Sub-Doppler cooling

Sub-Doppler cooling is a class of laser cooling techniques that reduce the temperature of atoms and molecules below the Doppler cooling limit. In experiment implementation, Doppler cooling is limited by the broad natural linewidth of the lasers used in cooling. ^[1] Regardless of the transition used, however, Doppler cooling processes have an intrinsic cooling limit that is characterized by the momentum recoil from the emission of a photon from the particle. This is called the recoil temperature and is usually far below the linewidth-based limit mentioned above. ^[2]

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, electromagnetically induced transparency (EIT) cooling, and the use of a dark magneto-optical trap. These techniques can be used independently or combined in an experimental sequence, depending on the minimum temperature needed and specifications of the individual setup. For example, an optical molasses time-of-flight technique was used to cool sodium (Doppler limit ${\displaystyle T_{D}\approx 240\ \mu K}$) to ${\textstyle 43\pm 20\ \mu K}$.^[3]

Motivations for sub-doppler cooling include motional ground state cooling, cooling to the motional ground state, a requirement for maintaining fidelity during many quantum computation operations.