| 1 | A stationary atom sees the laser neither red- nor blue-shifted and does not absorb the photon. |
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| 2 | An atom moving away from the laser sees it red-shifted and does not absorb the photon. |
| 3.1 | An atom moving towards the laser sees it blue-shifted and absorbs the photon, slowing the atom. |
| 3.2 | The photon excites the atom, moving an electron to a higher quantum state. |
| 3.3 | The atom re-emits a photon. As its direction is random, there is no net change in momentum over many photons. |
Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques.