Sisyphus cooling (or Sisyphos cooling) is a mechanism for laser cooling of atoms or ions using light forces. It was understood in 1985, after the observation that laser cooling experiments with cesium atoms can lead to temperatures well below the Doppler limit, that the simple mechanism of Doppler cooling is not a sufficient explanation. The mechanism of Sisyphus cooling is somewhat sophisticated. It involves a polarization gradient, as generated e.g. by two counterpropagating linearly polarized laser beams with perpendicular polarization directions (lin ⊥ lin configuration), and is therefore sometimes called polarization gradient cooling. The full explanation is based on the picture of dressed states . An essential ingredient is that when atoms in a certain dressed state “climb uphill”, i.e. reach a position where their potential energy is relatively large, it becomes likely that they are optically pumped into another state for which the potential energy at that position is close to a minimum. In such a way, the polarization gradient introduces non-conservative light forces, which can reduce the average kinetic energy of atoms.
Sisyphus cooling has become important for optical frequency standards, because it makes it possible to cool atoms to very low temperatures (“sub-Doppler temperatures”), so that the linewidth of certain forbidden transitions becomes very small.
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