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Alfred Kastler developed optical pumping, using selective absorption and emission of light to redistribute populations among atomic Zeeman sublevels. Angular momentum selection rules make some transitions accessible and others forbidden, so repeated illumination can produce a strongly unequal population. A subsequent magnetic resonance changes that distribution and therefore the detected light. Optical preparation and optical readout connect a weak radio frequency excitation with a sensitive optical signal.^[ref]

A physicist.^[ref]

Jean Brossel and Alfred Kastler developed optical preparation and detection of atomic magnetic resonance. Selectively absorbed light creates unequal populations among Zeeman sublevels; a radio frequency transition redistributes those populations. The resulting change in absorption or fluorescence supplies a visible signal of the resonance. The light thus has two distinct jobs: optical pumping prepares the initial quantum state distribution, while optical detection reads out the effect of a much weaker magnetic excitation.^[ref]

A physicist.^[ref]

A Zeeman sublevel is a magnetic-field-resolved component of an angular-momentum level, labeled by its projection quantum number $m$. In the weak-field regime, $\Delta E=g{\mu }_{B}mB$ for the relevant electronic angular momentum, with appropriate modifications for hyperfine levels.

For angular momentum one, the values $m=-1,0,1$ give three shifts. The states can be degenerate at zero field yet acquire different energies in a field. Light polarization and selection rules control which sublevels a laser addresses. Strong fields can change the angular-momentum coupling, so one cannot extrapolate every weak-field formula indefinitely.