Sub-Doppler cooling is a class of laser cooling techniques that reduce the temperature of atoms and molecules below the Doppler cooling limit. Doppler cooling processes have a cooling limit that is characterized by the momentum recoil from the emission of a photon from the particle.
Some methods of sub-Doppler cooling include optical molasses, Sisyphus cooling, evaporative cooling, free space Raman cooling, Raman side-band cooling, resolved sideband cooling, polarization gradient cooling, and the use of a dark magneto-optical trap.
For example, an optical molasses time-of-flight technique was used to cool sodium (Doppler limit ) to .
Some possible motivations for sub-doppler cooling include cooling to the motional ground state, a requirement for maintaining fidelity during many quantum computation operations.