where γ is the inverse upper-state lifetime, related to the natural linewidth of the atomic transition, and kB is the Boltzmann constant. Typical values for the Doppler limit are of the order of hundreds of microkelvins.
Temperatures below the Doppler limit can be reached, e.g. with Sisyphus cooling or with velocity-selective coherent population trapping. It is then possible to reach the lower recoil limit, or even lower temperatures.
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