SERF regime
Pump beam (x) Probe beam (y) Net spin P Atom spins
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SERF Atomic Magnetometer

This simulator models a spin-exchange-relaxation-free (SERF) atomic magnetometer — the sensor design behind the world's most sensitive magnetic-field measurements, from tests of fundamental physics to helmet-mounted brain-imaging (MEG) systems that no longer need liquid helium. A vapor of alkali atoms is optically pumped into a polarized spin state, an adjustable field Bz makes that polarization precess, and a probe beam reads out the transverse component as the sensor's output signal. Raise the vapor density and spin-exchange collisions between atoms — which would normally dephase the ensemble — instead suppress each other through motional narrowing whenever the precession is slow compared to the collision rate, carving out the narrow, steep dispersive response near zero field that gives SERF sensors femtotesla-class sensitivity. Every control drives the same live Bloch-equation solution feeding both the 3D spin ensemble and the response-curve readout.