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.
@article{arxiv.1909.12295,
title = {Quantum Microwave Radiometry with a Superconducting Qubit},
author = {Zhixin Wang and Mingrui Xu and Xu Han and Wei Fu and Shruti Puri and S. M. Girvin and Hong X. Tang and S. Shankar and M. H. Devoret},
journal= {arXiv preprint arXiv:1909.12295},
year = {2021}
}