Rydberg physics with cold strontium

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Rydberg physics with cold strontium
James Millen Durham University Atomic
Molecular Physics group
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Outline

• Rydberg physics
• Why strontium?
• Building a strontium Rydberg experiment
• The worlds first cold strontium Rydberg gas
• Probing a strontium Rydberg gas with two-electron
  excitation

Rydberg physics with cold strontium Seminar
October 2010
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The team
Dr. Matt Jones (2006)
Danielle Boddy (2010)
Graham Lochead (2008)
Benjamin Pasquiou Sarah Mauger Clémentine Javaux
Liz Bridge (NPL) (MSci)
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Rydberg physics
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Definition
A state of high principal quantum number n.
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Properties of Rydberg atoms

• Size scales as n2

• Lifetime scales as n3
• t5s5p 5ns
• t5s56d 25µs

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Properties of Rydberg atoms
Van der Waals interaction scales as n11
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Consequence of strong interactions
Interaction shift ?E
RB
Energy
R
Inter-atomic separation
Dipole Blockade can only have ONE Rydberg
excitation in a
certain radius RB.
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Consequence of dipole blockade
Leads to highly entangled states
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Many-body states
Can create many body entangled states
Superatoms!
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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.
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Cold plasma formation
Initial ionization ? creation of a cold plasma
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Cold plasmas

• Requires a certain amount of initial ionization
  (density dependence).
• Ecoulomb gt Ethermal (hence cold, or even
  ultra-cold).
• Stays bound for 10µs.
• Strongly correlated

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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?

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Why strontium?
Two valence electrons.
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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.

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Two electron excitation
Two valence electrons ? two electron excitation
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Autoionization
The overlap between the two electronic
wavefunction causes the atom to ionize
Autoionization
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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.
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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.

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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.

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Building a strontium Rydberg experiment
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From scratch
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Zeeman slower
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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

?1 461nm 32MHz
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Trapping strontium
106 atoms 1010 cm-3 density 5mK
Trap our atoms in a standard six beam
magneto-optical trap
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Internals
MOT coils and electrodes inside the chamber,
micro-channel plate (MCP) detector. Also CCD
camera outside.
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A cold strontium Rydberg gas J. Millen et. al. in
preparation
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Rydberg excitation

• Excite n 18 ? ionization threshold.
• Direct spontaneous ionization to detector with
  field pulse.
• Can perform high resolution spectroscopy

?2 420 nm or 413nm
?1 461nm 32MHz
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Rydberg spectroscopy

• Located a large range of Rydberg states

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Rydberg spectroscopy

• Can calculate dipole matrix elements to model
  data

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Now we understand the singly excited Rydberg
states, what can we learn through two electron
excitation?
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Probing a strontium Rydberg gas with two-electron
excitation J. Millen et. al., Phys. Rev. Lett.
(Accepted)
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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.

?2 413nm
?1 461nm 32MHz
Rydberg physics with cold strontium Seminar
October 2010
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Autoionization

• Excite the inner valence electron after delay ?t,
  atom autoionizes.
• Get greatly increased ionization

Field pulse directsions to detector
?3 408nm
?2 413nm
?1 461nm 32MHz
Rydberg physics with cold strontium Seminar
October 2010
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Autoionization

• Excite the inner valence electron after delay ?t,
  atom autoionizes.
• Can take the spectrum of this transition (?3 is
  detuning from the bare ion line, S is
  autoionization signal)

?3 408nm
?2 413nm
?1 461nm 32MHz
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Analysis
Low Rydberg density
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High density

• Increase the Rydberg density by increasing the
  power of ?2.

• A new, Rydberg density dependent feature appears

Low Rydberg density
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Evolution
At high density allow the Rydberg gas to evolve
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Transfer
Transfer where?
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Destination state
Look at the decay of signal at different spectral
points
60µs
25µs
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Destination state
The autoionization spectrum of the 5s54f 1F3
state coincides with the late-time spectrum of
the Rydberg gas
Black line ?t 100µs high Rydberg density
spectrum. Blue line spectrum of the 5s54f
1F3 state.
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Quantitative analysis
13 3 of the Rydberg population transferred to
5s54f state
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Plasma formation
The mechanism for population transfer is cold
plasma formation
Black data population transfer. Red
data spontaneous ionization.
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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
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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.

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Rydberg physics with cold strontium Seminar
October 2010