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.^[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 $|\gamma |/(2\pi )=7Hz/nT$, 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.