Laser cooling is a technique that uses light to cool atoms to a very low temperature. The simplest form of laser cooling is the so called Optical Molasses.

This technique works by tuning the frequency of light slightly below an electronic transition in the atom. Because the light is detuned to the red of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. So if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam with which direction they are counter-propagating. In each scattering event the atom loses a momentum equal to the momentum of the photon. If the atom, which is now in the excited state, emits a photon spontaneously, it will be kicked by the same amount of momentum but in a random direction. If this absorption and emission happens many times the average amount of momentum transferred to the atom by the spontaneous emission process is nearly zero and the atom cools down.

The atom performs a random walk in momentum space with steps equal to the photon momentum due to the spontaneous emission. This constitutes a heating effect which counteracts the cooling process and imposes a limit on the amount by which the atom can be cooled. The lowest temperature where these two opposing mechanisms balance each other is called the Doppler temperature. In Doppler laser cooling counter propagating sets of laser beams in all three Cartesian coordinates are usually used to cool all the three degrees of freedom of the atom.

See also

• Resolved sideband cooling