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SubDoppler cooling is a laser cooling technique used to cool atoms or molecules to temperatures far below the Doppler limit. The Doppler limit, typically around tens of microkelvins, arises from the random motion of atoms, which causes them to experience a Doppler shift in the frequency of the laser light. This shift limits the effectiveness of standard Doppler cooling, where atoms absorb photons from a laser tuned slightly below their resonance frequency and emit photons in random directions, leading to a net momentum change.

SubDoppler cooling, also known as Sisyphus cooling or polarization gradient cooling, overcomes this limit by exploiting the interaction of atoms with spatially varying light polarization. In this technique, the atoms are exposed to a laser beam with a specific polarization gradient, often created by interfering two laser beams with different polarizations. As atoms move through this gradient, they experience a cyclic process involving optical pumping and stimulated emission that effectively "pushes" them into states with lower kinetic energy.

The mechanism relies on the fact that atoms can be optically pumped into different internal states. In a polarization gradient, atoms in certain internal states will be preferentially excited by the laser light. As they absorb a photon, they move to a different internal state, and then spontaneously emit a photon. This process, repeated cyclically, leads to a cooling effect that can reach temperatures as low as a few nanokelvins, significantly below the Doppler limit. This technique is crucial for many experiments in atomic physics, quantum computing, and precision measurements, enabling the study of matter at extremely low temperatures.