© Daniel Kleppner.
Atoms moving with respect to a laser see light that is Doppler shifted. If an atom moves toward the light source, the light appears shifted to the blue. The laser must be tuned to the red side of an atomic resonance to be absorbed by the atom. Red-detuned lasers pointing along every axis can be used to slow down atoms in all directions, and thus cool them. If the atom is not moving, as shown on the left side of the diagram, the red-detuned lasers will not interact with the atom at all. If the atom is moving, however, it will absorb photons from the laser beam opposing its motion and slow down as the photon's momentum is transferred to the atom. Cooling continues until the atom cannot detect the Doppler shift. This occurs when the Doppler shift becomes so small that it is masked by the natural linewidth of the atomic transition. (Unit: 5)