An Optically Pumped Comagnetometer for High-sensitivity RF Detection without Magnetic Shielding

We develop an optically-pumped magnetometer (OPM) sensitive in the radiofrequency( RF) regime based on 85Rb in a natural abundance rubidium vapor cell to operate in dynamic geomagnetic field environments. Because high sensitivity operation of RF OPMs requires control of the magnetic field environment near DC, we develop an OPM comagnetometer that also utilizes the 87Rb present within the same vapor cell to implement a secondary OPM variometer that provides information on the field-environment near DC. This information is used to provide feedback via a set of tri-axial field control coils that counters the unwanted effects of external geomagnetic fields from DC to 60 Hz, which in turn allows RF OPM operation with an intrinsic sensitivity of around 9 fT/√Hz presence of external magnetic fields on the order of 10s of µT.