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arXiv:2604.18937

A robust laser cavity platform for NV-diamond singlet infrared absorption magnetometry

Shao Qi Lim, Alexander A. Wood, Brett C. Johnson, Qiang Sun, Jan Jeske, Hiroshi Abe, Takeshi Ohshima, David J. Ottaway, Heike Ebendorff-Heidepriem, Robert E. Scholten, Andrew D. Greentree, Brant C. Gibson

quant-phSubmitted Apr 21, 2026• Announced Apr 22, 2026• 0 citations
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Abstract

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 $7.6~\mathrm{nT/\sqrt{Hz}}$ (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.

Comments
10 pages, 5 figures

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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.
Nitrogen-vacancy (NV) centers have considerable promise as high sensitivity magnetometers; however, they are commonly limited by inefficient collection and low contrasts. Laser threshold magnetometry (LTM) enables efficient collection and high contrasts, providing a path toward higher sensitivity magnetometry. We demonstrate an infrared LTM using an ensemble of NV centers in a single-crystal diamond plate integrated into a vertical external cavity surface emitting laser. The laser was tuned to the spin-dependent absorption line of the NV centers, allowing for optical readout by monitoring the laser output power. We demonstrate a magnetic sensitivity of 7.5 nT/Hz in the frequency range between 10 and 50 Hz. Furthermore, the contrast and the projected photon shot noise limited (PSNL) sensitivity are shown to improve significantly by operating close to the lasing threshold, achieving 18.4% and 26.6 pT/Hz near the threshold. In addition, an unexpected saturable absorption phenomenon was observed near the threshold, which enhanced the contrast and projected PSNL sensitivity.
Nitrogen-vacancy centers (NV) in diamond are promising quantum systems for magnetic field sensing. The sensitivity and the linearity of such a quantum sensor can be greatly improved by using stimulated emission of the NV centers in the concept of laser threshold magnetometry (LTM), which is projected to reach the fT/sqrt(Hz) regime. Previous implementations of NV centers in optical cavities relied on external seed lasers, pulsed operation, or sensing via NVabsorption. In our work, we combine the NV centers with a second gain medium, a laser diode, within the same cavity, achieving self-sustainable continuous-wave lasing. This approach compensates the intrinsic losses of the cavity and the diamond, through a fixed additional gain below the threshold of the laser system. A continuous-wave laser threshold and a linewidth narrowing is observed with increasing pump power on the NV centers of the combined laser system. An improved laser system shows a magnetic field-dependent laser threshold, which is the basis for improved-sensitivity NV magnetometry via LTM.
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.
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.
Nitrogen vacancy (NV) centers in diamond have attracted considerable recent interest for use in quantum sensing, promising increased sensitivity for applications ranging from geophysics to biomedicine. Conventional sensing schemes involve monitoring the change in red fluorescence from the NV center under green laser and microwave illumination. Due to the strong fluorescence background from emission in the NV triplet state and low relative contrast of any change in output, sensitivity is severely restricted by a high optical shot noise level. Here, we propose a means to avoid this issue, by using the change in green pump absorption through the diamond as part of a semiconductor external cavity laser run close to the lasing threshold. We show that theoretical sensitivity to the magnetic field on the pT/Hz\sqrt{\mathrm{Hz}}Hz​ level is possible using a diamond with an optimal density of NV centers. We discuss the physical requirements and limitations of the method, particularly the role of amplified spontaneous emission near threshold and explore realistic implementations using current technology.