1. Rydberg physics with cold strontium James Millen Durham University – Atomic & Molecular Physics group

2. 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

3. The team Dr. Matt Jones (2006) Graham Lochead (2008) Danielle Boddy (2010) Benjamin Pasquiou Sarah Mauger Clémentine Javaux Liz Bridge (NPL) (MSci) Rydberg physics with cold strontium – Seminar October 2010

4. Rydberg physics

5. Rydberg physics with cold strontium – Seminar October 2010 Definition A state of high principal quantum number n. Ionization threshold Energy

6. Rydberg physics with cold strontium – Seminar October 2010 Properties of Rydberg atoms Size scales as n 2 : Lifetime scales as n 3 : τ 5s5p ≈ 5ns τ 5s56d ≈ 25μs

7. Rydberg physics with cold strontium – Seminar October 2010 Properties of Rydberg atoms M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)Rev. Mod. Phys. 82, 2313 (2010) Van der Waals interaction scales as n 11 :

8. Rydberg physics with cold strontium – Seminar October 2010 Consequence of strong interactions Dipole Blockade: can only have ONE Rydberg excitation in a certain radius R B. Inter-atomic separation R Energy or Interaction shift ΔE RBRB

9. 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) Nature Physics 5, 115 (2009) One atom Two atoms

10. Rydberg physics with cold strontium – Seminar October 2010 Many-body states Can create many body entangled states RBRB …”Superatoms”!

11. 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.

12. 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 Energy Separation

13. Rydberg physics with cold strontium – Seminar October 2010 Cold plasmas Requires a certain amount of initial ionization (density dependence). E coulomb > E thermal (hence cold, or even “ultra-cold”). Stays bound for ~10μs. Strongly correlated: T. Pohl et. al., Phys. Rev. Lett. 92, 155003 (2004)Phys. Rev. Lett. 92, 155003 (2004)

14. 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?

15. Rydberg physics with cold strontium – Seminar October 2010 Why strontium? Two valence electrons.

16. 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) Phy. Rev. Lett. 92, 143001 (2004) The Sr + ion has an optical transition (421.7nm). The expansion of the plasma can be studied.

17. Rydberg physics with cold strontium – Seminar October 2010 Two electron excitation Two valence electrons → two electron excitation:

18. Rydberg physics with cold strontium – Seminar October 2010 Autoionization The overlap between the two electronic wavefunction causes the atom to ionize: Ion “Autoionization”

19. 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

20. 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.

21. 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.

22. Rydberg physics with cold strontium – Seminar October 2010 Building a strontium Rydberg experiment

23. Rydberg physics with cold strontium – Seminar October 2010 From scratch… Strontium has no appreciable vapour pressure at room temperature: heat to 600˚C.

24. Rydberg physics with cold strontium – Seminar October 2010 Zeeman slower Strontium is now going very fast! Use a Zeeman slower.

25. Rydberg physics with cold strontium – Seminar October 2010 Trapping strontium λ 1 = 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)

26. Rydberg physics with cold strontium – Seminar October 2010 Trapping strontium Trap our atoms in a standard six beam magneto-optical trap ~ 10 6 atoms ~ 10 10 cm -3 density ~ 5mK

27. 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.

28. Rydberg physics with cold strontium – Seminar October 2010 A cold strontium Rydberg gas J. Millen et. al. in preparation

29. 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: λ 2 = 420 nm or 413nm λ 1 = 461nm 32MHz λ2λ2 Spontaneous ionization signal 0 2040 -20 -40 (MHz)

30. Rydberg physics with cold strontium – Seminar October 2010 Rydberg spectroscopy Located a large range of Rydberg states: n~125

31. Rydberg physics with cold strontium – Seminar October 2010 Rydberg spectroscopy Can calculate dipole matrix elements to model data:

32. Rydberg physics with cold strontium – Seminar October 2010 Now we understand the singly excited Rydberg states, what can we learn through two electron excitation?

33. 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)

34. 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. λ 2 = 413nm λ 1 = 461nm 32MHz

35. Rydberg physics with cold strontium – Seminar October 2010 Autoionization Excite the inner valence electron after delay Δt, atom autoionizes. Get greatly increased ionization: λ 2 = 413nm λ 1 = 461nm 32MHz λ 3 = 408nm Field pulse directs ions to detector Spontaneous ionization Autoionization

36. 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 (Δ 3 is detuning from the bare ion line, S is autoionization signal): λ 2 = 413nm λ 1 = 461nm 32MHz λ 3 = 408nm Low Rydberg density

37. Rydberg physics with cold strontium – Seminar October 2010 Analysis Low Rydberg density 6-channel MQDT fit Double peaked structure characteristic of the 5s56d 1 D 2 state in strontium

38. Rydberg physics with cold strontium – Seminar October 2010 High density Increase the Rydberg density by increasing the power of λ 2. Low Rydberg density A new, Rydberg density dependent feature appears: High Rydberg density

39. 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

40. 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 = 0.5μs low density Transfer of population very rapid. Δt = 0.5μs high density Transfer where?

41. 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 1 F 3 state. 54F state 25μs 60μs

42. Autoionization spectrum Rydberg physics with cold strontium – Seminar October 2010 Destination state The autoionization spectrum of the 5s54f 1 F 3 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 1 F 3 state. 56D Rydberg gas after 100μs evolution 54F Rydberg gas

43. Rydberg physics with cold strontium – Seminar October 2010 Quantitative analysis 13 ± 3% of the Rydberg population transferred to 5s54f state

44. 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 # Population transferred Spontaneous ionization M. P. Robinson et. al., Phy. Rev. Lett. 85, 4466 (2000) Phy. Rev. Lett. 85, 4466 (2000)

45. 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.

46. 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.

47. Rydberg physics with cold strontium – Seminar October 2010