Laser cooling only reduces the velocity component that is directed against the direction of the laser beam. For free atoms you therefore need three pairs of cou

Laser cooling only reduces the velocity component that is directed against the direction of the laser beam. For free atoms you therefore need three pairs of counter-propagating beams to achieve overall cooling. If the atoms are somehow confined (for example ions in a trap), their motion has to reverse periodically and a single cooling beam can be enough.