Simplified principle of Doppler laser cooling:
<p>(1) A stationary atom sees the laser neither red- nor blue-shifted and does not absorb the photon.
</p><p>(2) An atom moving away from the laser sees it red-shifted and does not absorb the photon.
</p><p>(3.1) An atom moving towards the laser sees it blue-shifted and absorbs the photon, slowing the atom.
</p><p>(3.2) The photon excites the atom, moving an electron to a higher quantum state.
</p>
(3.3) The atom re-emits a photon. As its direction is random, there is no net change in momentum over many atoms.
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