  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.
