The maser, a microwave (MW) analog of the laser, is a well-established method for generating and amplifying coherent MW irradiation with ultra-low noise. This is accomplished by creating a state of population inversion between two energy levels separated by MW frequency. Thermodynamically, such a state corresponds to a small but negative temperature. The reverse condition, where only the lower energy level is highly populated, corresponds to a very low positive temperature. In this work, we experimentally demonstrate how to generate such a state in condensed matter at moderate cryogenic temperatures. This state is then used to efficiently remove microwave photons from a cavity, continuously cooling it to the quantum limit, well below its ambient temperature. Such an "anti-maser" device could be extremely beneficial for applications that would normally require cooling to millikelvin temperatures to eliminate any MW photons. For instance, superconducting MW quantum circuits (such as qubits and amplifiers) could, with the use of this device, operate efficiently at liquid helium temperatures.
The maser, a microwave (MW) analog of the laser, is a well-established method for generating and amplifying coherent MW irradiation with ultra-low noise. This is accomplished by creating a state of population inversion between two energy levels separated by MW frequency. Thermodynamically, such a state corresponds to a small but negative temperature. The reverse condition, where only the lower energy level is highly populated, corresponds to a very low positive temperature. In this work, we experimentally demonstrate how to generate such a state in condensed matter at moderate cryogenic temperatures. This state is then used to efficiently remove microwave photons from a cavity, continuously cooling it to the quantum limit, well below its ambient temperature. Such an "anti-maser" device could be extremely beneficial for applications that would normally require cooling to millikelvin temperatures to eliminate any MW photons. For instance, superconducting MW quantum circuits (such as qubits and amplifiers) could, with the use of this device, operate efficiently at liquid helium temperatures.
The maser, a microwave (MW) analog of the laser, is a well-established method for generating and amplifying coherent MW irradiation with ultralow noise. This is usually accomplished by creating a state of population inversion between two energy levels separated by MW frequency. Thermodynamically, such a state corresponds to a small but negative temperature. The reverse condition, where only the lower energy level is highly populated, corresponds to a very low positive temperature. In this work, we experimentally demonstrate how to generate such a state in condensed matter at moderate cryogenic temperatures. This state is then used to efficiently remove microwave photons from a cavity, continuously cooling it, well below its ambient temperature. Such an “anti-maser” device could be extremely beneficial for applications that would normally require cooling to millikelvin temperatures to eliminate any MW photons. For instance, superconducting MW quantum circuits (such as qubits and amplifiers) could, with the use of this device, operate efficiently at liquid helium temperatures.
Reducing the thermal noises in microwave (MW) resonators can bring about significant progress in many research fields. In this study, we consider using three-level or four-level systems as “quantum refrigerators” to cool down MW resonators so as to reduce the thermal noises, and investigate their possible cooling limits. In such a quantum refrigerator system, the MW resonator is resonantly coupled with many three-level or four-level systems via two of their energy levels. A driving laser is applied on the multilevel systems, then together with the help of proper optical emissions, the populations of the multilevel systems can be concentrated into their ground states. Effectively that creates a system with zero temperature, which could continuously absorb the thermal photons in the MW resonator. By adiabatic elimination, we give a more precise description for this cooling process. For three-level systems, it turns out a too strong driving laser significantly perturbs the energy levels, which impedes the energy exchange between the multilevel systems and the resonator, and weakens the cooling effect. For four-level systems, by adopting an indirect pump, such an upper constraint for the driving strength can be released. In both cases, we obtain analytical results for the cooling limit of the MW resonator. Based on practical experimental parameters, our estimation shows the cooling limit could reach lower than the liquid helium temperature, without resorting to the traditional cryogenic systems.
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].
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
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.