Amplification of weak microwave signals with minimal added noise is of importance to science and technology. Artificial quantum systems, based on superconducting circuits, can now amplify and detect even single microwave photons. However, this requires operating at millikelvin temperatures. Natural quantum systems can also be used for low-noise microwave amplification using stimulated emission effects; however, they generate a higher noise, especially when operating above ~1 K. Here, we demonstrate the use of electron spins in diamond as a quantum microwave amplifier operating with quantum-limited internal noise, even above liquid nitrogen temperatures. We report on the amplifier's design, gain, bandwidth, saturation power, and noise. This capability can lead the way to previously unavailable quantum science, engineering, and physics applications.
Amplification of weak microwave signals with minimal added noise is of importance to science and technology. Artificial quantum systems, based on superconducting circuits, can now amplify and detect even single microwave photons. However, this requires operating at millikelvin temperatures. Natural quantum systems can also be used for low-noise microwave amplification using stimulated emission effects; however, they generate a higher noise, especially when operating above ~1 K. Here, we demonstrate the use of electron spins in diamond as a quantum microwave amplifier operating with quantum-limited internal noise, even above liquid nitrogen temperatures. We report on the amplifier's design, gain, bandwidth, saturation power, and noise. This capability can lead the way to previously unavailable quantum science, engineering, and physics applications.
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.
Current microwave quantum technologies require the amplification of weak signals with minimal added noise at millikelvin temperatures. To date, this stringent requirement has been met exclusively by superconducting technologies, such as Josephson or kinetic-inductance parametric amplifiers. A fundamentally distinct alternative approach could be offered by masers, the microwave counterpart of lasers, which were predicted as early as the 1950s to achieve quantum-limited noise performance under ideal conditions. However, their dependence on cryogenic operation historically limited further advancement. Here we demonstrate the first-ever non-superconducting, near-quantum-limited maser amplifier operating at millikelvin temperatures utilising nitrogen impurity spins (P1 centres) in diamond. Population inversion is achieved via microwave pumping, exploiting a four-spin cross-relaxation mechanism. We realise a maximum power gain exceeding 30 dB, an added noise of approximately 2.55 quanta above the standard quantum limit, and a maximum 1 dB output compression point of -63 dBm at 6.595 GHz. The ability to operate in strong static magnetic fields of arbitrary orientation may offer a complementary, non-superconducting route for applications such as semiconducting spin-qubit readout, magnetic-resonance spectroscopy, and dark-matter axion searches.
Abstract The tremendous progress in detecting tiny microwave signals from quantum devices under test at millikelvin temperatures has been enabled by superconducting parametric amplifiers [1–4], owing to their near-quantum-limited noise performance [5, 6]. Another fundamentally distinct approach could be offered by masers, the microwave counterpart of lasers, which have long been predicted to exhibit quantum-limited noise performance when operated in the absence of thermal noise and with sufficient population inversion [7–11]. Here, we demonstrate the first-ever non-superconducting, near-quantumlimited microwave amplifier operating at millikelvin temperatures based on a maser utilising impurity spins in diamond. We achieve power gains exceeding 30 dB, a minimum noise temperature of 0.86 K (corresponding to 2.2 noise photons), and a maximum 1 dB output compression point of −63 dBm at 6.595 GHz. The observed noise temperature is on par with superconducting parametric amplifiers, while the compression points exceed those of Josephson travellingwave parametric amplifiers by 20 to 30 dB. By revisiting masers at millikelvin temperatures, our work reestablishes them as robust low-noise amplifiers for emerging quantum technologies, including quantum computing, magnetic resonance spectroscopy [12, 13], and fundamental physics [14–16].
A quantum-limited amplifier enables the amplification of weak signals while introducing minimal noise dictated by the principles of quantum mechanics. Such amplifiers serve a broad spectrum of applications in quantum computing, including fast and accurate readout of superconducting qubits and spins, as well as various uses in quantum sensing and metrology. Parametric amplification, primarily developed with use of Josephson junctions, has evolved into the leading technology for highly effective microwave measurements within quantum circuits. Despite their significant contributions, these amplifiers face fundamental limitations, such as their inability to handle high powers, their sensitivity to parasitic magnetic fields, and particularly their limitation to operate only at millikelvin temperatures. To tackle these challenges, here we experimentally develop a novel quantum-limited amplifier based on superconducting kinetic inductance and present an extensive theoretical model to describe this nonlinear coupled-mode system. Our device surpasses the conventional constraints associated with Josephson-junction amplifiers by operating at much higher temperatures up to 4.5 K. With two distinct spectral modes and tunability through bias current, this amplifier can operate selectively in both the single-mode-amplification regime and the double-mode-amplification regime near the quantum noise limit. Using a nonlinear thin film exhibiting kinetic inductance, our device attains gain exceeding 50 dB in a single-mode configuration and 32 dB in a double-mode configuration, while adding 0.82 input-referred noise quanta. Importantly, this amplifier eliminates the need for Josephson junctions, resulting in the capability to handle significantly higher powers than Josephson junction--based amplifiers. It also demonstrates resilience in the presence of magnetic fields, offers a straightforward design, and increases reliability. This positions the amplifier as a versatile solution for quantum applications and facilitates its integration into future superconducting quantum computers.
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
Superconducting microwave amplifiers are essential for sensitive signal readout in superconducting quantum processors. Typically based on Josephson junctions, these amplifiers require operation at milli-Kelvin temperatures to achieve quantum-limited performance. Here, we demonstrate a quantum microwave amplifier that employs radiative cooling to operate at elevated temperatures and maintain near quantum-limited added noise. This kinetic-inductance-based parametric amplifier, patterned from a single layer of relatively high-Tc NbN thin film, maintains a high gain and meanwhile enables low added noise of 1.3 quanta when operated at 1.5 K. Remarkably, this represents only a 0.2 quanta increase compared to the performance at a base temperature of 0.1 K. Based on our findings, we also discuss the practicality of such an operating scheme for various quantum applications. By uplifting the parametric amplifiers from the mixing chamber without compromising readout efficiency, this work represents an important step toward more scalable microwave quantum technologies.