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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. The recoil temperature is
T r e c o i l = ℏ 2 k 2 m k B {\displaystyle T_{recoil}={\frac {\hbar ^{2}k^{2}}{mk_{B}}}} ,
since the photon's momentum is p = ℏ k {\displaystyle p=\hbar k} . The recoil temperature for the D2 lines of alkali atoms 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.