physics 551 presentation: doppler cooling zane shi princeton university november 6 th, 2007
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Physics 551 Presentation:
Doppler Cooling
Zane Shi Princeton University
November 6th, 2007
Photon Recoil
Absorbed photon reduces the atom’s velocity by ∆v = ћk/m
Emitted photon is in a random direction, and the time average of the momentum transfer is zero
Sodium Levels
Doppler EffectDue to the Doppler Effect, the laser must be tuned to a frequency ω=ω0-kvz
As the atom slows down, the driving frequency must be adjusted accordingly so that the atom still absorbs the photon
Two ways to experimentally achieve this:
Zeeman Tuning
Chirp Cooling
Zeeman Tuning
The tapered solenoid creates a variable magnetic field such that the transition frequency shift due to Zeeman splitting matches the frequency shift due to the Doppler effect
F’=3
F”=2
ħω
kvzBB
)(
0
Chirp Cooling
Another method of Doppler cooling is to rapidly sweep the driving laser frequency
If the frequency is swept fast enough, each atom will interact with the “right” photon; for sodium atoms, a sweep of 1GHz in a few milliseconds is required
The name comes from the fact that the driving frequency sweeps sounds like birds chirping
Optical Molasses
Six Laser beams from the three coordinate axis intersect at the origin to form a “trap”
Each atom with velocity v can absorb a frequency ofwhere is the natural line width
kv0
Due to the Doppler Effect, the force on the atom in the trap is F = -av, i.e., the atom is always pulled back in to the trap
Optical Molasses
Doppler Limit
The frequency of the six laser beams in the optical molasses can be gradually reduced so that only the slowest atoms are confined
However, since the emission of a photon from an excited atom is in a random direction, the atom can gain momentum from this emission
When the cooling effect from the laser balances the heating from emission, the Doppler Cooling Limit is reached:
BD kT
2
1997 Nobel Prize in Physics
William Phillips -Developed the Zeeman slowing technique First to slow atoms to an average velocity of zero in 1985
Steven Chu –Used chirp cooling to slow down atoms and was the first to confine atoms using an optical molasses trap
Claude Cohen-Tannoudji -Developed Sisyphus cooling
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