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