Photon Statistics of Propagating Thermal Microwaves

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 less than or similar to n less than or similar to 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.