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 with cold strontium – Seminar October 2010
Rydberg physics with cold strontium – Seminar October 2010
Rydberg physics
Rydberg physics with cold strontium – Seminar October 2010
Definition
A state of high principal quantum number n.
Ionization threshold
En
erg
y
Rydberg physics with cold strontium – Seminar October 2010
Properties of Rydberg atoms
• Size scales as n2:
• Lifetime scales as n3:
τ5s5p ≈ 5ns
τ5s56d ≈ 25μs
Rydberg physics with cold strontium – Seminar October 2010
Properties of Rydberg atoms
M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)
Van der Waals interaction scales as n11:
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
Consequence of dipole blockade
Leads to highly entangled states:
A. Gaëtan et. al.,Nature Physics 5, 115 (2009)
One atomTwo
atoms
Rydberg physics with cold strontium – Seminar October 2010
Many-body states
Can create many body entangled states
RB
…”Superatoms”!
Rydberg physics with cold strontium – Seminar October 2010
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.
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
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 with cold strontium – Seminar October 2010
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?
Rydberg physics with cold strontium – Seminar October 2010
Why strontium?
Two valence electrons.
Rydberg physics with cold strontium – Seminar October 2010
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.
Rydberg physics with cold strontium – Seminar October 2010
Two electron excitation
Two valence electrons → two electron excitation:
Rydberg physics with cold strontium – Seminar October 2010
Autoionization
The overlap between the two electronic wavefunction causes the atom to ionize:
Ion
“Autoionization”
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
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.
Rydberg physics with cold strontium – Seminar October 2010
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.
Rydberg physics with cold strontium – Seminar October 2010
Building a strontium Rydberg experiment
Rydberg physics with cold strontium – Seminar October 2010
From scratch…
Strontium has no appreciable vapour pressure at room temperature: heat to 600˚C.
Rydberg physics with cold strontium – Seminar October 2010
Zeeman slower
Strontium is now going very fast! Use a Zeeman slower.
Rydberg physics with cold strontium – Seminar October 2010
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)
Rydberg physics with cold strontium – Seminar October 2010
Trapping strontium
Trap our atoms in a standard six beam magneto-optical trap
~ 106 atoms
~ 1010 cm-3 density
~ 5mK
Rydberg physics with cold strontium – Seminar October 2010
Internals
MOT coils and electrodes inside the chamber, + micro-channel plate (MCP) detector. Also CCD camera outside.
Rydberg physics with cold strontium – Seminar October 2010
A cold strontium Rydberg gas
J. Millen et. al. in preparation
Rydberg physics with cold strontium – Seminar October 2010
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 physics with cold strontium – Seminar October 2010
Rydberg spectroscopy
• Located a large range of Rydberg states:
n~125
Rydberg physics with cold strontium – Seminar October 2010
Rydberg spectroscopy
• Can calculate dipole matrix elements to model data:
Rydberg physics with cold strontium – Seminar October 2010
Now we understand the singly excited Rydberg states, what can we learn through two electron excitation?
Rydberg physics with cold strontium – Seminar October 2010
Probing a strontium Rydberg gas with two-
electron excitationJ. Millen et. al., Phys. Rev. Lett. (Accepted)
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
Analysis
Low Rydberg density
6-channel MQDT fit
Double peaked structure characteristic of the 5s56d 1D2
state in strontium
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
Evolution
At high density allow the Rydberg gas to evolve:
Δt = 0.5 μs Δt = 60 μs Δt = 100 μs
Rydberg physics with cold strontium – Seminar October 2010
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?
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
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
Rydberg physics with cold strontium – Seminar October 2010
Quantitative analysis
13 ± 3% of the Rydberg population transferred to 5s54f state
Rydberg physics with cold strontium – Seminar October 2010
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)
Rydberg physics with cold strontium – Seminar October 2010
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.
Rydberg physics with cold strontium – Seminar October 2010
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 – Seminar October 2010