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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]

An experimental atomic physicist who coauthored a 1952 report on optical pumping with Jean Brossel and Alfred Kastler. Selective absorption of light redistributes population among ground-state Zeeman effect sublevels, preparing an atomic ensemble whose magnetic-state populations differ from thermal equilibrium. This connects control of incident light to preparation and readout of atomic angular momentum.^[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]

Alfred Kastler's 1966 lecture recalls his 1950 proposal and distinguishes the early ${}^{23}\mathrm{Na}$ atomic beam work from later sealed-vapour experiments. Its Fig. 5 shows subsequent sodium radio wave resonances, including multiple-photon transitions.^[ref]

The 1952 demonstration by Jean Brossel, Alfred Kastler and Jacques Winter used a sodium atomic beam, lamp light with circular polarization and fluorescence readout. Jean Brossel was a central experimental contributor; Alfred Kastler received the 1966 Nobel Prize in Physics alone.^[ref][ref]

The lecture also describes a ${}^{199}\mathrm{Hg}$ experiment: a pumping beam with circular polarization prepares the vapour, while a perpendicular probe converts transverse magnetic dipole moment Larmor precession into modulated absorption at a photodetector. Interrupting pump and RF together exposes decaying free Larmor precession. Its envelope measures transverse relaxation under that model; it is distinct from the sodium atomic beam fluorescence signal.^[ref]

MIT's later rubidium experiment pumps natural ${}^{85}\mathrm{Rb}$/${}^{87}\mathrm{Rb}$ with a discharge lamp filtered to D1 ($5S_{1/2}\to 5P_{1/2}$, near $795\mathrm{nm}$). A polarizer and quarter-wave plate precede a heated rubidium/neon cell. Helmholtz coils set a weak static magnetic field. Separate transverse RF mixing increases absorption, measured by photodiode transmission.^[ref]

Compare resonant and detuned RF with the optical beam fixed, then change the calibrated static magnetic field: a true magnetic sublevel resonance should move with its Zeeman splitting. Lamp drift, stray magnetic fields, imperfect circular polarization, optical attenuation and RF power broadening can change the observed signal. Consequently, a transmission dip alone does not determine every sublevel population. These are measurement-design considerations, not additional reported historical observations.

Awarded to Alfred Kastler for discovering and developing optical methods for studying radiofrequency resonances in chemical atoms.^[ref]