The named temperature bound for the ideal two-level-atom model of Doppler cooling is the Doppler limit. It is the lowest temperature predicted when laser cooling is balanced by heating from random spontaneous photon emissions.
In Doppler cooling, lasers are tuned slightly below an atom’s resonance frequency. Atoms moving toward an opposing laser see its light Doppler-shifted closer to resonance, absorb photons, and lose kinetic energy through the resulting radiation-pressure force.
Cooling cannot continue indefinitely. Each absorption event is followed by spontaneous emission in a random direction, producing momentum kicks that add heating. In the simplest model, the balance gives the Doppler temperature, commonly written as T_D = ℏγ/(2k_B), where γ is the transition linewidth.
The limit is an ideal benchmark, not an absolute floor for every laser-cooling method. Real atoms have hyperfine structure, and sub-Doppler mechanisms such as Sisyphus cooling can reach temperatures below it. The recoil limit is a different, generally lower scale.