rydberg physics with cold strontium james millen durham university – atomic & molecular...

Rydberg physics with cold strontium

James Millen

Durham University – Atomic & Molecular Physics group

Outline

Rydberg physics with cold strontium – Seminar October 2010

• Rydberg physics

• Why strontium?

• Building a strontium Rydberg experiment

• The world’s first cold strontium Rydberg gas

• Probing a strontium Rydberg gas with two-electron excitation

The team

Dr. Matt Jones(2006)

Graham Lochead(2008)

Danielle Boddy(2010)

Benjamin PasquiouSarah MaugerClémentine Javaux

Liz Bridge (NPL) (MSci)



Rydberg physics


Definition

A state of high principal quantum number n.

Ionization threshold

En

erg

y


Properties of Rydberg atoms

• Size scales as n2:

• Lifetime scales as n3:

τ5s5p ≈ 5ns

τ5s56d ≈ 25μs


Properties of Rydberg atoms

M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)

Van der Waals interaction scales as n11:


Consequence of strong interactions

Dipole Blockade: can only have ONE Rydberg excitation in a certain radius RB.

Inter-atomic separation

R

En

erg

y

or

Interaction shift ΔE

RB


Consequence of dipole blockade

Leads to highly entangled states:

A. Gaëtan et. al.,Nature Physics 5, 115 (2009)

One atomTwo

atoms


Many-body states

Can create many body entangled states

RB

…”Superatoms”!


Many-body systems

What happens when there is an ensemble of superatoms?

Correlated quantum many-body systems?

Rydberg gasses can also form correlated classical many-body systems: cold plasmas.


Cold plasma formation

Initial ionization → creation of a cold plasma

Fast ionization,some electrons leave.

Positive charge binds electrons. Electrons oscillate through gas

Ionizing and l-mixing electron Rydberg collisions

En

erg

y

Separation


Cold plasmas

• Requires a certain amount of initial ionization (density dependence).

• Ecoulomb > Ethermal (hence cold, or even “ultra-cold”).

• Stays bound for ~10μs.

• Strongly correlated:

T. Pohl et. al., Phys. Rev. Lett. 92, 155003 (2004)


Rydberg physics summary

• Rydberg systems exhibit greatly enhanced interatomic interactions.

• Strongly entangled states.

• Both quantum and classical correlated many-body systems.

• What can we add with our experiment?


Why strontium?

Two valence electrons.


Ion imaging

Two valence electrons → ion can be optically imaged:

C. E. Simien et. al.,Phy. Rev. Lett. 92, 143001

(2004)

• The Sr+ ion has an optical transition (421.7nm).

• The expansion of the plasma can be studied.


Two electron excitation

Two valence electrons → two electron excitation:


Autoionization

The overlap between the two electronic wavefunction causes the atom to ionize:

Ion

“Autoionization”


Autoionization as a probe

What can we do with autoionization?

• Amount of ionization ∝ number of Rydberg atoms→ probe of a Rydberg gas:

Spatial probe of the blockade effect.

Focussed autoionizing

beam


Rydbergs in a lattice

• Load Rydberg atoms into a 1-D optical lattice.

• Use a dipole trap far detuned from the INNER valence electron resonance.

• Get trapping without ionization, and without affecting the Rydberg electron.

• Investigate many body blockade in this ordered system.


Strontium Rydberg summary

• The extra valence electron is an exciting new handle.

• Rydberg gasses can be probed in a new way.

• Classical and quantum many-body systems can be studied.


Building a strontium Rydberg experiment


From scratch…

Strontium has no appreciable vapour pressure at room temperature: heat to 600˚C.


Zeeman slower

Strontium is now going very fast! Use a Zeeman slower.


Trapping strontium

λ1 = 461nm

32MHz

• Cool and trap using the 5s → 5p transition.

• Laser stabilization not trivial for strontium!

• Developed a unique strontium dispenser cell and a modulation-free spectroscopy technique:

E. M. Bridge et. al.,

Rev. Sci. Instrum. 80, 013101 (2009)

C. Javaux et. al.,

Eur. Phys. J. D 57, 151-154 (2010)


Trapping strontium

Trap our atoms in a standard six beam magneto-optical trap

~ 106 atoms

~ 1010 cm-3 density

~ 5mK


Internals

MOT coils and electrodes inside the chamber, + micro-channel plate (MCP) detector. Also CCD camera outside.


A cold strontium Rydberg gas

J. Millen et. al. in preparation


Rydberg excitation

• Excite n ≈ 18 → ionization threshold.

• Direct spontaneous ionization to detector with field pulse.

• Can perform high resolution spectroscopy:

λ2 = 420 nm or 413nm

λ1 = 461nm

32MHzλ2

Sp

on

tan

eou

s io

niz

ati

on

si

gn

al

0 20 40-20-40(MHz

)


Rydberg spectroscopy

• Located a large range of Rydberg states:

n~125


Rydberg spectroscopy

• Can calculate dipole matrix elements to model data:


Now we understand the singly excited Rydberg states, what can we learn through two electron excitation?


Probing a strontium Rydberg gas with two-

electron excitationJ. Millen et. al., Phys. Rev. Lett. (Accepted)


Rydberg excitation

• Excite to the 56D Rydberg state.

• Up to 10% of ground state population transferred to the Rydberg state.

• 1% of our Rydberg state population spontaneously ionizes.

λ2 = 413nm

λ1 = 461nm

32MHz


Autoionization

• Excite the inner valence electron after delay Δt, atom autoionizes.

• Get greatly increased ionization:

λ2 = 413nm

λ1 = 461nm

32MHz

λ3 = 408nm

Field pulse directsions to detector

Spontaneous ionization

Autoionization


Autoionization

• Excite the inner valence electron after delay Δt, atom autoionizes.

• Can take the spectrum of this transition (Δ3 is detuning from the bare ion line, S is autoionization signal):

λ2 = 413nm

λ1 = 461nm

32MHz

λ3 = 408nm

Low Rydberg density


Analysis

Low Rydberg density

6-channel MQDT fit

Double peaked structure characteristic of the 5s56d 1D2

state in strontium


High density

• Increase the Rydberg density by increasing the power of λ2.

Low Rydberg density

• A new, Rydberg density dependent feature appears:

High Rydberg density


Evolution

At high density allow the Rydberg gas to evolve:

Δt = 0.5 μs Δt = 60 μs Δt = 100 μs


Transfer

A change in shape→ a change of state.

Δt = 0.5 μs Δt = 100 μs

Δt = 0.5μslow

density

Transfer of populationvery rapid.

Δt = 0.5μshigh

density

Transfer where?


Destination state

Δt = 100 μs

Look at the decay of signal at different spectral points:

A

A

B

B

Blue line: The decay of the 5s54f 1F3 state.

54F state

25μs

25μs

60μs

60μs

Autoionization spectrum


Destination state

The autoionization spectrum of the 5s54f 1F3 state coincides with the late-time spectrum of the Rydberg gas:

Black line: Δt = 100μs high Rydberg density spectrum.

Blue line: spectrum of the 5s54f 1F3 state.

56D Rydberg gas after 100μs evolution

54F Rydberg gas


Quantitative analysis

13 ± 3% of the Rydberg population transferred to 5s54f state


Plasma formation

The mechanism for population transfer is cold plasma formation:

Black data: population transfer.

Red data: spontaneous ionization.

Plasma threshold

Initial Rydberg #

Pop

ula

tion

tr

an

sferr

ed S

pon

tan

eou

s io

niza

tion

M. P. Robinson et. al.,Phy. Rev. Lett. 85, 4466 (2000)


Summary

• We have probed our Rydberg gas in an entirely novel way.

• Excitation of the inner valence electron yields information on interactions in the gas.

• Identified, and quantitatively measured, population transfer, and identified mechanism.

• We have studied the very onset of plasma formation.


Outlook

• We will use autoionization as a probe of many-body blockaded systems.

• Use the inner valence electron to trap Rydberg atoms.

• Study charge delocalization in an optical lattice.

rydberg physics with cold strontium james millen durham university – atomic & molecular...

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