= Optical pumping
{wiki}
<Optical pumping> redistributes populations among internal <quantum states> by selective <photon absorption> followed by <spontaneous emission>. It can prepare atoms for measurement and participate in <sub-Doppler cooling>.
The practical aim is to make otherwise almost equally populated <magnetic sublevels> behave differently. Repeated absorption and emission can store a population imbalance in the long-lived <ground state>; a change in <photon absorption> or <fluorescence> then reveals a perturbation of that prepared distribution. This combines preparation and measurement: a weak <radio wave> can reorganize the populations, while abundant optical <photons> supply the readout.
An imbalance among <ground state> <magnetic sublevels> need not be a <population inversion> between optical <energy levels>. <Optical pumping> also supplies excitation in some <lasers>, where an appropriate <population inversion> matters for gain. The preparation problem here concerns the distribution within the <ground state>. A <two-level system> with only one lower <quantum state> cannot describe this redistribution.
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