Recoil temperature
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In laser cooling, the Boltzmann constant times the recoil temperature is equal to the recoil energy deposited in a single atom initially at rest by the spontaneous emission of a single photon. It is defined as
where is the wavevector of the light, is the mass of an atom, is Boltzmann's Constant and is Planck's Constant. It is typically on the order of 1 μK, and thus lower than the Doppler temperature. An example of a process where the recoil temperature can be reached is Sisyphus cooling.