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

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

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