Harish laser cooling
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LASER COOLING AND TRAPPING BY HARISH GOUD PULI 17 March 2010 University of Arkansas [email protected]
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laser cooling and trapping,, zeeman effect,,
Transcript of Harish laser cooling
LASER COOLING AND TRAPPING
BY HARISH GOUD PULI
17 March 2010University of Arkansas
It all started in 1975
1975 Hänsch/Schawlow and Wineland/Dehmelt : possibility of laser cooling1978 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 Prizes1989 Paul ion-trap1997 Chu, Cohen-Tannoudji, Philips laser cooling & trapping2001 Cornell, Ketterle, Wieman BEC2005 Glauber, Hall, Hänsch laser-based precision spectroscopy
Temperature Scale300K Room temperature
3K Liquid helium
30K Resonant Collisions
3µ K Recoil Limit
3mK Optical Cooling
300µK Doppler Limit
2nK Evaporative Cooling
0K Absolute zero
Atoms in gases
Hot gas
Cold gas
Atoms at room temperature have velocities ~ 666 m/s
TKmv Brms2
3
2
1 2
Atoms moving to left feel strong force
Atoms moving to right feel nothing
Laser
R
Doppler Effect
Viscous Drag Force
VF drag
dragF
V
Laser
LaserLa
ser
Laser
Optical Molasses
So no matter in what direction atom moves , it feels the force and cant escape.This situation called Optical Molasses.
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.
Doppler Temperature
D
B
DB
T
K
TK2
Boltzmann Constant
Doppler Temperature Limit
Energy width of optical transition.
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
Zeeman Cooling
Magnetic Field Compensates Doppler Shift
Atoms stays in resonance with the laser through out its journey from one end to another end
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
Sisyphus Cooling
• Superposition of the counter propagating fields gives rise to the position dependent polarization of the electric field... polarization gradient.
2
X-PolarizedY-Polarized
• Assume an atom with a doublet ground state
• Ground state manifold is coupled by the radiation field to the quadruplet excited state manifold.
2
1gJ
2
3eJ
2
3eJ
2
1gJ
2
1
2
1
2
1
2
1
2
3
2
3 Selection Rules
2
3eJ
2
1gJ
2
1
2
1
2
1
2
3
2
3
Field excites 1mField excites 1m
transition
transition
2
1
Atom in the light field
1 11/3 1/3 Clebsch –Gordan
Coefficients
Absorption
Further Selection Rules
Periodic modulation of atomic levels
Since the intensities of the polarization states vary inspace, 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 2
1
2
1
2
3eJ
2
1gJ
2
1
2
1
2
1
2
3
2
3
2
1
Atom in the light field
1
1/3
Squares of Clebsch –Gordan Coefficients
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
2/3
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
2/1g2/1g
2/1g
CCD camera
atom cloud
imaging lenses
UHV glass cell
laser beam
Atom Cloud Measurement
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
