Abstract
Microwave quantum technologies require amplification of weak signals with minimal added noise at millikelvin temperatures. This stringent demand has been met with superconducting parametric amplifiers. While masers offer another fundamental approach, their dependence on cryogenic operation has historically posed challenges for classical communication technologies -- a barrier that does not apply to microwave quantum technologies. In this work, we demonstrate an ultra-low-noise maser amplifier utilizing impurity spins in diamond at millikelvin temperatures. We achieve power gains exceeding 30dB, a minimum noise temperature of 0.86K (corresponding to 2.2 noise photons), and a maximum 1dB output compression point of -63dBm at 6.595GHz. Our results establish masers as viable components of microwave quantum technologies.