A near-quantum-limited diamond maser amplifier operating at millikelvin temperatures

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].