The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling. Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization is linear but the direction rotates along the direction of the beams at a very fast rate. Atoms moving in the spatially varying linear polarisation have a higher probability density of being in a state that is more susceptible to absorption of light from the beam coming head-on, rather than the beam from behind. This results in a velocity dependent damping force that is able to reduce the velocity of a cloud of atoms to near the recoil limit. The recoil limit, T r , is set by the energy of the photon emitted in the decay from the J’ to J state, where the J state is the ground state angular momentum and the J’ state is the excited state angular momentum. This temperature is given by k b T r = h 2 M λ λ 2 though practically the limit is a few times this value because of the extreme sensitivity to external magnetic fields in this cooling scheme. Atoms typically reach temperatures on the order of μ μ K , as compared to the doppler limit T D ≃ ≃ 240 μ μ K .