We propose using an optical cavity to enhance the sensitivity of a magnetometer relying on the detection of the spin state of a high-density nitrogen-vacancy ensemble in diamond using infrared optical absorption. The role of the cavity is to obtain a contrast in the absorption-detected magnetic resonance approaching unity at room temperature. We project an increase in the photon shot-noise limited sensitivity of two orders of magnitude in comparison with a single-pass approach. Optical losses can limit the enhancement to one order of magnitude, which could still enable room-temperature operation. Finally, the optical cavity also allows us to use less pumping power when the cavity is resonant at both the pump and the infrared probe wavelength.
We demonstrate a cavity-enhanced room-temperature magnetic field sensor based on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using absorption of light resonant with the 1042 nm spin-singlet transition. The diamond is placed in an external optical cavity to enhance the absorption, and significant absorption is observed even at room temperature. We demonstrate a magnetic field sensitivity of 2.5nT/Hz, and project a photon shot-noise-limited sensitivity of 70pT/Hz for a few mW of infrared light, and a quantum projection-noise-limited sensitivity of 250fT/Hz for the sensing volume of $\ensuremath{\sim}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}200\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}$.