Four-Channel Optically Pumped Magnetometer for MEG Sensor Arrays Background Optically Pumped Magnetometers (OPMs) operating in the Spin-Exchange Relaxation-Free (SERF) regime are highly sensitive magnetic field sensors, with sensitivities as low as 0.16 ft/√Hz and 0.54 ft/√Hz. OPMs are based on the interaction between spin-polarized atoms and magne...