High-Sensitivity rf Detection Using an Optically Pumped Comagnetometer Based on Natural-Abundance Rubidium with Active Ambient-Field Cancellation

Abstract

To detect a specific radio-frequency (rf) magnetic field, rf optically pumped magnetometers (OPMs) require a static magnetic field to set the Larmor frequency of the atoms equal to the frequency of interest. However, unshielded and variable magnetic field environments (e.g., an rf OPM on a moving platform) pose a problem for rf OPM operation. Here, we demonstrate the use of a natural-abundance rubidium vapor to make a comagnetometer to address this challenge. Our implementation builds upon the simultaneous application of several OPM techniques within the same vapor cell. First, we use a modified implementation of an OPM variometer based on ${}^{87}$$\mathrm{Rb}$ to detect and actively cancel unwanted external fields at frequencies $⪅60\phantom{\rule{0.2em}{0ex}}\mathrm{Hz}$ using active feedback to a set of field control coils. We exploit this stabilized field environment to implement a high-sensitivity rf magnetometer using ${}^{85}$$\mathrm{Rb}$. Using this approach, we demonstrate the ability to measure rf fields with a sensitivity of approximately $9\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.1em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ inside a magnetic shield in the presence of an applied field of approximately $20\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{T}$ along three mutually orthogonal directions. This demonstration opens up a path toward completely unshielded operation of a high-sensitivity rf OPM.