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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]

Henry Michael Foley and Polykarp Kusch compared atomic magnetic resonance frequencies to determine the electron magnetic moment. They selected quantum states whose orbital angular momentum and spin contributions enter with different weights. Ratios of the measured frequencies revealed a departure from the simplest electron spin g-factor of exactly two. Comparing responses in the same magnetic field reduces dependence on absolute field calibration, turning a small spectral difference into a precision test of the magnetic response.^[ref]

A physicist and professor of physics at Columbia University.^[ref] The Kusch-Foley magnetic moment experiment describes his work with Polykarp Kusch.

Isidor Rabi developed molecular-beam magnetic resonance for measuring nuclear magnetic moments. Two magnetic field gradient regions focus a selected beam onto a detector, while an intervening uniform magnetic field sets the resonance frequency. An oscillating transverse magnetic field drives transitions between magnetic quantum states. On resonance, the changed states no longer follow the focused path, reducing the detected intensity. Measuring the Larmor frequency in a known magnetic field thereby replaces a small static deflection with a more precise frequency comparison.^[ref]

Identity source: www.nobelprize.org/prizes/physics/1944/rabi/facts/

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]

Magnetic resonance is a resonantly driven transition between states whose energy separation depends on a magnetic field. A static field splits magnetic substates through the Zeeman effect, while an oscillating transverse field supplies energy near the transition frequency. For a simple spin with gyromagnetic ratio $\gamma$, the angular resonance frequency is ${\omega }_0=|\gamma |B_0$.

In the Rabi molecular beam magnetic resonance experiment, inhomogeneous magnets selected and refocused molecular trajectories. A radio-frequency field changed magnetic substates at resonance, altering the transmitted beam intensity. Scanning the frequency therefore measured magnetic moments through a frequency-dependent signal.^[ref]

Norman Ramsey Jr. developed precision magnetic resonance using separated oscillatory fields. A particle encounters two short coherent interactions with an interval of free evolution between them. The first interaction prepares a quantum superposition; the second converts the accumulated relative quantum phase into a measurable population difference. A small frequency mismatch accumulates quantum phase during the free interval, producing narrow interference fringes whose scale depends on that interval. Ramsey interferometry turns controlled coherent evolution into a precise frequency comparison.^[ref]

Identity source: news.harvard.edu/gazette/story/2015/07/norman-foster-ramsey-jr/

Polykarp Kusch and Henry Michael Foley measured atomic magnetic resonance frequencies to determine the electron magnetic moment. Different quantum states weight orbital angular momentum and spin contributions differently, allowing ratios of magnetic responses to isolate the spin contribution. The inferred electron spin g-factor differs slightly from two. The Kusch-Foley magnetic moment experiment thus probes a microscopic interaction through accurately counted frequencies, rather than a mechanical measurement of an isolated electron.^[ref]

A physicist.^[ref]