Recoil temperature

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

$T_{recoil}={\frac {\hbar ^{2}k^{2}}{2mk_{b}}}$

where $k$ is the wavevector of the light, $m$ is the mass of an atom, $k_{b}$ is Boltzmann's Constant and $\hbar$ 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.