Sisyphus Cooling

Sisyphus cooling (named after Sisyphus, a figure in the Greek mythology) is a mechanism through which atoms can be cooled using laser beams below the temperatures expected to be achieved by Doppler cooling. It comes about as a result of a polarization gradient created by two counter-propagating laser beams with orthogonal polarization. Atoms moving through the potential landscape created by the standing wave (created by the interference of the two counter-propagating beams) lose kinetic energy as they move to a potential maximum, at which point optical pumping moves them to a lower-energy state, thus losing the potential energy they had.

Repeated cycles of converting kinetic energy to potential energy, then losing potential energy via optical pumping allow the atoms to reach temperatures orders of magnitude below those available through simple Doppler cooling.