laser cooling and trapping,, zeeman effect,,

1. LASER COOLING AND TRAPPING BY HARISH GOUD PULI 17 March 2010 University of Arkansas harishfysx@gmail.com

2. It all started in 1975 1975 Hänsch/Schawlow and Wineland/Dehmelt : possibility of laser cooling 1978 First demonstration of laser cooling for trapped ions (Neuhauser et al.; Wineland et al.) 1982 First stopping of a thermal beam (Philips & Metcalf) 1985 First cooling of sodium atoms (Chu, Hollberg et al.) --- 240 μK 1987 Theory of magneto-optical trap (MOT) (Dalibard et al.) 1988 Sub-Doppler cooling (Cohen-Tannoudji et al.)--- 40 nK 1995 Laser + evaporative cooling (Anderson, Cornell et al.) --- 20 nK Nobel Prizes 1989 Paul ion-trap 1997 Chu, Cohen-Tannoudji, Philips laser cooling & trapping 2001 Cornell, Ketterle, Wieman BEC 2005 Glauber, Hall, Hänsch laser-based precision spectroscopy harishfysx@gmail.com

3. Temperature Scale 300K Room temperature 30K Resonant Collisions 3K Liquid helium 3mK Optical Cooling 300µK Doppler Limit 3µ K Recoil Limit 2nK Evaporative Cooling 0K Absolute zero harishfysx@gmail.com

4. Atoms in gases Hot gas Atoms at room temperature have velocities ~ 666 m/s Cold gas harishfysx@gmail.com

5. Light exerts force harishfysx@gmail.com

6. Resonance Force Frequency harishfysx@gmail.com

7. Frequency Mismatch Laser Atomic beam producer(OVEN) harishfysx@gmail.com

8. Frequency match Laser Atomic beam producer(OVEN) harishfysx@gmail.com

9. Doppler shifting of frequency Force Frequency harishfysx@gmail.com

10. Doppler Effect Atoms moving to left feel strong force Laser Atoms moving to right feel nothing harishfysx@gmail.com

11. Viscous Drag Force Laser Optical Molasses Laser Laser So no matter in what direction atom moves , it feels the force and cant escape. This situation called Optical Molasses. Laser harishfysx@gmail.com

12. Magneto Optical Trapping m=+1 m=-1 m=0 harishfysx@gmail.com

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14. Equilibrium in this process atom first absorbs photons and then emits it. When atom emits photon it gets kicked . This is heating process So there is competition between heating and cooling process. Finally atoms come to an equilibrium temperature. harishfysx@gmail.com

15. Doppler Temperature Boltzmann Constant Doppler Temperature Limit Energy width of optical transition. harishfysx@gmail.com

16. Problems Beam of atoms have huge spread of velocities. All atoms can not be slowed down. Once the atom slows down it cant absorb light any more Andreev,Balykin,Letokhov and Minogin 1981 harishfysx@gmail.com

17. Zeeman Cooling Magnetic Field Compensates Doppler Shift Atoms stays in resonance with the laser through out its journey from one end to another end harishfysx@gmail.com

18. Theory≠Practical Expected Temperature 240 µK Measured Temperature 40 µK harishfysx@gmail.com

19. SOLUTION Resolution of this problem was published by Dalibard and Cohen-Tannoudji The sources of this additional cooling is fortuitous combination of multilevel atoms,polrization gradients, light shifts and optical pumping. They named this phenomena as “Sisyphus” cooling harishfysx@gmail.com

20. Sisyphus Cooling Superposition of the counter propagating fields gives rise to the position dependent polarization of the electric field... polarization gradient. X-Polarized Y-Polarized harishfysx@gmail.com

21. Assume an atom with a doublet ground state Ground state manifold is coupled by the radiation field to the quadruplet excited state manifold. Selection Rules harishfysx@gmail.com

22. Atom in the light field Field excites transition Field excites transition Further Selection Rules 1/3 1/3 Clebsch –Gordan Coefficients 1 1 Absorption harishfysx@gmail.com

23. Periodic modulation of atomic levels Since the intensities of the polarization states vary in space, the energy shifts are also position dependent. We will get Stark shifts for the ground states as function of sine for states Or function of cosine for state harishfysx@gmail.com

24. Atom in the light field Consider a point where the field is left circularly polarized The related weights for the transitions are given by Clebsch Gordon Coefficients . From Clebsch Gordon coefficients we can conclude that the excited state with m=1/2 is more likely to decay back to ground state with m=1/2 1 Squares of Clebsch –Gordan Coefficients 2/3 1/3 harishfysx@gmail.com

25. The net result of this process is pumping of atoms from into which has at this location lower energy than Thus atom climbs up these potential hills and it would be put back to bottom of the hill by spontaneous emission. This is effective way of losing kinetic energy harishfysx@gmail.com

26. Its movie time harishfysx@gmail.com

27. Laser cooling : demonstrated species harishfysx@gmail.com

28. Evaporative cooling harishfysx@gmail.com

29. Atom Cloud &Temperature Measurement harishfysx@gmail.com

30. Atom Cloud Measurement CCD camera atom cloud imaging lenses laser beam UHV glass cell harishfysx@gmail.com

31. Temperature measurement harishfysx@gmail.com

32. Applications For making better atomic clocks High resolution spectroscopic measurements Studying ultra cold gases, ex: BEC Lithography with cold atomic beams to build accurately controlled structures Ultra precise measurements of gravitational fields Navigation, ex: GPS harishfysx@gmail.com

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