Laser Cooling 2

In Zwierlein's apparatus, the laser beam shoots photons in the opposite direction of the atomic beam. When an atom absorbs the photon, momentum is transferred and it slows down. When an atom re-emits that photon, it is emitted in a random direction. As the atoms are continuously absorbing photons coming from one direction, and re-emitting photons in random directions, the net result is a loss of momentum in that direction.