In experiments with superconducting quantum circuits, characterizing the photon statistics of propagating microwave fields is a fundamental task. We quantify the $n^2+n$ photon number variance of thermal microwave photons emitted from a blackbody radiator for mean photon numbers, $0.05\lesssim n\lesssim 1.5$. We probe the fields using either correlation measurements or a transmon qubit coupled to a microwave resonator. Our experiments provide a precise quantitative characterization of weak microwave states and information on the noise emitted by a Josephson parametric amplifier.

• Received 29 September 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.103602