Laser cooling

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. Because each of the scattered photons gives a small momentum kick to the atoms, they are effectively slowed down. By using counter propagating sets of laser beams in all three Cartesian coordinates, we get a force which drives the velocity of all the atoms to zero. This way, the atoms are cooled.

The lowest temperature one can reach with this technique is the so called Doppler temperature[?]. This temperature limit is due to the fact that the light not only cools the atoms, but also heats them. The light that is absorbed is emitted by spontaneous emission into a random direction. This means that the atoms get a lot of random momentum kicks from the light, which causes heating. The lowest temperature where these two opposing mechanisms balance each other is called the Doppler temperature.