Masers once represented the state of the art in low-noise microwave amplification technology but eventually became obsolete due to their need for cryogenic cooling. Masers based on solid-state spin systems perform most effectively as amplifiers, since they provide a large density of spins and can, therefore, operate at relatively high powers. While solid-state maser oscillators have been demonstrated at room temperature, continuous-wave amplification in these systems has only ever been realized at cryogenic temperatures. Here, we report on a continuous-wave solid-state maser amplifier operating at room temperature. We achieve this feat using a practical setup that includes an ensemble of nitrogen-vacancy center spins in a diamond crystal, a strong permanent magnet, and a simple laser diode. We describe important amplifier characteristics including gain, bandwidth, compression power, and noise temperature and discuss the prospects of realizing a room-temperature near-quantum-noise-limited amplifier with this system. Finally, we show that in a different mode of operation the spins can be used to reduce the microwave noise in an external circuit to cryogenic levels, all without the requirement for physical cooling. Published by the American Physical Society 2024
Masers once represented the state of the art in low-noise microwave amplification technology but eventually became obsolete due to their need for cryogenic cooling. Masers based on solid-state spin systems perform most effectively as amplifiers, since they provide a large density of spins and can, therefore, operate at relatively high powers. While solid-state maser oscillators have been demonstrated at room temperature, continuous-wave amplification in these systems has only ever been realized at cryogenic temperatures. Here, we report on a continuous-wave solid-state maser amplifier operating at room temperature. We achieve this feat using a practical setup that includes an ensemble of nitrogen-vacancy center spins in a diamond crystal, a strong permanent magnet, and a simple laser diode. We describe important amplifier characteristics including gain, bandwidth, compression power, and noise temperature and discuss the prospects of realizing a room-temperature near-quantum-noise-limited amplifier with this system. Finally, we show that in a different mode of operation the spins can be used to reduce the microwave noise in an external circuit to cryogenic levels, all without the requirement for physical cooling. Published by the American Physical Society 2024
The first solid-state masers to operate at room-temperature and ambient air-pressure were recently demonstrated using optically pumped spin-triplet states as the gain medium. In this Perspective, we briefly review the previous state-of-the-art in cryogenic solid-state masers and then discuss the development of the room-temperature solid-state maser: from the organic pentacene pulsed maser to the diamond nitrogen-vacancy continuous-wave maser. We characterize the operation of these masers as coherent microwave sources and ultra-low noise amplifiers before outlining how they can be adapted to act as model systems in which to explore room-temperature cavity quantum electrodynamics. After discussing challenges facing current embodiments of the room-temperature solid-state maser, we explore how they might be addressed or by-passed altogether through the development of alternative materials and masing mechanisms. Finally, we speculate on how the advent of masers that can operate in ambient conditions might lead to novel applications in metrology and quantum technologies.
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.
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 coherent microwave sources that can act as oscillators or quantum-limited amplifiers. Masers have historically required high vacuum and cryogenic temperatures to operate, but recently, masers based on diamond have been demonstrated to operate at room temperature and pressure, opening a route to new applications as ultra-low noise microwave amplifiers. For these new applications to become feasible at a mass scale, it is important to optimize diamond masers by minimizing their size and maximizing the power of signals that can be amplified. Here, we develop and numerically solve an extended rate equation model to present a detailed phenomenology of masing dynamics and determine the optimal properties required for the copper cavity, dielectric resonator, and gain medium in order to develop portable maser devices. We conclude by suggesting how the material parameters of the diamond gain media and dielectric resonators used in diamond masers can be optimized, and how rate equation models could be further developed to incorporate the effects of temperature and nitrogen concentration on spin lifetimes.
Advances in microwave quantum technologies increasingly demand amplifiers that are capable of boosting extremely weak signals with minimal added noise at millikelvin temperatures. To date, this requirement has been met almost exclusively by superconducting devices such as Josephson- and kinetic-inductance-based parametric amplifiers. Masers—the microwave counterparts of lasers—were predicted in the 1950s to achieve quantum-limited noise performance. However, their development later declined because their operation required low temperatures, making them difficult to deploy widely. In this thesis, I demonstrate a diamond spin maser amplifier operating at millikelvin temperatures, marking the first non-superconducting amplifier approaching the quantumnoise limit. The device employs substitutional nitrogen spins (P1 centers) in diamond, where population inversion is achieved through active microwave pumping via a four-spin crossrelaxation mechanism, assisted by fast-relaxing spins within the same crystal. The amplifier exhibits a maximum power gain exceeding 30 dB, a gain-bandwidth product of 2.8 MHz, and an output 1-dB compression point of −65 dBm (at a gain of 20 dB), measured at 6.595 GHz. The noise performance of the maser amplifier is quantitatively characterized using an impedance-matched, temperature-variable noise source integrated into a dilution refrigerator. This approach enables a direct and reliable determination of the input-referred added noise, revealing a minimum noise of approximately 3 quanta. Furthermore, a semi-classical rateequation model is developed to describe the coupled spin–photon dynamics and to provide insight into the gain behavior and population inversion process. By revisiting masers in the millikelvin regime, this work highlights their potential as a solid-state platform for low-noise microwave amplification with high compression power applicable to quantum computing, magnetic resonance spectroscopy, and fundamental physics.
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.