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arXiv:2607.07124

Room-temperature inversionless diamond nitrogen-vacancy electronic spin maser

Ali Fawaz, Sarath Raman Nair

quant-phSubmitted Jul 8, 2026• Announced Jul 9, 2026• 0 citations
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Abstract

We propose a method to create a room-temperature maser operating at approximately 2.9~GHz frequency using an ensemble of negatively charged nitrogen-vacancy electronic spins (NV) in diamond, without requiring population inversion. Our method considers a DC magnetic field of a few milli-Tesla (mT) applied along the perpendicular direction of an ensemble of NV spins aligned along a common axis. This perpendicular magnetic-field creates superposition states of $|m_{\mathrm{s}}=-1\rangle$ and $|m_{\mathrm{s}}=+1\rangle$ of the NV spin's ground state triplet levels and thereby makes it possible to drive all three transitions in the NV spin ground state. We model the system by including optical pumping of the NV spins, near-resonant driving of two transitions, and coupling the third transition to a near-resonant microwave resonator. Numerical estimates using experimentally realizable parameters show that inversionless masing can be achieved inside the microwave resonator using our method. As an application, we show that the output intensity of an inversionless maser ($1.1\times10^{14}$ spins) can be used for magnetic field sensing with a DC sensitivity on the order of a hundred pT/$\sqrt{\mathrm{Hz}}$. Our study opens a new direction in room-temperature diamond NV maser devices for quantum technological applications without the requirement of a strong bias magnetic field, as in conventional NV diamond masers.

Comments
6 pages, 3 figures, supplementary material (10 pages, 7 figures)

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We propose a method to create a room-temperature maser operating at approximately 2.9~GHz frequency using an ensemble of negatively charged nitrogen-vacancy electronic spins (NV) in diamond, without requiring population inversion. Our method considers a DC magnetic field of a few milli-Tesla (mT) applied along the perpendicular direction of an ensemble of NV spins aligned along a common axis. This perpendicular magnetic-field creates superposition states of ∣ms=−1⟩|m_{\mathrm{s}}=-1\rangle∣ms​=−1⟩ and ∣ms=+1⟩|m_{\mathrm{s}}=+1\rangle∣ms​=+1⟩ of the NV spin's ground state triplet levels and thereby makes it possible to drive all three transitions in the NV spin ground state. We model the system by including optical pumping of the NV spins, near-resonant driving of two transitions, and coupling the third transition to a near-resonant microwave resonator. Numerical estimates using experimentally realizable parameters show that inversionless masing can be achieved inside the microwave resonator using our method. As an application, we show that the output intensity of an inversionless maser (1.1×10141.1\times10^{14}1.1×1014 spins) can be used for magnetic field sensing with a DC sensitivity on the order of a hundred pT/Hz\sqrt{\mathrm{Hz}}Hz​. Our study opens a new direction in room-temperature diamond NV maser devices for quantum technological applications without the requirement of a strong bias magnetic field, as in conventional NV diamond masers.
Recent experiments demonstrated the cooling of a microwave mode of a high-quality dielectric resonator coupled to optically cooled nitrogen-vacancy (NV) spins in diamond. Our recent theoretical study [arXiv:2110.10950] pointed out the cooled NV spins can be used to realize cavity quantum electrodynamics effects (C-QED) at room temperature. In this article, we propose to modify the setup used in a recent diamond maser experiment [Nature 55, 493-496 (2018)], which features a higher spin transition frequency, a lower spin-dephasing rate and a stronger NV spins-resonator coupling, to realize better microwave mode cooling and the room-temperature CQED effects. To describe more precisely the optical spin cooling and the collective spin-resonator coupling, we extend the standard Jaynes-Cumming model to account for the rich electronic and spin levels of the NV centers. Our calculations show that for the proposed setup it is possible to cool the microwave mode from 293293293 K (room temperature) to 116116116 K, which is about 727272 K lower than the previous records, and to study the intriguing dynamics of the CQED effects under the weak-to-strong coupling transition by varying the laser power. With simple modifications, our model can be applied to, e.g., other solid-state spins or triplet spins of pentacene molecules, and to investigate other effects, such as the operations of pulsed and continuous-wave masing.
The application of masers is limited by its demanding working conditions (high vacuum or low temperature). A room-temperature solid-state maser is highly desirable, but the lifetimes of emitters (electron spins) in solids at room temperature are usually too short (∼ns) for population inversion. Masing from pentacene spins in p-terphenyl crystals, which have a long spin lifetime (∼0.1 ms), has been demonstrated. This maser, however, operates only in the pulsed mode. Here we propose a room-temperature maser based on nitrogen-vacancy centres in diamond, which features the longest known solid-state spin lifetime (∼5 ms) at room temperature, high optical pumping efficiency (∼10(6) s(-1)) and material stability. Our numerical simulation demonstrates that a maser with a coherence time of approximately minutes is feasible under readily accessible conditions (cavity Q-factor ∼5 × 10(4), diamond size ∼3 × 3 × 0.5 mm(3) and pump power <10 W). A room-temperature diamond maser may facilitate a broad range of microwave technologies.
Masers could transform medical sensing and boost qubit readout detection due to their superb low-noise amplification. The negatively charged nitrogen-vacancy diamond maser is the only continuous-wave solid-state maser operable at room temperature; however, it requires cumbersome magnets, which prevent its widespread use. We present a significant size reduction of the diamond maser oscillator using a much lighter electromagnet with a small footprint, reducing the weight from 2000 kg to a more portable 30 kg. We achieve a maximum oscillator output power near − 80 dBm, ten times higher than the first implementation, and discover techniques to reduce the magnetic field strength required for masing by 30 mT through precise manipulation of spin orientation. With the diamond maser now shrunk to a size that can fit on a benchtop, we move continuous-wave room-temperature masers away from the confines of research laboratories and closer to transforming readouts in quantum computing, frequency standards, and quantum-limited medical sensing.
Masers, the microwave analog of lasers, are an important class of devices that can be used to generate highly coherent and stable microwave signals as well as quantum-limited amplification of microwave photons. Recently, the interest in such devices has increased, probably owing to experiments demonstrating the successful operation at room temperature of solid-state maser employing defects in diamond. These defects, called nitrogen vacancy (NV) centers, are comprised of a negatively charged substitutional nitrogen that replaces one of the diamond crystal lattice’s carbons and a vacancy adjacent to the excluded carbon atom. The performance of diamond-based masers, in terms of gain, noise, bandwidth, frequency of operation, and saturation, greatly depends upon the specific diamond crystal composition and its geometry in the respective microwave device. The currently available data on these issues are scarce and sometimes conflicting. It is, therefore, important to provide additional experimental data, complemented by theoretical analysis, to further optimize the required diamond material and thus enhance the capabilities of diamond-based maser technology. The latter is currently limited to operate only as an oscillator at very small bandwidths, with low saturation power. Here, we provide experimental results on a set of important parameters affecting diamond maser operation, such as the population and linewidth of the electron spin energy levels of the NVs under light illumination, as well as their relaxation times for several different diamond material compositions. These results are then used to point out which diamond material compositions and crystal geometry may be best suitable for maser-type applications.
Nitrogen-Vacancy (NV) centres in diamond have been shown to possess a quantised energy structure that allows for a number of macroscopic parameters to be measured with a high level of sensitivity. The process of sensing magnetic fields can be achieved through the behaviour of the NV centre’s electron spin triplet states using the separation of field-dependent and independent states, inferred by applying a microwave frequency to drive transitions. Observation of the states is achieved through optical pumping and stimulated emission, where the intensity of the fluorescent emission and frequency of the applied microwaves relates to the magnitude of the magnetic field the NV centre experiences. The optical method of sensing can be facilitated using optical fibres in which NV diamond particles are embedded between the core and outer fibre layers, leading to a robust platform for further practical applications with strong resistance to external interference.
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.