= Optical pumping preparation and readout
<Circular polarization> selects the sign of the absorbed <photon>'s <angular momentum> along the beam. Choose that direction as $z$ and define $\sigma^+$ by the absorption rule $m'=m+1$, where $m$ is the <magnetic quantum number>. <Spontaneous emission> can return an atom to a different <magnetic sublevel>, so absorption followed by emission is a population-transfer cycle. A sublevel that the chosen beam cannot excite is dark to that beam. Darkness is relative to the selected transition and <electromagnetic polarization>, rather than a claim that the atom cannot interact with any <light>.
A static <magnetic field> along $z$ defines a stable axis and gives <Zeeman splitting>. A transverse oscillating <magnetic field> can redistribute <magnetic sublevel> populations when its <frequency> matches an allowed splitting. With the appropriate optical transition, that redistribution restores <photon absorption>: transmitted intensity decreases, or collected <fluorescence> increases. These are alternative readouts whose detailed sensitivity depends on the transition, viewing direction and <electromagnetic polarization>.
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