Progress towards laser cooling strontium atoms on the intercombination transition

1. Progress towards laser cooling strontium atoms on the intercombination transition Danielle Boddy Durham University – Atomic & Molecular Physics group

2. The team Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

3. Motivation: Rydberg physics Ionization threshold Energy States of high principal quantum number n. Exaggerated size and lifetimes. Can be prepared through laser excitation. Greatly enhanced inter-atomic interactions. Strong, tunable, long-range dipole-dipole interactions among the atoms. Applications include quantum computation. M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

4. Motivation: Dipole blockade Y Miroshnychenko et al., Nat. Phys. 5, 115-118 (2009) E Urban et al., Nat. Phys. 5, 110-114 (2009) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

5. Motivation: An introduction to strontium Doubly excited state Rydberg state Sr88 is an alkaline earth metal with no hyperfine structure. Two valence electrons permits two electron excitation. Ground state Spectroscopy of strontium Rydberg states using electromagnetically induced transparencyS. Mauger, J. Millen and M. P. A. Jones J. Phys. B: At. Mol. Opt Phys. 40, F319 (2007) Two-electron excitation of an interacting cold Rydberg gasJ. Millen, G. Lochead and M. P. A. Jones Phys. Rev. Lett. 105, 213004 (2010) At present, we’re investigating the spatial excited state distribution. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

6. Motivation: Dipole blockade regime Blockaded No blockade rB T ~ 400 nK Density ~ 1 x 1012 cm-3 T ~ 5 mK Density ~ 1 x 109 cm-3 How do we enter the dipole blockade regime? Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

7. Motivation: Laser cooling of strontium 1S0→3P1 intercombination transition → TD ≈ 180 nK. Photon recoil limits TD → Tmin ≈ 460 nK. 1P1 3P2 Introduce two stages of cooling: First cool on the (5s2) 1S0 → (5s5p) 1P1. Second cool on the narrow-line (5s2) 1S0 → (5s5p) 3P1 . 3P1 λ = 461 nm Γ = 2π x 32 MHz 1st stage cooling 3P0 λ = 689 nm Γ = 2π x 7.5 kHz 2nd stage cooling 1S0 Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

8. Outline Simple laser stabilization set-up Laser system Pound-Drever-Hall (PDH) Locking to an atomic transition Fluorescence Electron shelving Summary Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

9. Simple laser stabilization set-up Atomic signal Fabry-Perot cavity Laser system Red MOT Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

10. Simple laser stabilization set-up Atomic signal Fabry-Perot cavity Laser system Laser system Red MOT Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

11. Laser system Compared old and new designs. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

12. Laser system frequency frequency time time 10 s OLD 1 Wavemeter 2 NEW NEW OLD 1 Wavemeter 2 10 s Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

13. Laser system New Old New design fluctuates more in the short term. Little difference between the long term stability. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

14. Simple laser stabilization set-up Atomic signal Fabry-Perot cavity Fabry-Perot cavity Laser system Red MOT Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

15. Pound-Drever-Hall (PDH) technique Require the laser linewidth < 7.5 kHz. Noise broadens the linewidth to the MHz regime. Uses Fabry-Perot cavity as a frequency reference. Cavity peaks are spaced by the free spectral range : Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

16. Pound-Drever-Hall (PDH) technique Phase modulator adds sidebands to the laser. High-finesse Fabry-Perot cavity measures the time-varying frequency of the laser input. An electronic feedback loop works to correct the frequency error and maintain constant optical power. Phase modulator Etalon Laser Current modulation Piezo FPD Lock Box Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

17. Pound-Drever-Hall (PDH) technique Atomic signal FPD Feedback to cavity piezo Lock Box Fast feedback to diode PS Slow feedback to piezo Laser A crystal oscillator phase modulates the 689 nm beam at a frequency of 10 MHz. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

18. Pound-Drever-Hall (PDH) technique Laser locks to the central feature of the PDH error signal (b) (a) (d) (c) Increasing the gradient of the error signal strengthens the lock and reduces the linewidth. Gradient depends on sideband power: carrier power ratio. Gradient steepest when Ps = 0.42 Pc Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

19. Pound-Drever-Hall (PDH) summary Generate PDH signal Gradient of error signal → strength of lock and laser linewidth NEXT STEP: Finish high bandwidth servo IMPROVEMENTS: Build high-finesse cavity Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

20. Simple laser stabilization set-up Atomic signal Atomic signal Fabry-Perot cavity Laser system Red MOT Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

21. Locking to an atomic transition CHALLENGE: Detecting the transition. Two detection methods: 1. Electron Shelving 2. Fluorescence Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

22. Electron shelving Excite atoms to the 3P1 and measure the rate at which these atoms decay out of the state. Photon scattering rate is proportional to the linewidth of the transition. 1P1 3P1 λ = 461 nm λ = 689 nm 1S0 Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

23. Electron shelving photodiode 1P1 atomic beam 3P1 λ = 461 nm The amount of scattered light is proportional to the number of atoms initially in the 1S0 ground state. 1S0 Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

24. Electron shelving photodiode 1P1 atomic beam 3P1 λ = 461 nm λ = 689 nm 1S0 Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

25. Electron shelving: Experiment photodiode atomic beam Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

26. Electron shelving: Experiment photodiode atomic beam Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

27. Electron shelving: Experiment photodiode atomic beam Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

28. Electron shelving: Experiment ≈ 32 MHz Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

29. Electron shelving: Lifetime measurement Gradient: (8.9 ± 0.2) x 10-2 mm-1 Using a velocity of 500 ms-1 Lifetime of 3P1 is (23 ± 1) μs Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

30. Electron shelving: Crossed beams photodiode atomic beam FWHM crossed beams is ≈ 20 MHz. Linewidth has reduced by 1/3. This is not narrow enough for the Fabry-Perot to lock to! Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

31. Electron shelving: Summary • Detected the transition indirectly via electron shelving. • Determined the lifetime of the 3P1 state. • And the lineshape? • Tried crossing the beams: • Did the linewidth reduce? • Is this narrow enough for the laser to lock to? Work in progress Try a direct method of detection. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

32. Fluorescence: The experiment Strontium has negligible vapour pressure at room temperature → heated to 900 K. CCD camera takes spatially resolved images of the fluorescence. Exposure length set to 65.5 ms. atomic beam Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

33. Fluorescence: The experiment • Slice along direction of laser beam → absorption and decay. • Slice along direction of atomic • beam → transverse velocity • distribution. (a) (b) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

34. Fluorescence: The experiment Gradient: (9.0 ± 0.3) x 10-2 mm-1 Using a velocity of 500 ms-1 Lifetime of 3P1 is (22.2 ± 0.7) μs Other time resolved fluorescence detection: τ= (21.3 ± 0.5) μs See R Drozdowski., Phys. D. 41:125 (1997) Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

35. Fluorescence: The experiment BUT what about the absorption? Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

36. Fluorescence: The model Solves optical bloch equations (OBEs) for a two level atom as a function of distance. Velocity distribution of atoms α Randomly selects a value of and . If the value of is kept. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

37. Fluorescence: The model Assuming the laser is on resonance the only other unknown in the OBEs is the Rabi frequency. Top hat pulse: x Gaussian pulse: x Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

38. Fluorescence: The model Velocity of 500 ms-1 Top hat pulse Gaussian pulse Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

39. Fluorescence: The model 2 x waist Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

40. Fluorescence: Summary Detected the transition directly. Determined the lifetime of the 3P1 state. Written code to model absorption and decay. Data and theory don’t quite agree. Need to find source of problem. NEXT: Try locking to this fluorescence signal. Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

41. Summary 689 nm laser built and tested. Need to finish PDH high bandwidth servo circuit. Build high-finesse cavity. Tested an indirect and direct method to detect the transition. Measured lifetime of 3P1 state from both methods. Try locking to fluorescence signal. If this works….GREAT! If it doesn’t work….try pump-probe spectroscopy Red MOT → colder atoms Progress towards laser cooling strontium atoms on the intercombination transition - May 2011

42. Questions? Thanks for listening  Any questions? Progress towards laser cooling strontium atoms on the intercombination transition - May 2011