  The interaction of photons and coherent quantum systems can be employed to
detect electromagnetic radiation with remarkable sensitivity. We introduce a
quantum radiometer based on the photon-induced-dephasing process of a
superconducting qubit for sensing microwave radiation at the sub-unit-photon
level. Using this radiometer, we demonstrated the radiative cooling of a 1-K
microwave resonator and measured its mode temperature with an uncertainty ~0.01
K. We have thus developed a precise tool for studying the thermodynamics of
quantum microwave circuits, which provides new solutions for calibrating hybrid
quantum systems and detecting candidate particles for dark matter.
