Doppler Cooling
Photons have a momentum given by ħk (where ħ is the reduced Planck constant and k the photon wavenumber), which is conserved in all atom-photon interactions. Thus, when an atom absorbs a photon, it is given a momentum kick in the direction of the photon before absorption. By detuning a laser beam to a frequency less than the resonant frequency (also known as red detuning), laser light is only absorbed if the light is frequency up-shifted by the Doppler effect; which occurs whenever the atom is moving towards the laser source. This applies a friction force to the atom whenever it moves towards a laser source.
For cooling to occur along in all directions, the atom must see this friction force along all 3 cartesian axis; this is most easily achieved by illuminating the atom with 3 orthogonal laser beams, which are then reflected back along the same direction.
Read more about this topic: Magneto-optical Trap
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