We demonstrate magnetic-field sensing using the intrinsic nitrogen-vacancy concentration of a single-crystal diamond placed in an optical cavity resonant with the pump field. We investigate two approaches based on fluorescence detection and pump absorption, respectively.
We demonstrate magnetic-field sensing using an ensemble of nitrogen-vacancy centers by recording the variation in the pump-light absorption due to the spin-polarization dependence of the total ground-state population. Using a 532 nm pump laser, we measure the absorption of native nitrogen-vacancy centers in a chemical-vapor-deposited diamond placed in a resonant optical cavity. For a laser pump power of 0.4 W and a cavity finesse of 45, we obtain a noise floor of $\ensuremath{\sim}100 \mathrm{nT}/\sqrt{\mathrm{Hz}}$ spanning a bandwidth up to 125 Hz. We project a photon shot-noise-limited sensitivity of $\ensuremath{\sim}1 \mathrm{pT}/\sqrt{\mathrm{Hz}}$ by optimizing the nitrogen-vacancy concentration and the detection method.
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 $\ensuremath{\sim}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}200\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}$.
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 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.
Magnetometry with nitrogen-vacancy (NV) centers in diamond has shown great promise in recent years. In particular, absorption-based magnetometry techniques, employing a cavity to enhance the absorption length, can improve the contrast and sensitivity compared to conventional techniques based on reading out the NV− triplet fluorescence. The absorption techniques rely on magnetic-field-dependent absorption at the NV− singlet zero phonon line at 1042nm and its phonon sideband. In a cavity-enhanced spectroscopy approach, we study pump-laser- and microwave-induced cavity signal changes at room temperature over a spectral range of 680-1050nm. Through normalization, we eliminate the cavity-enhancement effect and provide quasi-single-pass values for the absorption and optically detected magnetic resonance (ODMR) contrast. The highest contrast is found at 1042nm, but multiple points of high contrast are found at the peaks of the phonon sideband. Additionally, cavity-enhanced ODMR contrasts in the range of 50-80% are presented. We further measure the broadband singlet absorption cross section at room temperature with a novel method through microwave-induced signal changes. This method is insensitive to pump-laser-induced signal changes by other defects and quantifies the room-temperature absorption strength of the singlet transition and its entire phonon sideband. We determine the absorption cross section at 1042nm to be σ1042★=(0.89±0.14)⋅10−21m2 or σ1042▲=(2.9±0.5)⋅10−21m2. depending on the employed 532nm NV− absorption cross section.