H. J. Metcalf, P. Straten, Laser Cooling and Trapping

Beam deceleration with photon recoil

• Absorb a photo to receive momentum kick

• Spontaneous emission is symmetric

• Random walk theory assumed

• Recoil limit

k

1Mv

k

M

kTk recB 2

)( 2

• A two-photon process: a simultaneous absorption

and a stimulated emission by an atom

• Two counter-propagating beams

• Resonant excitation condition

• Linewidth determined by interaction time

• At 600nm, pulse duration 0.6ms , equivalent to

Raman transitions

122211 )( vkvk LL

)(2 2112 LLkv

pulse

v

1

smmv /1 900/recT

Raman cooling

J. Foot, Atomic Physics

• Using Sodium with hyperfine ground states

• Atoms were pre-cooled using beam

deceleration magneto-optical trap, and

3D polarization gradient molasses.

• Applied a sequence of Raman pulses with

controlled linewidth and line shape.

• Using Blackman pulse envelope to avoid

unwanted off-resonant excitation

• Achieve T=100nK, less than 1/10 recoil limit

Experimental demonstration by S. Chu et al.

M. Kasevich, S. Chu, Laser cooling below a photon recoil with 3-level atoms, Phys. Rev. Lett. 69, 12 (1992)

• In theory one can arbitrarily narrow the linewidth

• However it takes longer time for narrower linewidth, which limit the Raman cooling in practice.

Fundamental limit