Fig 1. Optically-pumped-magnetometer’s principle of operation.

(1) The rubidium atoms of the vapor cell have randomly oriented atomic spins. (2) Using the circularly polarized pump-laser (795 nm), the spins are oriented in the propagation direction of the pump-laser, generating a macroscopic magnetization of Rb vapor (dotted arrow). (3) Due to an external magnetic field (B) the atomic spins precess, causing the magnetization vector of the ensemble of atoms to have a constant angle relative to the pump beam axis approximately proportional to B. Through rotation of the probe beam polarization due to the Faraday effect, one can deduce the component of magnetization along the pump/probe beam axis.