= Optical magnetometry
<Optical magnetometry> infers a <magnetic field> from the response of optically prepared and observed atomic spins. In a typical vapor-cell apparatus, resonant pump light polarizes alkali <atoms>; a field makes that polarization precess at a <Larmor frequency> $\omega_L=|\gamma|B$. Probe-light absorption or rotation converts the spin evolution into an optical signal.<Optical Magnetometry by Dmitry Budker and Michael Romalis>[Principles of optical magnetometry and atomic-vapor implementations]{ref}
A motivating experiment uses a sealed vapor cell, pump and probe beams, photodetection, magnetic shielding and coils producing a known test field. Applying an oscillating field and sweeping its frequency reveals a resonance near $\omega_L$. Its shift when the coil current changes calibrates the field response, while its width reflects spin relaxation. If a sensor has a calibrated gyromagnetic ratio $|\gamma|/(2\pi)=7\,\mathrm{Hz/nT}$, a 70 Hz resonance corresponds to 10 nT in this simple model. This numerical example illustrates conversion of frequency to field; actual calibration must control light shifts, field gradients and relaxation. Optical readout detects the spin response, not a direct image of the magnetic field.
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