The nitrogen vacancy (NV) center in diamond is a versatile color center used for magnetometry, quantum computing, and quantum communications. In this article, using a single laser beam as a pump and probe, we measure the spin states of the NV center using the Faraday effect and use such measurements to develop a novel magnetic field sensor. Using the spin-state-dependent effect on the left and right circularly polarized light, we probe and confirm the existence of spin-orbit coupling in the NV center at room temperature. The sensitivity of our magnetometer is 350nT/Hz, limited by the background produced by the laser trapped inside the diamond. We argue that by using an optical cavity and a high-purity diamond, sensitivities in the femtotesla level can be achieved.
We demonstrate 4π-steradian vector magnetic field sensing using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond coupled to a microwave (MW) cavity. The MW cavity enhances the spin-photon coupling which enables efficient, high-contrast spin-state readout via MW interrogation and removes the need for bulky optical collection components. An applied AC bias magnetic field lifts the zero-field degeneracy of the four crystallographic NV orientations, allowing each orientation to be individually addressed and used for vector reconstruction of the magnetic field. The resulting magnetometer has a 40\% contrast (20x higher than typical for optical spin-ensemble readout) and achieves a single-axis sensitivity of 250 pT/Hz which is flat from DC to 1 kHz. Noise models of the composite spin-cavity system establish MW amplitude noise as the dominant noise source and predict a thermal noise limit of 2 pT/Hz.
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.