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Physical Review Applied

Towards high-sensitivity magnetometry with nitrogen-vacancy centers in diamond using the singlet infrared absorption

Tayefeh Younesi, Ali, Omar, Muhib, Arne Wickenbrock, Budker, Dmitry, Ulbricht, Ronald

Published Jan 1, 2025• 0 citations
Publisher

Abstract

The negatively charged nitrogen vacancy center in diamond is widely used for quantum sensing due to the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields, which makes it an excellent probe for changes in these perturbations.The spin state can be measured through optically detected magnetic resonance, which is most commonly achieved by detecting the photoluminescence after exciting the spin-triplet transition.Recently, methods have been proposed and demonstrated that use the absorption of the infrared singlet transition at 1042 nm instead.These methods, however, require cryogenic temperatures or external cavities to enhance the absorption signal.Here, we report on our efforts to optimize the magnetometer sensitivity at room temperature and without cavities.We reach a sensitivity of 18 pT/ √ Hz, surpassing previously reported values, and a calculated shot-noise limit of 5 pT/ √ Hz .We also report on a defect that is native to diamond grown by chemical vapor deposition and thus absent in high-pressure high-temperature diamond, the excitation of which impacts the measured singlet absorption signal.

DOI
10.15120/gsi-2025-01317

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The negatively charged nitrogen vacancy center in diamond is widely used for quantum sensing due to the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields, which makes it an excellent probe for changes in these perturbations.The spin state can be measured through optically detected magnetic resonance, which is most commonly achieved by detecting the photoluminescence after exciting the spin-triplet transition.Recently, methods have been proposed and demonstrated that use the absorption of the infrared singlet transition at 1042 nm instead.These methods, however, require cryogenic temperatures or external cavities to enhance the absorption signal.Here, we report on our efforts to optimize the magnetometer sensitivity at room temperature and without cavities.We reach a sensitivity of 18 pT/ √ Hz, surpassing previously reported values, and a calculated shot-noise limit of 5 pT/ √ Hz .We also report on a defect that is native to diamond grown by chemical vapor deposition and thus absent in high-pressure high-temperature diamond, the excitation of which impacts the measured singlet absorption signal.
The negatively charged nitrogen vacancy center in diamond is widely used for quantum sensing due to the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields, which makes it an excellent probe for changes in these perturbations. The spin state can be measured through optically detected magnetic resonance, which is most commonly achieved by detecting the photoluminescence after exciting the spin-triplet transition. Recently, methods have been proposed and demonstrated that use the absorption of the infrared singlet transition at 1042 nm instead. These methods, however, require cryogenic temperatures or external cavities to enhance the absorption signal. Here, we report on our efforts to optimize the magnetometer sensitivity at room temperature and without cavities. We reach a sensitivity of 18 pT / Hz , surpassing previously reported values, and a calculated shot-noise limit of 5 pT / Hz . We also report on a defect that is native to diamond grown by chemical vapor deposition and thus absent in high-pressure high-temperature diamond, the excitation of which impacts the measured singlet absorption signal.
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 2.5  nT/Hz2.5\text{ }\text{ }\mathrm{nT}/\sqrt{\mathrm{Hz}}2.5  nT/Hz​, and project a photon shot-noise-limited sensitivity of 70  pT/Hz70\text{ }\text{ }\mathrm{pT}/\sqrt{\mathrm{Hz}}70  pT/Hz​ for a few mW of infrared light, and a quantum projection-noise-limited sensitivity of 250  fT/Hz250\text{ }\text{ }\mathrm{fT}/\sqrt{\mathrm{Hz}}250  fT/Hz​ for the sensing volume of $\ensuremath{\sim}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}90\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}\ifmmode\times\else\texttimes\fi{}200\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}$.
Magnetometry with nitrogen-vacancy (NV) centers in diamond has so far been measured via emission of light from NV centers or via absorption at the singlet transition at 1042 nm. Here, we demonstrate a phenomenon of broadband optical absorption by the NV centers starting in the emission wavelength and reaching up to 1000 nm. The measurements are enabled by a high-finesse cavity, which is used for room-temperature continuous wave pump-probe experiments. The red to infrared probe beam shows the typical optically detected magnetic resonance (ODMR) signal of the NV spin with contrasts up to 42 %. This broadband optical absorption has not yet been reported in terms of NV magnetometry. We argue that the lower level of the absorbing transition could be the energetically lower NV singlet state, based on the increased optical absorption for a resonant microwave field and the spectral behavior. Investigations of the photon-shot-noise-limited sensitivity show improvements with increasing probe wavelength, reaching an optimum of 7.5pT/Hz. The results show significantly improved ODMR contrast compared to emission-based magnetometry. This opens a new detection wavelength regime with coherent laser signal detection for high-sensitivity NV magnetometry.
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 1042 \,nm 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 680-1050 \,nm. 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 1042 \,nm, 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 1042 \,nm to be σ1042 ★=(0.89±0.14)⋅10−21 m2σ^{\,\bigstar}_{1042}=(0.89\pm0.14)\cdot 10^{-21}\,\text{m}^2σ1042★​=(0.89±0.14)⋅10−21m2 or σ1042 ▲=(2.9±0.5)⋅10−21 m2σ^{\,\blacktriangle}_{1042}=(2.9\pm0.5)\cdot 10^{-21}\,\text{m}^2σ1042▲​=(2.9±0.5)⋅10−21m2. depending on the employed 532 \,nm NV−^-− absorption cross section.
The negatively charged nitrogen-vacancy center (NV−^-−) in diamond is a versatile platform for quantum magnetometry under ambient conditions. Recently, laser threshold magnetometry (LTM) has been proposed as a means to significantly enhance the sensitivity of NV-based magnetometers by incorporating a diamond hosting NV−^-− centers within a laser cavity and operating near threshold. While demonstrations have validated the concept, practical implementations remain technically demanding, requiring high pump powers and precise alignment of free-space cavities. It remains unclear whether the benefits of operating near threshold will outpace increased laser noise. In this work, we integrate an NV-diamond with a high NV−^-− content into a compact external cavity diode laser and demonstrate singlet infrared absorption optically detected magnetic resonance (ODMR). The system exhibits exceptional threshold current stability, enabling ODMR using the threshold current as the read-out parameter. We report a five-fold enhancement in the ODMR contrast by operating near threshold. The best magnetic field sensitivity of 7.6 nT/Hz7.6~\mathrm{nT/\sqrt{Hz}}7.6 nT/Hz​ (DC-500 Hz) is achieved well above threshold, while near threshold sensitivity is limited by increased probe laser noise. These results establish a compact and mechanically robust platform for singlet absorption-based NV−^-− magnetometry and highlight key trade-offs between contrast enhancement and laser noise near threshold.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.