What phenomenon uses the Doppler effect to trap and cool atoms?

The phenomenon that uses the Doppler effect to trap and cool atoms is Doppler cooling. It slows atoms by using carefully detuned laser light so that moving atoms preferentially absorb photons opposing their motion.

The lasers are tuned slightly below an atom’s resonant frequency, a setting called red detuning. An atom moving toward an incoming beam sees the light shifted closer to resonance and is therefore more likely to absorb a photon from that direction. Photon absorption pushes against the atom’s motion; later spontaneous emission occurs randomly, so its kicks average out while repeated absorptions provide a net slowing force.

Doppler cooling was proposed independently in 1975 by David Wineland and Hans Dehmelt, and by Theodor Hänsch and Arthur Schawlow. Experiments followed in 1978. In three dimensions, counter-propagating beams can create optical molasses, and a magnetic field can be added to form a magneto-optical trap.

Doppler cooling is not identical to every form of laser cooling. Sisyphus cooling and evaporative cooling can reach lower temperatures under suitable conditions, while Doppler cooling has a characteristic limit set by the balance between cooling and photon-recoil heating.