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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.
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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) Danielle Boddy (2010) Graham Lochead (2008) Benjamin Pasquiou Sarah Mauger Clémentine Javaux Liz Bridge (NPL) (MSci) Rydberg physics with cold strontium – Seminar October 2010
Rydberg physics Rydberg physics with cold strontium – Seminar October 2010
Definition Ionization threshold Energy A state of high principal quantum number n. 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 or Interaction shift ΔE RB Energy 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 One atom Two atoms A. Gaëtan et. al.,Nature Physics 5, 115 (2009) Leads to highly entangled states: Rydberg physics with cold strontium – Seminar October 2010
Many-body states RB Can create many body entangled states …”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 Positive charge binds electrons. Electrons oscillate through gas Separation Ionizing and l-mixing electron Rydberg collisions Initial ionization → creation of a cold plasma Rydberg physics with cold strontium – Seminar October 2010
Cold plasmas T. Pohl et. al., Phys. Rev. Lett. 92, 155003 (2004) • Requires a certain amount of initial ionization (density dependence). • Ecoulomb > Ethermal (hence cold, or even “ultra-cold”). • Stays bound for ~10μs. • Strongly correlated: 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 C. E. Simien et. al.,Phy. Rev. Lett. 92, 143001 (2004) 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. 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 Ion The overlap between the two electronic wavefunction causes the atom to ionize: “Autoionization” Rydberg physics with cold strontium – Seminar October 2010
Autoionization as a probe Focussed autoionizing beam What can we do with autoionization? • Amount of ionization ∝number of Rydberg atoms→ probe of a Rydberg gas: 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 C. Javaux et. al., Eur. Phys. J. D 57, 151-154 (2010) E. M. Bridge et. al., Rev. Sci. Instrum. 80, 013101 (2009) • 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 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 Spontaneous ionization signal -20 20 40 0 -40 λ2 (MHz) • 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 32MHz Rydberg physics with cold strontium – Seminar October 2010
Rydberg spectroscopy n~125 • Located a large range of Rydberg states: 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 Autoionization Spontaneous ionization • Excite the inner valence electron after delay Δt, atom autoionizes. • Get greatly increased ionization: Field pulse directsions to detector λ3 = 408nm λ2 = 413nm λ1 = 461nm 32MHz Rydberg physics with cold strontium – Seminar October 2010
Autoionization Low Rydberg density • 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): λ3 = 408nm λ2 = 413nm λ1 = 461nm 32MHz Rydberg physics with cold strontium – Seminar October 2010
Analysis Double peaked structure characteristic of the 5s56d1D2 state in strontium 6-channel MQDT fit Low Rydberg density Rydberg physics with cold strontium – Seminar October 2010
High density High Rydberg density • Increase the Rydberg density by increasing the power of λ2. • A new, Rydberg density dependent feature appears: Low Rydberg density Rydberg physics with cold strontium – Seminar October 2010
Evolution Δt = 0.5 μs Δt = 60 μs Δt = 100 μs At high density allow the Rydberg gas to evolve: Rydberg physics with cold strontium – Seminar October 2010
Transfer Δt = 100 μs Δt = 0.5 μs A change in shape→ a change of state. Δt = 0.5μshigh density Δt = 0.5μslow density Transfer of populationvery rapid. Transfer where? Rydberg physics with cold strontium – Seminar October 2010
Destination state Δt = 100 μs 54F state 25μs B A 60μs A B Blue line: The decay of the 5s54f 1F3 state. Look at the decay of signal at different spectral points: 60μs 25μs Rydberg physics with cold strontium – Seminar October 2010
Destination state Autoionization spectrum 56D Rydberg gas after 100μs evolution 54F Rydberg gas 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. 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 Initial Rydberg # M. P. Robinson et. al.,Phy. Rev. Lett. 85, 4466 (2000) The mechanism for population transfer is cold plasma formation: Black data: population transfer. Red data: spontaneous ionization. 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
