Doppler cooling is a technique for laser cooling of small particles (typically atoms or ions). The basic principle is that absorption and subsequent spontaneous emission of photons lead to light forces, and these forces become velocity-dependent through the Doppler effect: an absorption resonance of an atom or ion is shifted, e.g., towards lower frequencies when the particle is moving towards the light source. For example, a beam of atoms in a vacuum chamber can be stopped and cooled with a counterpropagating single-frequency laser beam, the optical frequency of which is first chosen to be somewhat higher than the atomic resonance, so that only the fastest atoms can absorb photons. Subsequently, the laser frequency is reduced so that slower and slower atoms participate in the interaction, and finally all atoms have a greatly reduced speed (at least in one dimension). This corresponds to a lower temperature, assuming that thermal equilibrium can be reestablished.
An alternative to sweeping the laser frequency is sweeping the atomic resonances via a spatially varying magnetic field (Zeeman slowing).
Doppler cooling can also be used in an arrangement called optical molasses, where cooling occurs in all three dimensions.
The minimum temperature achievable with Doppler cooling is the Doppler limit. In some cases, however, cooling well below the Doppler limit (down to the region of the recoil limit) has been observed and explained as Sisyphus cooling.
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