  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. Whilst solid-state masers
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 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
cool the system noise in an external circuit to cryogenic levels, all without
the requirement for physical cooling.
