril juin 2010

A PEEK INSIDE THEPERIMETER INSTITUTE

A LINTRIEUR DELINSTITUT PERIMETER

ffordSticky NoteThis is an official electronic offprint of the full issue of Physics in Canada, Vol. 66 No. 2 (April-June, 2010).

Copyright 2010, CAP/ACPAll rights reserved / Tous droits de reproduction rservs.

F.M. FordManaging Editor, PiC

PHYSICS IN CANADA / VOL. 66, NO. 2 ( Apr.-June 2010 ) C i

PHYSICS IN CANADALA PHYSIQUE AU CANADAVolume 66 No. 2 (Apr.-June 2010 / avr. juin 2010)

73 Why Physics Needs Quantum Foundations, by L. Hardy and R. Spekkens

77 Quantum Bayesianism at the Perimeter, by C.A. Fuchs

83 A Triple Slit Test for Quantum Mechanics, by U. Sinha, C. Couteau, F. Dowker, T. Jennewein, G. Weihs, and R. Laflamme

87 Spin Systems and Computational Complexity, by D. Gottesman

91 Warped Views: Observing Black Holes, by L. Boyle and L. Lehner

95 Analog Gravity and Black Holes, by W.G. Unruh

99 Phenomenological Quantum Gravity, by S. Hossenfelder and L. Smolin

103 Getting a Big Bang from String Theory, by C. Burgess

107 Reviving Gravitys Aether in Einsteins Universe, by N. Afshordi

111 Dark Forces, by B. Batell and M. Pospelov

115 The Early LHC Era, by M. Trott

119 The Geometry of Trees, by F. Cachazo

123 Quark Soup: Applied Superstring Theory, by A. Buchel, R.C. Myers and A. Sinha

FEAT

URE

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127 Perimeter Scholars International, by J. Berlinsky130 Mission: Outreach - The Why and the How of It,

by J. Matlock and G. Dick

135 Perimeter Institute - Outreach- Measuring Plancks Constant- Black Box Demonstration

Advertising Rates and Specifications (effective January 2010) can be found on the PiC website(www.cap.ca - Physics in Canada).Les tarifs publicitaires et dimensions (en vigueur depuis janvier 2010) se trouvent sur le site internet deLa Physique au Canada (www.cap.ca - La Physique au Canada).

67 Foreword / Prface by/par R. Myers and N. Turok69 Editorial - An Experiment in Theoretical Physics, by N. Turok71 ditorial - Une exprience en physique thorique, par N. Turok

Cover / Couverture :

Architects rendering of TheStephen Hawking Centre atPerimeter Institute, now underconstruction and expected toopen in fall, 2011. Coverdesign by Perimeter Institute,image courtesy of TeepleArchitects Inc.

Croquis darchitecte du CentreStephen Hawking lInstitutPerimeter, maintenant sous con-struction et dont lachvement estprvu pour lautomne 2011.Couverture conue par lInstitutPerimeter. Image courtoisie deTeeple Architects Inc.

EDUCATION AND EDUCATION CORNERENSEIGNEMENT ET ESPACE DUCATIF

Canadian Associationof Physicists

Association canadienne des physiciens et physiciennes

www.cap.ca

II C LA PHYSIQUE AU CANADA / Vol. 66, No. 2 ( avr. juin 2010 )

82 Departmental, Sustaining, Corporate and Institutional Members / Membres dpartemen-taux, de soutien, corporatifs et institutionnels

90 Congratulations / Addendum98 News - Killam Prizes102 News - Canada Excellence Research

Chairholders

106 Art of Physics / LArt de la physique114 PiC welcomes articles / Invitation

soumettre des articles

141 Science Policy Corner (NSERC) /Le coinde la politique scientifique (CRSNG)

142 Second Science Policy Symposium held 12-14 May 2010 in Gatineau, Quebec

144 In Memoria- H. Roy Krouse (1935-2010)- B.P. Stoicheff (1924-2010)

146 Books Received / Livres reus147 Book Reviews / Critiques de livres152 Advertisement / Publicit

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Normand MousseauChaire de recherche du Canada, Dpartement de physiqueUniversit de Montral, C.P. 6128, Succ. centre-villeMontral, Qubec H3C 3J7(514) 343-6614; Fax: (514) 343-2071Email: [email protected]

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e were delighted to be invited to prepare thistheme issue of Physics in Canada, and we hopeyou will enjoy the glimpse provided here intothe diverse research activities at Perimeter

Institute (PI).

From its inception, spacetime and quantum theory have beenat the heart of PI research. While these topics might at firstsight seem abstract and somewhat remote from the real world,we hope that you will see in these pages that here at PI, westill live by the maxim that nature and experiment are a theo-rists best guide.

One of Perimeters unique features is its group working onquantum foundations. Rob Spekkens and Lucien Hardy elo-quently lay out the case for their field, describing both themotivations and the impact it has had. Among these was therecent experimental confirmation of Hardys Paradox bygroups at both the University of Toronto and OsakaUniversity, which may in time yield practical applications.Christopher Fuchs also gives his perspective on a freshapproach to unraveling the conundrums of quantum mechan-ics with the help of Bayesian probability theory.

Quantum information science emerged as an offshoot ofquantum foundations, and now flourishes with the promise ofnew technologies which may transform our society. Perimeterwas instrumental in launching the Institute for QuantumComputing (IQC) at the nearby University of Waterloo, bothpartners with which PI enjoys strong synergistic relations. Intwo articles here, we see the synergies between quantuminformation and other areas of theoretical physics. In theirarticle, Urbasi Sinha, Raymond Laflamme and colleaguesgive an account of a new triple slit experiment which tests thebasic tenets of quantum mechanics following a proposal byPIs Rafael Sorkin. Then Daniel Gottesman describes howwork on quantum computational complexity classes has pro-vided unexpected insights into spin glasses.

On PIs spacetime theme, we begin with two thoroughly dif-ferent accounts of black holes. Once an exotic toy in the the-orists playground, these phenomena of ultra-strong gravityhave become the workhorses of modern astrophysics. In theirarticle, Luis Lehner and Latham Boyle describe new astro-physical observations which may teach us more about blackholes through their impact on light and gases caught in theirimmense gravitational fields. In contrast, Bill Unruh, one ofPIs Distinguished Research Chairs, hopes to tame thesedynamos in his bathtub. His article tells a remarkable storyabout how doing experiments in a tank of water may providea better understanding of Hawking radiation leaking fromblack holes and, perhaps, even of quantum gravity.

W

Every great advance in science has issued from a new audacity of imagination. John Dewey

It doesnt matter how beautiful your theory is, it doesnt matter how smart you are.If it doesnt agree with experiment, its wrong.

Richard P. Feynman

Rob Myers ,Senior Faculty mem-ber

Neil Turok ,Director

Perimeter Institute forTheoretical Physics,31 Caroline St. N.,Waterloo, ONN2L 2Y5

While quantum gravity delves into spacetime at unimagin-ably small scales, Lee Smolin and Sabine Hossenfelderdescribe how we may nevertheless find fingerprints of thequantum nature of short distance physics via experimentsmeasuring phenomena over cosmological scales. Similarly,cosmological observations provide important clues about thephysics of the very early universe, which in turn may providehints as to the ultimate theory of nature. With this motivation,Cliff Burgess describes how we might find fingerprints ofstring theory through its impact on cosmic inflation.

Dark energy and the present acceleration of the universe rep-resent one of the greatest puzzles in cosmology today. Theyprovide vital clues as to the ultimate theory and call for a rad-ical reworking of the standard theory of cosmology,Einsteins theory of gravity, or both. Niayesh Afshordi out-lines his own efforts to rethink gravity at the largest scales,which have intriguing cosmological signatures. Particle theo-rists Maxim Pospelov and Brian Batell relate exciting newproposals for the properties of dark matter, the other greatenigma dominating large scale phenomena in the universe,which may explain several curious new observational results.

Particle physics is entering a particularly exciting new era asCERNs Large Hadron Collider (LHC) is beginning toexplore physics at a new energy frontier. Michael Trott givesus a theorists perspective on the motivations, challenges andpossible discoveries for physics at the LHC. Freddy Cachazodescribes his work on the foundations of quantum field theo-ry which, unexpectedly, provides new tools for analyzingupcoming accelerator experiments. Alex Buchel, Rob Myersand Aninda Sinha tell us how novel techniques developed instring theory are helping us to understand a remarkable newphase of nuclear matter.

The lifeblood of physics, as we all know, is brilliant youngpeople. John Berlinsky writes about Perimeter ScholarsInternational, an innovative graduate research training coursewhich we launched in the fall of 2009. As you read this issue,the first class of 28 students, drawn from 16 countries, isgraduating. With PSI we are attempting to reinvigorate thetraining of young theorists and the initial results of our exper-iment are highly promising.

Aside from research, an equally vital part of PIs mission is toshare the joy and the power of scientific discovery with thewider community through educational programs and events,and Greg Dick and John Matlock explain why we feel out-reach is so important.

We hope you enjoy this peek inside the Perimeter. Happyreading!

Rob Myers and Neil Turok, Guest Editors

FOREWORD

PHYSICS IN CANADA / VOL. 66, NO. 2 ( Apr.-June 2010 ) C 67

The contents of this journal, including the views expressed above, do not necessarily represent the views orpolicies of the Canadian Association of Physicists. Le contenu de cette revue, ainsi que les opinionsexprimes ci-dessus, ne reprsentent pas ncessairement les opinions et les politiques de lAssociationcanadienne des physiciens et des physiciennes.

A PEEK INSIDE THE PERIMETER INSTITUTE

PRFACE

68 C LA PHYSIQUE AU CANADA / Vol. 66, No. 2 ( avr. juin 2010 )

Nous avons t ravis daccepter linvitation de prparer cette di-tion thmatique de La Physique au Canada et nous esprons quevous apprcierez cet aperu des diffrentes activits de recherchede lInstitut Perimeter (PI).

Depuis ses dbuts, lespace-temps et la thorie des quanta ont tau cour des recherches de PI. Bien que ces sujets puissent voussembler abstraits au premier abord et un peu loign du monde telquon le connat, nous souhaitons quau fil de ces pages vousvoyiez qu PI nous avons toujours pour maxime que la nature etlexprimentation sont le meilleur guide du thoricien.

Une des caractristiques uniques de lInstitut est son groupeouvrant sur les fondements quantiques. Rob Spekkens et LucienHardy expliquent avec loquence les enjeux de leur domaine, endcrivant la fois les motivations et les impacts de ces recherch-es. Parmi celles-ci, on souligne la rcente confirmation expri-mentale du paradoxe de Hardy par des groupes aux universits deToronto et dOsaka, qui avec le temps pourraient engendrer desapplications pratiques. Christopher Fuchs prsente aussi sa per-spective sur une toute nouvelle approche sur le dcodage desnigmes de la mcanique quantique laide de la thorie desprobabilits baysienne.

La science de linformation quantique a merg en tant que rami-fication des fondements quantiques; son essor actuel nous prometde nouvelles technologies qui pourraient transformer notresocit. PI a jou un rle dterminant dans le lancement delInstitute for Quantum Computing (IQC) luniversit voisine deWaterloo, deux partenaires avec lesquels PI jouit de solides rela-tions synergiques. Dans deux des articles de cette dition, nouspouvons constater la synergie existant entre linformation quan-tique et les autres champs de la physique thorique. Dans leur arti-cle, Urbasi Sinha, Raymond Laflamme et leurs collgues de lIQCprsentent un compte rendu de leur exprience triples faisceauxau cours de laquelle ils ont vrifi les principes de base de lamcanique quantique en rponse une demande de Rafael Sorkinde PI. Un peu plus loin, vous pourrez lire larticle de DanielGottesman dans lequel il dcrit comment son travail sur les class-es de complexit algorithmiques quantiques lui a fourni des rsul-tats inattendus sur les verres de spin.

lgard du thme de lespace-temps, cher PI, nous prsentonsdeux premiers rapports totalement diffrents traitant des astresocclus. Considrs une certaine poque comme un jouet exo-tique dans le terrain de jeu des thoriciens, ces phnomnes degravit ultra puissante sont devenus le cheval de bataille des astro-physiciens modernes. Dans leur article, Luis Lehner et LathamBoyle dcrivent de nouvelles observations astrophysiques quipourraient nous dvoiler davantage sur les astres occlus grce leur impact sur la lumire et aux gaz emprisonns au sein de leursimmenses champs gravitationnels. En contraste, Bill Unruh, lundes titulaires mrites de la chaire de recherche de PI, souhaitedompter ces dynamos dans sa baignoire . Son article raconteremarquablement bien comment ses expriences entreprises dansun bassin deau pourraient fournir une meilleure comprhensionde la radiation de Hawking qui schappe des astres occlus etpeut-tre mme de la gravit quantique.

Bien que la gravit quantique explore la notion despace-temps une chelle infiniment petite, Lee Smolin et Sabine Hossenfelderdcrivent comment nous pourrions nanmoins y trouver des

empreintes de nature quantique en physique des courtes distancespar le biais dexpriences au cours desquelles on mesure lephnomne des chelles cosmologiques. De la mme faon, lesexpriences cosmologiques nous fournissent de prcieux indicesconcernant la physique de lunivers son stade primaire, qui deleurs cts peuvent nous fournir des pistes sur la thorie ultime dela nature. Avec cette motivation, Cliff Burgess dcrit comment ilpourrait trouver des empreintes de la thorie des cordes par lebiais de son impact sur linflation cosmique.

Lnergie sombre et lacclration actuelle de lunivers prsententune des plus grandes nigmes de la cosmologie actuelle. Ellesprocurent des indices vitaux lendroit de la thorie ultime et sup-posent une approche radicalement remodele de la thorie stan-dard en cosmologie, de la thorie sur la gravit de Einstein, ou desdeux. Niayesh Afshordi souligne ses propres travaux lendroitdun regard nouveau sur la gravit de grandes chelles,lesquelles ont des signatures cosmologiques intrigantes. Lesthoriciens des particules Maxim Pospelov et Brian Batell nousrelatent de passionnantes nouvelles propositions sur les propritsde la matire noire, lautre grande nigme qui domine lesphnomnes de grande envergure dans notre univers et qui pour-rait expliquer plusieurs nouveaux rsultats dobservations assezcurieux.

La physique des particules entre dans une re particulirementpassionnante au moment o le grand collisionneur de hadrons(LCN) du CERN commence explorer les nouvelles frontires delnergie en physique. Michael Trott nous dvoile la perspectivedun thoricien lgard des motivations, des dfis et de possiblesdcouvertes pour la physique que pourrait gnrer le LCN.Freddy Cachazo pour sa part nous dcrit son travail dans ledomaine de la thorie sur les fondements quantiques, lequel afourni, alors quon ne sy attendait pas, de nouveaux outils perme-ttant lanalyse des futures expriences en acclration. AlexBuchel, Rob Myers et Aninda Sinha nous relatent comment destechniques originales dveloppes dans le cadre de la thorie descordes nous permettront de comprendre une nouvelle phaseremarquable de la matire nuclaire.

Llment vital de la physique, on le sait, est lhumain, surtout nosjeunes gens talentueux. John Berlinsky nous propose un texteconcernant le programme de bourses dtudes internationales delInstitut Perimeter (PSI), un cours novateur de formation enrecherches pour tudiants de troisime cycle lanc lautomne2009. Au moment o vous lisez cette dition, le premier groupede 28 tudiants venant de 16 pays ont obtenu leurs diplmes. Avecle PSI nous tentons de revigorer la formation de jeunes thoricienset les premiers rsultats de notre exprience sont trs prometteurs.

part la recherche, une autre partie trs importante de la missionde PI est de partager la joie et la puissance de la dcouverte scientifique avec la communaut largie par lentremise de programmes ducatifs et dvnements; et Greg Dick etJohn Matlock expliquent pourquoi cest si important.

Nous souhaitons que vous apprciez cette incursion au sein delInstitut Perimeter.

Bonne lecture!

Rob Myers et Neil Turok, Rdacteurs honoraires

Chaque perce importante en science est ne dune imagination audacieuse. John Dewey

Peu importe la beaut de votre thorie et peu importe que vous soyez brillant; si a ne passe pas le stade de lexprimentation, cest que vous faites fausse route.

Richard P. Feynman

A LINTRIEUR DE LINSTITUT PERIMETER

erimeter Institute (PI) was founded just ten yearsago. At the time, to outsiders, its success seemedunimaginable. Why in Canada? Why choose suchan ambitious scientific focus? What could an

upstart young institute contribute to such a well-estab-lished field as theoretical physics? Where is Waterloo,anyway?

The best opportunities are often only obvious after thefact. With hindsight, it is clear that Canada has everythingneeded to create a world leading centre for theoreticalphysics. It has excellent universities and a strong physicscommunity. There is consensus in government around theneed for investment in basic research and highly qualifiedpersonnel. Canada is exceptionally welcoming to peoplefrom overseas and has a deep tradition of internationalism.And, of course, Canada is an underappreciated, vast andbeautiful country.

Why invest in theoretical physics? Taking the big pictureview, the argument is simple. Theoretical physics is a highimpact, low cost field. The breakthroughs made byNewton, Maxwell, Einstein, Bohr, and their descendantsnourished all the other sciences and spawned innumerabletechnologies, many of which form the basis for modernsociety. The field continues to drive the search for newquantum technologies, and a better understanding of theuniverse. All the researchers need is food, coffee, black-boards, computers and other researchers to talk to. Theirwork motivates and drives big science experiments likeLHC and LIGO, and helps analyse and interpret the mas-sive data sets generated. Theoretical physics is the mostcost-effective field in all of science, for the simple reasonthat the human mind is simultaneously the most powerfulpiece of apparatus we know of and the cheapest to oper-ate!

But is there much one can do to improve the odds ofprogress in such a fundamental field? The great discover-ies are almost always completely unplanned, resultingfrom a combination of daring, luck and new technical ortechnological opportunities or unexpected observations.Perhaps we can do no better than wait, for another Einsteinor Bohr to make the next big breakthrough. PIs foundersthought we could do better. Mike Lazaridis and PIs firstBoard members and supporters, and Howard Burton, theInstitutes first Director, saw a giant opportunity forWaterloo, for Canada, and for the world, precisely becauseno-one else had the audacity to try. Drawing on the wis-dom of the worlds top theorists, they built the institutionon foundations of excellence and the highest ambitions.

P

AN EXPERIMENT IN THEORETICAL PHYSICS

Neil Turok ,Director, PerimeterInstitute forTheoretical Physics,31 Caroline St. N.,Waterloo, ONN2L 2Y5

From the start PI took as its scientific focus a better under-standing, and reconciliation, of the two pillars of twentiethcentury physics: quantum theory and spacetime. The insti-tute took the unusual step of deliberately promoting com-peting approaches within, because it is precisely throughthe clash of different approaches that one learns mostquickly about the strengths and weaknesses of each. ThusPI has strong groups both in string theory and in quantumgravity, and the lively interaction between the two hasearned the institute a reputation as an open-minded andstimulating place to visit and to work.

Next, while we cannot anticipate exactly where the nextbreakthroughs might occur, we can certainly try to focusour efforts on the most promising areas. Here, PIs flexi-bility is a huge asset. Areas which dont fit within the tra-ditional boundaries of a university department or centrecan be easily accommodated within PIs highly interdisci-plinary community. As an example, PIs first focus onfoundational quantum theory proved remarkably far-sight-ed, making the institute a natural hub for the new field ofquantum information and allowing us to help foster ourexperimental partner institute, the Institute for QuantumComputing (IQC) at the University of Waterloo. Today, PIand IQC together form a powerful magnet attracting thebest researchers in this exciting field.

Looking forward, I believe there are several naturalresearch foci in which PI can become world-leading. Oneis what one might call high-powered quantum field the-ory, namely the attempt to develop more powerfulapproaches to our fundamental understanding of quantumfields. The latter describe all of nuclear and particle theo-ry, condensed matter, and early universe cosmology.Therefore, foundational progress in quantum field theorywill have an impact across all of physics. We have grow-ing strength at PI in this area, and are well on our way tomaking this a world-leading effort.

A second strongly emerging theme is the connectionbetween theoretical work at PI and large experimentalefforts like LHC and LIGO. Even a small number of the-orists, working in a focused way, can have an enormousimpact on these massive international experiments, bypointing out new signals to look for, better ways to analyseand interpret the data, and key physics targets to guide thedesign of new experiments.

A third emerging theme is the study of black holes andgravitational wavesthe next great frontier in astronomyand cosmology. Ten years from now, we hope to be rou-tinely detecting bursts of gravitational waves emitted by

EDITORIAL


The contents of this journal, including the views expressed above, do not necessarily represent theviews or policies of the Canadian Association of Physicists. Le contenu de cette revue, ainsi que lesopinions exprimes ci-dessus, ne reprsentent pas ncessairement les opinions et les politiques delAssociation canadienne des physiciens et des physiciennes.

DITORIAL


colliding black holes, and planning even more ambitiousexperiments, using gravitational waves to look all the way backto the beginning of the Universe. Which, of course, is anotherfocus of our work at PI.

The fundamental unity and coherence of theoretical physics isa source of research strength, as PI expands to draw on comple-mentary insights from across the whole spectrum of physics.We have nascent efforts in particle physics and cosmology. Weare also looking to grow in condensed matter, especially in therealm of strongly quantum-correlated systems, an area whichconnects well to our existing strengths as well as to emergingtechnological frontiers.

Science has become an increasingly collaborative venture.Within this context, PI seeks to be a resource for physics inCanada, and internationally. We are looking to grow our linkswith the strong community of physicists in Canada, withworld-class theory centers like the Canadian Institute forTheoretical Astrophysics, and world-class experimental proj-ects such as those at TRIUMF and SNOLAB. PIs AffiliateMember program, which draws in physics faculty from acrossCanada to visit and participate in the institutes research activ-ities, now counts 96 members. By working together, I believewe can create a win-win situation which allows Canada toobtain maximum benefit from its support of basic physics.

In a wider sense, PI is striving to be a global center andresource. It is clearly important that we collaborate with otheradvanced centers, but I believe it is even more vital that wesupport emerging centers in the developing world, where enor-mous pools of talent lie waiting to be unlocked. By helping topromote the cause of these centers, and by offering to sharePIs substantial institution-building expertise, I believe we canmake a substantial contribution both to the future of physicsand to international development.

The progress of theoretical physics rests, more than anything,on brilliant young people. One of our key objectives is there-fore to support a flow of youthful talent through PI. For thisreason, we launched Perimeter Scholars International (PSI), aninnovative Masters program designed to attract talented stu-dents from around the world into theoretical physics, and tobring them to the cutting edge of research as quickly as possi-ble. This year, 28 students from 16 countries will graduate, andwe are delighted with their progress. Many Canadian facultyhave been involved in lecturing, and in supervising projects. Inthe future, we hope PSI will become seen as a valuable newmodel for teaching theoretical physics, and a global stimulusfor the field.

PI already hosts the largest group of independent postdoctoralresearchers in theoretical physics in the world. We are nowrecruiting at the highest level, competing successfully for tal-ent with the strongest institutions internationally. We are alsobuilding our strength in terms of senior, establishedresearchers. Over the last eighteen months, we have recruited20 of the worlds top theoretical physicists as DistinguishedResearch Chairs (DRCs) at Perimeter Institute. They includeboth bright young stars such as Patrick Hayden (McGill) andGuifre Vidal (Queensland) and world leading figures such asYakir Aharonov (Tel Aviv), Stephen Hawking (Cambridge) andAshoke Sen (Harish Chandra Institute, Allahabad). They spanan enormous range of expertise, from quantum foundations

through particle physics, condensed matter, cosmology toquantum gravity and black holes. While retaining their perma-nent positions at home, our DRCs visit PI for extended periods(typically one to two months per year) to do research, collabo-rate and in some cases to teach on PSI. There has been excel-lent uptake of these positions, and the continuous flow of topinternational researchers adds to the excitement of working atPI. We were especially delighted that two of our DRCs werewidely touted as potential Nobel prize winners last year. In theend, neither won but Willard Boyles prize was certainly greatcompensation, and his remarks about the importance of curios-ity driven research, and of special places for science like BellLabs in its heyday inevitably evoked comparisons with PI.

To accommodate all of this growth, PIs iconic black boxbuilding is now being substantially expanded with the StephenHawking Centre at Perimeter Institute (see cover), which willbe completed next summer. Our expanded facility will allow PIto accommodate around 250 researchers, making it by somemargin the largest facility for foundational theoretical physicsin the world.

One of the smart things that Mike Lazaridis and HowardBurton did in founding PI was to give public outreach a veryhigh priority. The scale of these effortsfor students, teachers,and the publicis something that really sets PI apart. Just oneexample is our public lecture series, which attracts an audienceof six hundred plus to a local high school auditorium eachmonth. Last fall, we held a science festival called Quantum toCosmos: Ideas for the Future. It was a huge, risky undertaking:a giant tent in Waterloo town square filled with hands-onexhibits and 3-D film narrated by Stephen Hawking, the worldpremiere of The Quantum Tamers, a PI-produced documentaryon quantum mechanics that is anything but the usual documen-tary fare, concerts, and a science film festival. Last but notleast, TV Ontario came and broadcast five nights of their cur-rent affairs program, The Agenda with Steve Paikin, rightfrom PIs atrium. Oh yes, and they filmed and helped broadcast30 talks and panels with 80 presenters over 10 days, all liveonline in high definition, something never before attempted onthis scale.

As I walked down the hill to PI, to introduce the first lecture,I must admit I was nervous. So much could go wrong, but allwe could do was trust to the professionalism of our team.I neednt have worried. The festival exceeded our wildesthopesover 40,000 people came to events here, and over amillion viewers on TV and online (if you havent seen any ofthe talks yet, you can still see them at www.q2cfestival.com).At every event, questions were encouragedwhether fromthose present, or those online. It was an all out celebration ofwhere curiosity can lead.

Quantum to Cosmos was a stunning success, and I see it asemblematic of what PI is trying to do and where it is going: tocarefully create the best initial conditions, disregard the limitsof common wisdom and convention and shoot for the stars.Sometimes, magic can happen.

Neil TurokGuest Editor, Physics in Canada

Readers comments on this editorial are more than welcome.

EDITORIAL


UNE EXPRIENCE EN PHYSIQUE THORIQUEIl ny a que dix ans que lInstitut Perimeter (PI) a t cr. cemoment-l, pour les gens de lextrieur, son succs semblaitinimaginable. Pourquoi au Canada? Pourquoi se concentrer sur unobjectif scientifique aussi ambitieux? Quest-ce quun institutnophyte pouvait apporter un secteur aussi bien tabli que laphysique thorique? Au fait, o se trouve Waterloo?

Les meilleures perspectives ne se peroivent souvent que devantle fait accompli. Avec le recul, il ne fait aucun doute que leCanada dispose de tout ce quil faut pour crer un centre dephysique thorique denvergure mondiale. Le pays est dot dex-cellentes universits et dune solide communaut dans le secteurde la physique. Un consensus se dgage au gouvernement sur lesbesoins dinvestir dans la recherche fondamentale et dans unemain-doeuvre hautement qualifie. Le Canada est particulire-ment accueillant lendroit de gens venant doutre-mer et a unetradition internationaliste bien tablie. Et bien sr, le Canada estun pays vaste et magnifique que lon napprcie pas toujours sajuste valeur.

Pourquoi investir dans la physique thorique? Vue dans sonensemble, la raison est simple. La physique thorique est unchamp assez peu coteux, dont les impacts importants se fontnanmoins sentir. Les perces russies par Newton, Maxwell,Einstein et Bohr et leurs descendants ont aliment toutes les autressciences et fait natre une multitude de technologies, dontplusieurs forment aujourdhui la base de notre socit moderne.Le champ est lavant-garde de la recherche de nouvelles tech-nologies quantiques et dune meilleure comprhension de lu-nivers. Tout ce que les chercheurs ont besoin, cest de nourriture,de caf, de tableaux noirs, dordinateurs et dautres chercheursavec qui discuter. Leur travail motive et mne terme des expri-ences comme celles du LCN et du LIGO, et aide lanalyse et linterprtation de lensemble des donnes qui y sont gnres.La physique thorique est le champ scientifique le plus efficientpour la simple raison que le cerveau humain est la fois lappareille plus puissant que lon connaisse et le moins cher utiliser!

Par contre, y a til quelque chose quon peut faire pour amliorerles chances du progrs dans un tel champ fondamental? Lesgrandes dcouvertes se sont presque toutes produites sans quellesnaient t planifies, rsultant de laudace, de la chance, de nou-velles occasions fournies par la technologie ou dobservationsimprvues. Peut-tre que nous ne pouvons quattendre la venuedun nouvel Einstein ou dun Bohr pour parvenir aux prochainesgrandes perces. Les fondateurs de PI croient plutt quon peutfaire mieux que . Mike Lazaridis et les membres du premierconseil dadministration avec ceux qui les appuient, ainsi queHoward Burton, le premier directeur de lInstitut, y ont vuune grande occasion pour Waterloo, pour le Canada et pour lemonde, prcisment parce que personne ne sy tait risqu. Ensappuyant sur la sagesse des thoriciens mondiaux mrites, ilsont bti cette institution sur des bases dexcellence et dambitionsleves.

Ds le dpart, PI a choisi comme concentration scientifique unemeilleure comprhension et une rconciliation des deux piliers dela physique du vingtime sicle : la thorie des quanta et de le-space-temps. LInstitut a choisi une voie inhabituelle et dlibr-ment audacieuse mettant de lavant en son sein deux points de vuequi sopposent, car cest prcisment en confrontant desapproches diamtralement opposes quon peut apprendre plusrapidement les forces et les faiblesses de chacune de ces options.

Par consquent, lInstitut est dot de groupes forts autant du ctde la thorie des cordes que du ct de la gravit quantique; lavivacit des interactions ainsi engendre a permis lInstitut de secrer la rputation dtre un endroit stimulant et desprit ouvertquil est agrable de frquenter et o il fait bon travailler.

De plus, bien que ne puissions pas prvoir prcisment o se pro-duirons les prochaines perces, nous pouvons coup sr concen-trer nos efforts dans les domaines les plus prometteurs, et cest ce niveau que la souplesse de PI lui procure un avantage de taille.Les domaines qui ne se trouvent pas dans le registre traditionneldes dpartements de physique des universits ou des centres peu-vent facilement trouver leur place au sein de la communauthautement interdisciplinaire de lInstitut. Par exemple, la vocationprincipale de lInstitut lendroit de la thorie des fondementsdes quanta a prouv quelle tait tourne vers lavenir, faisant delInstitut la pierre angulaire du nouveau domaine de linformationquantique et nous permettant daider lintgration de notre insti-tut exprimental partenaire, lInstitute for Quantum Computing(IQC) luniversit de Waterloo. LIQC et PI forment aujourdhuiun puissant ple dattraction auprs des meilleurs chercheurs dece domaine passionnant.

Quand je regarde ce que lavenir nous rserve, je crois que PIpourra tenir un rle davant-garde dans plusieurs domainesnaturels de recherches. Lun de ceux-ci, que lon pourrait qualifi-er de domaine de la thorie des champs quantifis de haute puis-sance , tente de dvelopper des approches plus nergiques l-gard de notre comprhension de base des champs quantifis. Cedernier dcrit lensemble des thories nuclaires, des particules,de la matire condense et de la cosmologie de lunivers primaire.Par consquent, le progrs des bases de la thorie des champsquantifis aura un impact sur la physique dans son ensemble.LInstitut est dot dune force croissante dans ce domaine est surla voie den devenir le fer de lance mondial.

Un deuxime thme mergeant avec vigueur est la connexionentre le travail thorique de PI et les grands efforts exprimentauxcomme ceux du LCN et du LIGO. Mme un nombre restreint dethoriciens, travaillant en convergence, peut avoir un impactnorme sur ces expriences internationales en signalant de nou-veaux lments surveiller, en tablissant de meilleures faonsdanalyser et dinterprter les donnes et en prcisant les objectifsphysiques principaux permettant de guider la forme que prendrontles nouvelles expriences.

Un troisime thme qui se profile est ltude des astres occlus etdes ondes de gravit la prochaine grande frontire en astronomieet en cosmologie. Nous souhaitons que dans une dizaine dannes,nous puissions dtecter de faon routinire les vagues dondes degravit mises par la collision dastres occlus et nous planifionsmme des expriences encore plus ambitieuses qui nous permet-tront dutiliser les ondes de gravit afin de pousser les observa-tions aussi loin quau dbut de lunivers. Il sagit bien sr duneautre facette du travail effectu lInstitut.

Lunit des fondements et la cohrence de la physique thoriquesont la source dnergie de la recherche qui permet lInstitut depoursuivre son dveloppement et son avance dans des sphrescomplmentaires venant du spectre complet de la physique. Nousfaisons actuellement nos premiers pas en physique des particuleset en cosmologie; nous comptons aussi voluer du ct de laphysique de la matire condense, particulirement dans ledomaine des systmes fortement quantifis, une zone qui corre-

DITORIAL


spond bien nos forces actuelles ainsi quaux frontires tech-nologiques en mergence.

La science est de plus en plus une entreprise de collaborations.Dans ce contexte, PI cherche se positionner comme ressourcedans le domaine de la physique la fois lchelle canadienne etinternationale. Nous voulons tisser des liens avec la vigoureusecommunaut canadienne de la physique, avec des centres dephysique thorique denvergure mondiale comme lInstitut cana-dien dastrophysique thorique et avec des projets exprimentauxde calibre international comme ceux du TRIUMF et du SNOLAB.Le programme pour membres affilis de PI, qui incite les facultsde physique des universits partout au Canada visiter lInstitutet participer ses activits en recherche, compte maintenant 96membres. Cest en travaillant ensemble que nous pourrons crerune situation gagnante pour tous qui permettra au Canada de tirerle maximum de bnfices de son soutien la physique fondamen-tale.

Dune faon gnrale, PI sefforce devenir un centre et uneressource mondiale dans son domaine. Il est trs important quenous collaborions avec les autres centres de notre calibre, mais jecrois par-dessus tout quil est vital que nous soutenions les centresen mergence travers le monde o se trouve une norme sourcede chercheurs talentueux qui ne demandent qu spanouir. Enaidant ces centres prendre leur envol et en offrant le savoir-fairedtablissement institutionnel de notre organisation, je crois quenous pouvons apporter une solide contribution lavenir de laphysique ainsi quau dveloppement international.

Le progrs de la physique thorique repose avant tout sur de bril-lants jeunes gens. Un des principaux objectifs de lInstitut estnotamment de soutenir larrive de ces nouveaux talents. Cestpourquoi nous avons lanc le programme de bourses interna-tionales Perimeter Scholars International (PSI), un programmeinternational de Matrise novateur conu pour attirer des tudiantstalentueux do quils viennent la physique thorique et de rapi-dement les amener la fine pointe de la recherche. Cette anne,28 tudiants venant de 16 pays obtiendront leur diplme et noussommes enchants de leurs progrs. Plusieurs facults de nos uni-versits canadiennes ont offert des projets de confrences et desupervision. Nous esprons qu lavenir, le PSI sera perucomme un nouveau modle valable pour lenseignement de laphysique thorique et comme un lment de stimulation dans ledomaine en gnral.

LInstitut accueille dj le plus important groupe de chercheurspostdoctoraux indpendants en physique thorique au monde.Nous recrutons actuellement avec succs des gens talentueux dansles hautes sphres, en concurrence avec les meilleures institutionsinternationales. Nous btissons aussi notre force en matire dechercheurs tablis bien aguerris. Au cours des derniers dix-huitmois, nous avons recrut 20 des meilleurs physiciens au monde enphysique thorique comme titulaires mrites de la chaire derecherche de l'Institut Perimeter. On y retrouve Patrick Hayden(McGill), Guifre Vidal (Queensland) ainsi que des figures mar-quantes du domaine comme Yakir Aharonov (Tel-Aviv), StephenHawking (Cambridge) et Ashoke Sen (Harish Chandra Institute,Allahabad). Ensemble, ils embrassent une norme tendue dex-pertise, des fondements quantiques la physique des particules, la physique de la matire condense, la cosmologie, la gravitquantique et aux astres occlus. Bien quils maintiennent tous leurposte permanent chez eux, nos titulaires sont en visite lInstitutpour de longs sjours (gnralement dune dure allant de deuxmois une anne complte) pour y faire de la recherche, y colla-borer et parfois mme enseigner aux PSI. Lintrt suscit par cestitulaires a t excellent, et le flot constant de chercheurs de cali-

bre international a renchri la passion de travailler lInstitut.Nous sommes particulirement fiers que deux de nos titulairesaient t approchs avec insistance comme rcipiendaires poten-tiels du prix Nobel de lanne passe. Aucun deux na finalementobtenu le prix, mais le prix qua obtenu Willard Boyle a certaine-ment compens; ses remarques lgard de la recherche stimulepar la curiosit et des endroits de science spciaux comme aucours de lge dor des Bell Labs, ont invitablement voqu lacomparaison avec PI.

Afin daccommoder tout cette croissance, la bote noire , lim-meuble icne de lInstitut prend maintenant une expansion impor-tante avec lajout du Centre Stephen Hawking lInstitutPerimeter (voir la couverture). Ce nouveau centre, qui sera achevlt prochain, permettra PI daccueillir environ 250 chercheurs,ce qui en fera dans une certaine mesure, le plus grand centre dephysique thorique fondamentale du monde.

Un des cts brillants du projet de Mike Lazaridis et de HowardBurton lorsquils ont fond lInstitut a t d'accorder beaucoupdimportance la proximit avec le public. La valeur de cesefforts auprs des tudiants, des professeurs et du public, posi-tionne PI dans une classe part. Un exemple de cette situation estnotre srie de confrences publiques qui attirent un auditoire deplus de six cents personnes lauditorium de lcole secondairelocale chaque mois. Lautomne dernier, nous avons tenu un festi-val des sciences intitul Du quantum au cosmos : des idesdavenir . Ctait une norme entreprise, assez risque : unchapiteau mont au square municipal rempli de stands interactifs,un film en 3D comment par Stephen Hawking, la premire mon-diale du film documentaire The Quantum Tamers produit parlInstitut et traitant des mcanismes quantiques (qui navait riendun documentaire classique), des concerts et un festival de filmsscientifiques; et pour finir, rien de moins que TV Ontario qui taitsur place pendant 5 soires pour la diffusion de son mission daf-faires publiques The Agenda with Steve Paikin en direct delatrium de lInstitut. Ils ont dailleurs film et aid diffuser 30interviews avec 80 personnes au cours des 10 journes, toutes endirect et en haute dfinition, rien de si important navait jamais tentrepris!

Je dois admettre qualors que je descendais vers PI pour introduirema premire confrence la nervosit ma gagn. Tant de chosespouvaient draper, mais tout ce que lon pouvait faire tait de sefier au professionnalisme de notre quipe. Je naurais pas d menfaire. Le festival a dpass nos attentes les plus optimistes, plus de40 000 personnes ont particip nos activits et plus dun millionde tlspectateurs ou dinternautes ont pu voir les interviews (sivous ne lavez pas fait, vous pouvez les regarder au www.q2cfes-tival.com). Lors de toutes les activits, on incitait les participants poser des questions, que ce soit sur place ou en ligne. Ce fut uneclbration totale de la curiosit.

Le festival Du quantum au cosmos : des ides davenir aobtenu un succs stupfiant et je le considre maintenantcomme le symbole de la vocation de PI et de la direction quilveut tracer : crer avec soin les meilleures conditions de dpart,ignorer les limites habituelles des conventions et de la sagesse etviser les toiles! Il arrive que la magie soit au rendez-vous.

Neil TurokRdacteur honoraire, La Physique au Canada

Les commentaires de nos lecteurs au sujet de cet ditorial sont bien-venus.

NOTE: Le genre masculin na t utilis que pour allger le texte.

uantum theory is a peculiar creature. It was bornas a theory of atomic physics early in the twen-tieth century, but its scope has broadened overtime, to the point where it now underpins all of

modern physics with the exception of gravity. It has beenverified to extremely high accuracy and has never beencontradicted experimentally. Yet despite its enormous suc-

SUMMARY

Quantum foundations is the field ofphysics that seeks to understand what quan-tum theory is telling us about the nature ofreality. Researchers hope to answer ques-tions such as: What do the elements of themathematical formalism of quantum theoryrepresent? From what physical principlescan the formalism be derived? What are theprecise ways in which a quantum world dif-fers from a classical world and other possi-ble worlds? Progress on these questions islikely to come both from an operationalapproach, wherein one characterizes a theo-ry entirely in terms of the predictions formacroscopic experiments described usingeveryday concepts, and from a realistapproach, wherein one seeks to find deeperexplanations for these predictions in termsof simple entities and abstract concepts. Weillustrate the practical significance of foun-dational research by recalling the role that itplayed as a spawning ground for the field ofquantum information science, and weexplain why we think that it will have a simi-lar role to play in unifying quantum theorywith general relativity.

Q

BY LUCIEN HARDY AND ROBERT SPEKKENS

WHY PHYSICS NEEDS QUANTUM FOUNDATIONS

Lucien Hardy ([email protected]),

and

Robert Spekkens(rspekkens@ perimeterinstitute.ca),

Perimeter Institute forTheoretical Physics,31 Caroline St. N.,Waterloo, ON,N2L 2Y5

cess, there is still no consensus among physicists aboutwhat this theory is saying about the nature of reality. Thereis no question that quantum theory works well as a tool forpredicting what will occur in experiments. But just asunderstanding how to drive a car is different from under-standing how it works or how to fix it should it breakdown, so too is there a difference between understandinghow to use quantum theory and understanding what itmeans. The field of quantum foundations seeks to achievesuch an understanding. In particular, it seeks to determinethe correct interpretation of the quantum formalism. It alsoseeks to determine the principles that underlie quantumtheory. Why do we have a quantum world instead of aclassical world or some other kind of world entirely?

There are many motivations for pursuing foundationalresearch. One is the development of quantum technolo-gies, such as quantum computation and quantum cryptog-raphy. A better understanding of the theory facilitates theidentification and development of these new technologies,the harnessing of the power of nonclassicality. Anothermotivation is that quantum theory is likely not the end ofthe road. If we are to move beyond it, then it is importantto know which parts can be changed, generalized or aban-doned. Finally, there are the personal motivations of indi-vidual researchers: because quantum theory is very myste-rious and counterintuitive and surprising and seems todefy us to understand it. And so we take up the challenge.

OPERATIONALISM AND REALISM

Broadly speaking, researchers in quantum foundations canbe divided into two camps. There are the operationalistsand there are the realists. For the operationalist, operatorsin Hilbert space represent preparation and measurementprocedures, specified as lists of instructions of what to do

FEATURE ARTICLE


WHY PHYSICS NEEDS ... (HARDY AND SPEKKENS)


in the lab. They are recipes with macroscopic activities asingredients. The theory merely specifies what probabilities ofoutcomes will be observed when a given measurement followsa given preparation. For the realist, there is some deeper reali-ty underlying the equations of quantum theory that ultimatelyaccounts for why we see the relative frequencies we do. Doesthe wave function describe this reality? Or are there extra hid-den variables in addition to the wave function needed to fullydescribe a quantum system? These are the sorts of questions therealist ponders.

A classic example of the power of applying operational think-ing is Einsteins approach to special relativity. By carefullyconsidering how to synchronise distant clocks, he was led toabandon the hitherto cherished notion of absolute simultaneity.A good example of the successful application of realism is theatomic hypothesis. In this case, John Dalton and others wereright to insist on the reality of atoms (in opposition to opera-tionalists such as Ernst Mach). It led to a theory for Brownianmotion (Einstein again), the theory of statistical mechanics,and ultimately much of modern physics.

Historically, both approaches were important in the develop-ment of quantum theory. Heisenbergs 1925 paper on matrixmechanics, which ushered in the modern age of quantum theo-ry, began with the sentence, The present paper seeks to estab-lish a basis for theoretical quantum mechanics founded exclu-sively upon relationships between quantities which in principleare observable. This was operational thinking. In parallel tothis, de Broglie posited the existence of waves to describequantum phenomena and Schrdinger found an equation fortheir motion. This was realist thinking.

In modern research into the foundations of quantum theory,both operationalism and realism are alive and well. By think-ing operationally, a general mathematical framework has beendeveloped which can accommodate a wide variety of proba-bilistic theories. Quantum theory fits very comfortably into thisframework as a special case and so can be easily understood inoperational terms. Much progress has been made recently inunderstanding the deeper mathematical structure of quantumtheory in the context of this mathematics of operationalism. Forexample, many features of quantum theory (such as the impos-sibility of building a machine that can clone quantum states)turn out to be features of any non-classical probabilistic the-ory. These tools also contribute to the program of reconstruct-ing quantum theory, that is, deriving its abstract mathematicalformalism from natural postulates, just as the Lorentz transfor-mations are derived from Einsteins postulates for special rela-tivity.

But operationalism is not enough. Explanations do not endwith detectors going click. Rather, the existence of detectorsthat click is the sort of thing that we can and should look to sci-ence to explain. Indeed, science seeks to explain far more thanthis, such as the existence of human agents to build these detec-tors, the existence of an earth and a sun to support these agents,

and so on to the existence of the universe itself. The only wayto meet these challenges is if explanations do not bottom outwith complex entities and everyday concepts, but rather withsimple entities and abstract concepts. This is the view of therealist. Without adopting some form of realism, it is unclearhow one can seek a complete scientific world-view, incorporat-ing not just laboratory physics, but all scientific disciplines,from evolutionary biology to cosmology. It is true of coursethat all of our evidence will come to us in the form of macro-scopically observable phenomena, but we need not and shouldnot restrict ourselves to these concepts when constructing sci-entific theories. For the realist, then, we need an interpretationof quantum theory.

There are already plenty of candidates to choose from. There isthe pilot wave model of Louis de Broglie and David Bohm inwhich the wave function guides the motion of actual particlesaccording to a well defined equation. There is the many worldsinterpretation of Hugh Everett III in which the universe isregarded as splitting into many copies every time the wave-function evolves into a superposition of distinct situations.There are also collapse models in which extra terms are addedto the Schrdinger equation to cause a collapse of the wave-function when sufficiently macroscopic possibilities becomesuperposed. Many more ideas for interpretations are in themaking today. Cases have been made for each by their respec-tive proponents, but none has yet proven sufficiently com-pelling to achieve a scientific consensus. So research on theseissues continues.

Ultimately, we expect that both operationalism and realism willplay an important methodological role in future research.Operationalism is, at least, a useful exercise for freeing themind from the baggage of preconceptions about the world, asEinstein did when he showed that the notion of absolute simul-taneity was unfounded. As such it can provide a minimal inter-pretation, some conceptual and mathematical scaffolding onwhich to build. On the other hand, the extra commitments, con-straints and details of a realist model can also be a virtue.Realist models are more falsifiable, they typically suggest newand interesting questions (questions that may uncover novelconsequences of a theory), and they often suggest avenues formodifying and generalizing the theory.

THE FOUNDATIONAL ROOTS OF QUANTUMINFORMATION THEORY

The field of quantum foundations provides many examples ofhow basic research guided by a desire for deeper understand-ing can lead to discoveries of great practical interest. Quantuminformation science serves as the best example. To first approx-imation, it was born of two communities: on the one hand,computer scientists and information theorists, and on the other,physicists thinking about the foundations of quantum theory.If the name of a field indicated its parentage, then the quan-tum in quantum information would refer to quantum foun-dations.

WHY PHYSICS NEEDS ... (HARDY AND SPEKKENS)AA


Since those early days, there has been a slow but steady marchtowards quantum technologies becoming practical. Quantumcryptographic systems, for instance, are now available com-mercially. Meanwhile, progress on the theoretical side hasshown how one can achieve stronger forms of security thanpreviously conceived. One of the most celebrated cryptograph-ic applications of quantum theory is key distribution: the abili-ty to establish a shared secret key among distant parties over apublic channel in such a way that one can reliably detect thepresence of an eavesdropper. Recent work has shown thatunder the very conservative assumption that superluminal sig-nalling is impossible, one can achieve key distribution even ifthe would-be eavesdropper has the advantage of providing thevery devices that are used by the communicating parties [1,2].

This is practical stuff, but the path that led to such results startswith foundational research. In 1964, John Bell was consideringthe question of whether there is an interpretation of quantumtheory in terms of hidden variables. He had been pondering theargument by Einstein, Podolsky and Rosen in favor of theincompleteness of the quantum description and thinking aboutvarious theorems that purported to show the impossibility ofsuch completions. He was also studying the pilot wave modelof de Broglie and Bohm. He noted that this theory postulatedsuperluminal causal influences and wondered whether thismight be true of all realist models of quantum theory. Once thequestion was asked, it was not long before he was able to provethat this is indeed the case C a theorem that now bears hisname [3].

Bells theorem is a profound result because it demonstrates atension between the two pillars upon which modern physics isbuilt C quantum physics and relativity theory. Since its discov-ery, physicists have been puzzling over it. One such person wasArtur Ekert. In 1991, he realized that the statistical correlationscentral to Bells theorem could be used to achieve secure keydistribution [4]. Although a different quantum protocol for keydistribution had been developed seven years earlier by CharlieBennett and Gilles Brassard [5], it was Ekerts protocol that ulti-mately led to the results mentioned above C the possibility ofachieving security regardless of the provenance of the devices.

The theory of entanglement C the property of quantum statesthat is critical to the Einstein-Podolsky-Rosen argument andBells theorem C is another example of the practical payoff offoundational thinking. In 1980, William Wootters had just com-pleted a Ph.D. thesis on a foundational question: from whatprinciples can the Born rule of quantum theory be derived?Important to his considerations was a task known as quantumstate tomography. This is an attempt to infer the identity of aquantum state by implementing many different measurementson a large number of samples of it. In the fall of 1989, AsherPeres, another foundational researcher, asked whether jointmeasurements on a pair of systems might yield better tomogra-phy than separate measurements. They were able to find strongnumerical evidence that this was indeed the case [6]. It seemed,therefore, that if a pair of similarly prepared particles was sep-arated in space, an experimenter would be less able to identify

their state than if they were together. In other words, there is alimit to how much information about the state can be accessedby local means C a kind of nonlocality. In 1992, CharlieBennett heard a talk by Wootters on the subject and askedwhether the nonlocality that seemed to be inherent in entangledstates might provide a way of achieving state tomography onseparated systems with the same success that could be achievedif they were proximate.

Again, once the question was asked, it took only a few days forWootters, Bennett, Peres and their co-workers (Gilles Brassard,Claude Crpeau and Richard Jozsa) to answer it. Yes, it couldbe done [7]. The key insight was that by consuming a maximal-ly entangled state (i.e. using it in a manner that ultimatelydestroys it), the quantum state of a system could be transferredfrom one party to another distant party using only local opera-tions and classical communication. The trick was dubbedquantum teleportation by its authors. Several discoveries inquantum information theory (including Ekerts key distri-bution protocol) had shown that entanglement was useful, andwith the discovery of teleportation, it became especially obvi-ous: entanglement was a resource. This change in perspectiveprompted researchers to ask many new and interesting ques-tions about entanglement. The result has been a dramaticincrease in our understanding of the phenomena, leading toapplications across all subdisciplines of quantum informationscience (cryptography, communication and computation) andfurther afield (for instance, in new density matrix renormaliza-tion group methods for simulating quantum many-body sys-tems).

One final story. Early in the history of quantum informationtheory, when most researchers were still thinking about quan-tum theory as imposing upon us additional limitations relativeto what we would face in a world that was governed by a clas-sical theory, David Deutsch was thinking differently. He waslooking to identify tasks for which quantum theory provided anadvantage. In the mid-eighties, his unique perspective led himto write one of the very first articles on quantum computation,an article that prepared the ground for important subsequentdiscoveries [8]. What led Deutsch to perform this seminalwork? He was thinking about the information-processing con-sequences of Everetts many worlds interpretation of quantumtheory.

QUANTUM FOUNDATIONS MEETS QUANTUMGRAVITY

Perhaps the holy grail of modern physics is a theory of quan-tum gravity. We need to find a theory that reduces to quantumtheory in one limit and to general relativity in another, and thatmakes new predictions which are subsequently verified inexperiments. This has been an open problem since the birth ofquantum theory, yet we still do not have a theory of quantumgravity. The problem is difficult because there are deep concep-tual differences between general relativity and quantum theory.Consequently, the two theories have very different mathemati-cal structures.

WHY PHYSICS NEEDS ... (HARDY AND SPEKKENS)


In the past, when two less fundamental theories have been uni-fied into a deeper, more fundamental theory, the unification hastypically required an entirely new mathematical framework,motivated by conceptual insights from the two component the-ories. If this is the case for quantum gravity, then foundationalthinking is likely to be useful. Does quantum gravity call for anew type of probabilistic theory? Which of the postulates ofquantum theory (in whatever formulation) will have to be mod-ified or abandoned, if any? A similar type of conceptual think-ing about the foundations of general relativity is also likely tobe significant. If we have a mathematical framework that isrich enough to contain a theory of quantum gravity (in muchthe same way that the mathematics of Hilbert space is sufficientfor quantum theory and the mathematics of tensor calculus issufficient for general relativity) then we could expect that a fewsuitably chosen postulates would narrow us down to the righttheory. It is in the construction of this framework and the selec-tion of these postulates that the conceptual and mathematicaltools of quantum foundations are likely to be useful.

SEND OFF

The field of quantum foundations does not merely exist to tidyup the mess left behind after the physics has been done. Ratherit should be regarded as part and parcel of the great project oftheoretical physics: to gain an ever better understanding of theworld around us.

In particular, researchers in the field are striving to achieve adeeper understanding of the conceptual and mathematicalstructure of quantum theory. It is a testament to the importanceof this sort of pure enquiry that the ideas of quantum founda-tions have found such a compelling application in the field ofquantum information science. It was John Bell thinking abouthidden variables that ultimately led to many practical results inquantum cryptography; it was William Wootters asking Whythe Born rule? that guided us down the last stretch of the paththat culminated in understanding entanglement as a resource,and it was David Deutsch thinking about the many worldsinterpretation of quantum theory that laid the foundations ofquantum computing.

We should not expect that quantum information theory will bethe only substantial application of ideas from quantum founda-tions. They may well play a significant role in the constructionof a theory of quantum gravity. They may even spawn entirelynew fields of research that we cannot currently predict.Thinking about foundations pays off in the long run. DavidMermin once summarized a popular attitude towards quantumtheory as Shut up and calculate!. We suggest a different slo-gan: Shut up and contemplate!

REFERENCES

1. J. Barrett, L. Hardy, and A. Kent, No Signaling and Quantum Key Distribution, Phys. Rev. Lett. 95, 010503 (2005).2. A. Acn, N. Gisin, Ll. Masanes, From Bells Theorem to Secure Quantum Key Distribution, Phys. Rev. Lett. 97, 120405 (2006).3. J.S. Bell, On the Einstein-Podolsky-Rosen paradox, Physics (Long Island City, N.Y.) 1, 195 (1964).4. A.K. Ekert, Quantum cryptography based on Bells theorem, Phys. Rev. Lett. 67, 661 (1991).5. C.H. Bennett and G. Brassard, Quantum Cryptography: Public key distribution and coin tossing, in Proceedings of the IEEE

International Conference on Computers, Systems, and Signal Processing, Bangalore, p. 175 (1984).6. A. Peres and W.K.Wootters, Optimal detection of quantum information, Phys. Rev. Lett. 66, 1119 (1990).7. C.H. Bennett, G. Brassard, C. Crpeau, R. Jozsa, A. Peres, W.K. Wootters, Teleporting an Unknown Quantum State via Dual Classical

and Einstein-Podolsky-Rosen Channels, Phys. Rev. Lett. 70, 1895 (1993).8. D. Deutsch, Quantum theory, the Church-Turing principle and the universal quantum computer, Proc. R. Soc. Lond. A 400, 97

(1985).

A FEARED DISEASE

he start of the new decade has just passed and sohas the media frenzy over the H1N1 flu pandem-ic. As misplaced as the latter turned out to be, itdid serve to remind us of a basic truth: That a

healthy body can be stricken with a fatal disease which tooutward appearances is nearly identical to a common year-ly annoyance. There are lessons here for quantum mechan-ics. In the history of physics, there has never been ahealthier body than quantum theory; no theory has everbeen more all-encompassing or more powerful. Its calcu-lations are relevant at every scale of physical experience,from subnuclear particles, to table-top lasers, to the coresof neutron stars and even the first three minutes of the uni-verse. Yet since its founding days, many physicists havefeared that quantum theorys common annoyance C thecontinuing feeling that something at the bottom of it doesnot make sense C may one day turn out to be the symptomof something fatal.

There is something about quantum theory that is differentin character from any physical theory posed before. To puta finger on it, the issue is this: The basic statement of thetheory C the one we have all learned from our textbooksC seems to rely on terms our intuitions balk at as havingany place in a fundamental description of reality. Thenotions of observer and measurement are taken asprimitive, the very starting point of the theory. This is anunsettling situation! Shouldnt physics be talking aboutwhat is before it starts talking about what will be seen andwho will see it? Perhaps no one has put the point moreforcefully than John Stewart Bell [1]:

What exactly qualifies some physical systems toplay the role of measurer? Was the wavefunctionof the world waiting to jump for thousands of mil-lions of years until a single-celled living creature

SUMMARY

This article summarizes the QuantumBayesian view of quantum mechanics devel-oped by the author and collaborators over anumber of years. Present work at PerimeterInstitute is focused on streamlining a repre-sentation of quantum mechanics purely interms of probabilities, without amplitudes orHilbert-space operators.

T

BY CHRISTOPHER A. FUCHS

QUANTUM BAYESIANISM AT THE PERIMETER

Christopher Fuchs ([email protected]),Perimeter Institute forTheoretical Physics,31 Caroline St. N.,Waterloo, ON,N2L 2Y5

appeared? Or did it have to wait a little longer, forsome better qualified system ... with a PhD?

One sometimes gets the feeling that until this issue is set-tled, fundamental physical theory has no right to move on.Worse yet, that to the extent it does move on, it does soonly as the carrier of something insidious, something thatwill eventually cause the whole organism to stop in itstracks. Dark matter and dark energy? Might these bethe first symptoms of something systemic? Might theproblem be much deeper than getting our quantum fieldswrong? C This is the kind of fear at work here.

So the field of quantum foundations is not unfounded; it isabsolutely vital to physics as a whole. But what constitutesprogress in quantum foundations? Throughout theyears, it seems the most popular criterion has derived fromthe tenor of Bells quote: One should remove the observerfrom the theory just as quickly as possible. In practice thishas generally meant to keep the mathematical structure ofquantum theory as it stands (complex Hilbert spaces, etc.),but find a way to tell a story about the mathematical sym-bols that involves no observers.

Three examples suffice to give a feel: In the de Broglie-Bohm pilot wave version of quantum theory, there areno fundamental measurements, only particles flyingaround in a 3N-dimensional configuration space, pushedaround by a wave function regarded as a physical field.In spontaneous collapse versions, systems are endowedwith quantum states that generally evolve unitarily,but from time-to-time collapse without any need formeasurement. In Everettian or many-worlds quantummechanics, it is only the world as a whole C they call it amultiverse C that is really endowed with an intrinsicquantum state. That quantum state evolves deterministi-cally, with only an illusion from the inside of probabilisticbranching.

The trouble with all these interpretations as quick fixes toBells complaint is that they look to be just that, reallyquick fixes. They look to be interpretive strategies hardlycompelled by the details of the quantum formalism. Thisexplains in part why we could exhibit three such differentstrategies, but it is worse: Each of these strategies givesrise to its own set of incredibilities C ones for which, ifone were endowed with Bells gift for the pen, one couldmake look just as silly. Take the pilot-wave theories: Theygive instantaneous action at a distance, but not actions thatcan be harnessed to send detectable signals. If there were

FEATURE ARTICLE


QUANTUM BAYESIANISM ... (FUCHS)


no equations to give the illusion of science, this would havebeen called counting angels on the head of a pin.

QUANTUM STATES DO NOT EXIST

There is another lesson from the H1N1 virus. To some perplex-ity, it seems people over 65 C a population usually more sus-ceptible to fatalities with seasonal flu C fare better thanyounger folk with H1N1. No one knows exactly why, but theleading theory is that the older population, in its years of otherexposures, has developed various latent antibodies. The anti-bodies are not perfect, but they are a start. And so it may be forquantum foundations.

Here, the latent antibody is the concept of information, and theperfected vaccine, we believe, will arise in part from the theo-ry of single-case, personal probabilities C the branch of proba-bility theory called Bayesianism. Symbolically, the older pop-ulation corresponds to some of the founders of quantum theo-ry (Heisenberg, Pauli, Einstein) and some of the younger disci-ples of the Copenhagen school (Rudolf Peierls, John Wheeler,Asher Peres), who, though they disagreed on many details,were unified on one point: That quantum states are not some-thing out there, in the external world, but instead are expres-sions of information. Before there were people using quantumtheory as a branch of physics, there were no quantum states.The world may be full of stuff, composed of all kinds of things,but among all the stuff and things, there is no observer-inde-pendent, quantum-state kind of stuff.

The immediate payoff of this strategy is that it eliminates theconundrums arising in the various objectified-state interpreta-tions. James Hartle [2] put the point decisively, The reductionof the wave packet does take place in the consciousness of theobserver, not because of any unique physical process whichtakes place there, but only because the state is a construct of theobserver and not an objective property of the physical system.The real substance of Bells fear is just that, the fear itself. Tosuccumb to it is to block the way to understanding the theory.Moreover, the harsher notes of Bells rhetoric are the least ofthe worries: The universe didnt have to wait billions of yearsto collapse its first wave function C wave functions are not partof the observer-independent world.

But this much of the solution is only a somewhat ineffectiveantibody. Its presence is mostly a call for more research.Luckily the days for this are ripe, and it has much to do withthe development of the field of quantum information C thatmultidisciplinary field that includes quantum cryptography andquantum computation. Terminology can say it all: A practition-er in that field is just as likely to call any *, quantum infor-mation as a quantum state. What does quantum teleporta-tion do? It transfers quantum information from Alice toBob. What we have here is a change of mindset [3].

What the protocols and theorems of quantum informationpound home is the idea that quantum states look and feel likeinformation in the technical sense of the word. There is no

more beautiful demonstration of this than Robert Spekkensstoy model mimicking various features of quantum mechan-ics [4]. In this model, the toys are each equipped with fourpossible mechanical configurations; but the players, the manip-ulators of the toys, are consistently impeded from having morethan one bit of information about each toys actual configura-tion (two bits about two toys, etc.). The only things the playerscan know are their states of uncertainty. The wonderful thing isthat these states of uncertainty exhibit many of the characteris-tics of quantum information: from the no-cloning theorem toanalogues of quantum teleportation, quantum key distribution,and even interference in a Mach-Zehnder interferometer. Morethan two dozen quantum phenomena are reproduced qualita-tively, and all the while one can pinpoint the cause: The phe-nomena arise in the uncertainties, not in the mechanical config-urations.

What considerations like this tell the objectifiers of quantumstates is that, far from being an appendage cheaply tacked on tothe theory, the idea of quantum states as information has a uni-fying power that goes a significant way toward explaining whythe theory has the mathematical structure it does. There are,however, aspects of Bells challenge that remain a worry. Andupon these, all could still topple. Particularly, the questionsWhose information? and Information about what? must beaddressed before any vaccine can be declared a success.

Good immunology does not come easily. But this much is sure:The glaringly obvious (that a large part of quantum theory isabout information) should not be abandoned rashly: To do so isto lose grip of the theory, with no better grasp on reality inreturn. If on the other hand, one holds fast to the central pointabout information, initially frightening though it may be, onemay still be able to construct a picture of reality from theperimeter of vision.

QUANTUM BAYESIANISM

Every area of human endeavor has its bold extremes. Ones thatsay, If this is going to be done right, we must go this far.Nothing less will do. In probability theory, the bold extremeis personalist Bayesianism [5]. It says that probability theoryis of the character of formal logic C a set of criteria for testingconsistency. The key similarity is that formal logic does nothave within it the power to set the truth values of the proposi-tions it manipulates. It can only show whether various truthvalues are inconsistent; the actual values come from anothersource. Whenever logic reveals a set of truth values inconsis-tent, one must return to their source to alleviate the discord.Precisely in which way to alleviate it, though, logic gives noguidance.

The key idea of personalist Bayesian probability theory is thatit too is a calculus of consistency (or coherence as the prac-titioners call it), but this time for ones decision-makingdegrees of belief. Probability theory can only show whethervarious degrees of belief are inconsistent. The actual beliefscome from another source, and there is nowhere to pin their

QUANTUM BAYESIANISM ... (FUCHS)AA


responsibility but on the agent who holds them. A probabilityassignment is a tool an agent uses to make gambles and deci-sions, but probability theory as a whole is not about a singleisolated belief C rather it is about a whole mesh of them. Whena belief in the mesh is found to be incoherent with the others,the theory flags the inconsistency. However, it gives no guid-ance for how to mend any incoherences it finds. To alleviatediscord, one must return to the source of the assignments in thefirst place C the very agent who is attempting to sum up all hishistory and experience with those assignments.

Where personalist Bayesianism breaks from other develop-ments of probability theory is that it says there are no externalcriteria for declaring an isolated probability assignment right orwrong. The only basis for a judgment of adequacy comes fromthe inside, from the greater mesh of beliefs the agent accesseswhen appraising his coherence. Similarly for quantum mechan-ics.

The defining feature of Quantum Bayesianism [3,6-11] is that itsays, If this is going to be done right, we must go this far.Specifically, there can be no such thing as a right and truequantum state, if such is thought of as defined by criteria exter-nal to the agent making the assignment: Quantum states mustinstead be like personalist Bayesian probabilities. The connec-tion between the two foundational issues is this. Quantumstates, through the Born Rule, can be used to calculate proba-bilities. On the other hand, if one assigns probabilities for theoutcomes of a well-selected set of measurements, then this ismathematically equivalent to making the quantum-state assign-ment itself. Thus, if probabilities are personal in the Bayesiansense, then so too must be quantum states.

What this buys interpretatively is that it gives each quantumstate a home. Indeed, a home localized in space and time Cnamely, the physical site of the agent who assigns it! By thismethod, one expels once and for all the fear that quantummechanics leads to spooky action at a distance, and expels aswell any hint of a problem with Wigners friend. It does thisbecause it removes the very last trace of confusion overwhether quantum states might still be objective, agent-inde-pendent, physical properties.

The innovation of Quantum Bayesianism is that, for most ofthe history of trying to take an informational point of viewabout quantum states, the supporters of the idea have tried tohave it both ways: that on the one hand quantum states are notreal physical properties, yet on the other there is a right quan-tum state after all. One hears things like, The right quantumstate is the one the agent should adopt if he had all the informa-tion. The tension in this statement, however, leaves its holderopen to immediate attack: If theres a right quantum state afterall, then why not just be done with all this squabbling and callit a physical fact independent of the agent? And if it is a phys-ical fact, what recourse does one have for declaring that thereis no action at a distance when delocalized quantum stateschange instantaneously?

The Quantum Bayesian dispels these difficulties by being con-scientiously forthright. Whose information? Mine!Information about what? The consequences (for me) of myactions upon the physical system! The point of view here isthat a quantum measurement is nothing other than a well-placed kick upon a system C a kick that leads to unpredictableconsequences for the very agent who did the kicking. What ofquantum theory? It is a universal single-user theory in muchthe same way that Bayesian probability theory is. It is a usersmanual that any agent can pick up and use to help make wisedecisions in this world of inherent uncertainty. In my case, aworld in which I am forced to be uncertain about the conse-quences of my actions; in your case, a world in which you areforced to be uncertain about the consequences of your actions.In a quantum mechanics with the understanding that eachinstance of its use is strictly single-user C My measurementoutcomes happen right here, to me, and I am talking about myuncertainty of them. C there is no room for most of the peren-nial quantum mysteries.

With this we finally pin down the way in which quantum theo-ry is different in character from any physical theory before.For the Quantum Bayesian, quantum theory is not somethingoutside probability theory C it is not a picture of the world asit is C but rather an addition to probability theory itself. Asprobability theory is a normative theory, not saying what onemust believe, but offering rules of consistency an agent shouldstrive to satisfy within his overall mesh of beliefs, so it is thecase with quantum theory. To embrace this is all the vaccina-tion quantum theory needs.

Fig. 1 In contemplating a quantum measurement, one makes a con-ceptual split in the world: one part is treated as an agent, andthe other as a kind of catalyst. A quantum measurement con-sists first in the agent taking an action on the quantum sys-tem. The action is represented formally by a set of operators{Ei } C a positive-operator valued measure (POVM). Theaction generally leads to an incompletely predictable conse-quence Ek for the agent. The quantum state *, appears nextto the agents head because it captures his degrees of beliefconcerning the consequences of his actions.

QUANTUM BAYESIANISM ... (FUCHS)


SEEKING SICS C THE BORN RULE AS FUNDA-MENTAL

Yet, if quantum theory is a users manual, one cannot forgetthat the world is its author. And from its writing style, one maystill be able to tell something of the author herself. The ques-tion is how to tease out the motif.

Something that cannot be said of the Quantum Bayesian pro-gram is that it has not had to earn its keep in the larger worldof quantum interpretations. Since the beginning, the promotersof the view have been on the run proving technical theoremswhenever required to close a gap in its logic or negate an awk-wardness in its new way of speaking. A case in point is thisquestion: If quantum theory is so closely allied with probabili-ty theory, why is it not written in a language that starts withprobability, rather than a language that ends with it? Why doesquantum theory invoke the mathematical apparatus of Hilbertspaces and linear operators, rather than probabilities outright?This brings us to present-day research at Perimeter Institute.

The answer we seek hinges on a hypothetical structure called asymmetric informationally complete positive-operator-valuedmeasure, or SIC for short. This is a set of d 2 rank-one projec-tion operators i = *i , +i* on a d-dimensional Hilbert spacesuch that

*+i *j ,*2 = whenever i =/ j . (1)

Because of their extreme symmetry, it turns out that such setsof operators, when they exist, have remarkable properties.Among these, two powerful ones are that they must be linear-ly independent (spanning the space of Hermitian operators) andsum to d times the identity.

This is significant because it implies that an arbitrary state can be expressed as a linear combination of the i. Moreover,because the operators Hi = 1i are positive-semidefinite andsum to the identity, these can be interpreted as labeling the out-comes of a quantum measurement device C not a vonNeumann measurement device, but a measurement device ofthe most general kind allowed in quantum theory [12]. Finally,the is symmetry gives a simple relation between the proba-bilities P(Hi ) = tr(Hi ) and the expansion coefficients for :

(2)

The extreme simplicity of this formula suggests it is the bestplace for the Quantum Bayesian to seek his motif.

Before proceeding, we must reveal what is so consternatingabout the SICs: It is whether they exist at all. Despite 10 yearsof growing effort since the definition was introduced [13,14], noone has been able to show that they exist in general dimension.All that is known firmly is that they exist in dimensions 2through 67: dimensions 2-15, 19, 24, 35, and 48 by analyticproof, and the remainder through numerical simulation [15].

How much evidence is this that SICs exist? The reader mustanswer this for himself, but for the remainder of the art

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