Transcript Rydberg physics with cold strontium

Rydberg physics with cold
strontium
James Millen
Durham University – Atomic & Molecular Physics group
Outline
• 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
Rydberg physics with cold strontium – Seminar October 2010
The team
Dr. Matt Jones
(2006)
Graham Lochead
(2008)
Benjamin Pasquiou
Sarah Mauger
Clémentine Javaux
Danielle Boddy
(2010)
Liz Bridge (NPL) (MSci)
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.
Energy
Ionization threshold
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
Van der Waals interaction scales as n11:
M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)
Rydberg physics with cold strontium – Seminar October 2010
Consequence of strong interactions
Interaction shift ΔE
Energy
RB
or
R
Inter-atomic separation
Dipole Blockade: can only have ONE Rydberg excitation in a
certain radius RB.
Rydberg physics with cold strontium – Seminar October 2010
Consequence of dipole blockade
Leads to highly entangled states:
Two atoms
One atom
A. Gaëtan et. al.,
Nature Physics 5, 115 (2009)
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
Fast ionization,
some electrons leave.
Energy
Initial ionization → creation of a cold plasma
Separation
Positive charge binds
electrons. Electrons
oscillate through gas
Ionizing and l-mixing
electron Rydberg collisions
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:
• The Sr+ ion has an optical
transition (421.7nm).
• The expansion of the plasma
can be studied.
C. E. Simien et. al.,
Phy. Rev. Lett. 92, 143001 (2004)
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:
Focussed
autoionizing beam
Spatial probe of the blockade effect.
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
• 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:
λ1 = 461nm
32MHz
C. Javaux et. al.,
Eur. Phys. J. D 57, 151-154 (2010)
E. M. Bridge et. al.,
Rev. Sci. Instrum. 80, 013101 (2009)
Rydberg physics with cold strontium – Seminar October 2010
Trapping strontium
~ 106 atoms
~ 1010 cm-3 density
~ 5mK
Trap our atoms in a standard six beam magneto-optical trap
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.
λ2 = 420 nm or 413nm
λ1 = 461nm
32MHz
Spontaneous
ionization signal
• Can perform high resolution spectroscopy:
-40
-20
0
λ2
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
excitation
J. 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:
λ3 = 408nm
Field pulse directs
ions to detector
λ2 = 413nm
Autoionization
λ1 = 461nm
32MHz
Rydberg physics with cold strontium – Seminar October 2010
Spontaneous
ionization
Autoionization
• Excite the inner valence electron after
delay Δt, atom autoionizes.
λ3 = 408nm
• Can take the spectrum of this transition
(Δ3 is detuning from the bare ion line, S
is autoionization signal):
λ2 = 413nm
λ1 = 461nm
32MHz
Low Rydberg density
Rydberg physics with cold strontium – Seminar October 2010
Analysis
Double peaked structure characteristic
of the 5s56d 1D2 state in strontium
6-channel MQDT fit
Low Rydberg density
Rydberg physics with cold strontium – Seminar October 2010
High density
• Increase the Rydberg density by increasing the power of λ2.
• A new, Rydberg density dependent feature appears:
Low Rydberg density
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
Δt = 0.5 μs
Δt = 100 μs
Δt = 0.5μs
low density
Δt = 0.5μs
high density
A change in shape
→ a change of state.
Transfer of population
very rapid.
Transfer where?
Rydberg physics with cold strontium – Seminar October 2010
Destination state
Look at the decay of signal at different spectral points:
Δt = 100 μs
54F state
60μs
25μs
A
B
25μs
A
B
Blue line: The decay of the 5s54f 1F3 state.
Rydberg physics with cold strontium – Seminar October 2010
60μs
Destination state
The autoionization spectrum of the 5s54f 1F3 state coincides
with the late-time spectrum of the Rydberg gas:
Autoionization spectrum
56D Rydberg gas
after 100μs
evolution
Black line: Δt = 100μs high
Rydberg density spectrum.
54F Rydberg gas
Blue line: spectrum of the
5s54f 1F3 state.
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
Plasma threshold
Spontaneous ionization
Population transferred
The mechanism for population transfer is cold plasma formation:
Black data: population
transfer.
Red data: spontaneous
ionization.
Initial Rydberg #
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
Rydberg physics with cold strontium – Seminar October 2010