Title: A Room-Temperature Solid-State Maser Amplifier

The detection of weak microwave signals is a challenge that lies at the heart of many modern technologies, including deep-space satellite communication systems, radio telescopes and radar. To detect such faint signals, microwave receivers must add as little noise as possible, taking advantage of ultra-low noise amplifiers to first boost the signals before measurement. Maser amplifiers are devices that exploit stimulated emission in an inverted ensemble of microwave-frequency emitters - often in the form of paramagnetic centers such as spins - to achieve low-noise amplification of microwave signals. The noise temperature of a maser amplifier can theoretically reach the quantum mechanical limit, which for a system operating in the microwave X-band (~ 10 GHz) can be as low as 0.5 K.

Maser amplifiers based on solid-state spin systems such as ruby once represented the gold-standard in low-noise microwave amplification technology. The requirement for solid-state maser amplifiers to be cooled to cryogenic temperatures (typically < 4.2 K) saw these systems eventually replaced by modern transistor-based amplifiers. However, pioneering experiments using organic [1] or diamond-based [2] spin systems have demonstrated that solid-state masers can operate at room temperature, generating a resurgence in their interest.

Here we present a continuous-wave maser amplifier based on a solid-state system that operates at room temperature [3]. We use an ensemble of nitrogen-vacancy (NV) center spins in a bulk diamond crystal as the gain medium and couple this to a high quality factor microwave dielectric resonator. We measure important amplifier characteristics such as gain, bandwidth and noise temperature. Our results demonstrate that NV spin ensembles in diamond coupled to microwave resonators constitute a system with exceptional promise for performing ultra-low noise detection of microwave signals at room temperature.

[1] M. Oxborrow, et al., Room-temperature solid-state maser, Nature 488, 353 (2012)

[2] J. D. Breeze, et al., Continuous-wave room-temperature diamond maser, Nature 555, 493–496 (2018)

[3] T. Day, et al., A Room-Temperature Solid-State Maser Amplifier, arXiv:2405.07486, Physical Review X (in press) (2024)