  We present a theoretical model of an on-chip three-level maser in a
superconducting circuit based on a single artificial atom and pumped by a
temperature gradient between thermal baths coupled to different interlevel
transitions. We show that maser powers of the order of a few femtowatts, well
exceeding the resolution of the sensitive bolometry, can be achieved with
typical circuit parameters. We also demonstrate that population inversion in
the artificial atom can be detected without measuring coherent radiation output
of the maser. For that purpose, the system should operate as a three-terminal
heat transport device. The hallmark of population inversion is the influx of
heat power into the weakly coupled output terminal even though its temperature
exceeds the temperatures of the two other terminals. The proposed method of
on-chip conversion of heat into microwave radiation and control of energy-level
populations by heating provide additional useful tools for circuit quantum
electrodynamics experiments.
