Tomography of thermal microwave photons with a cryogenic particle detector*

Microwave photons can be routinely generated, controlled, and teleported, providing a variety of applications. However, their detection may be challenging because the energy of several photons is considerably smaller than what can be measured with the room-temperature detectors. Recently, we introduced a new tool to circuit QED, namely, an ultrasensitive bolometer which we employed at millikelvin to measure a superconducting transmon qubit in a single shot without preamplification. Here, we extend this novel power measurement tool to overcome the limitations of previous bolometric measurements of propagating microwave fields. We can clearly differentiate powers corresponding to 0.17 photon/(s Hz). This allows us to investigate the quantum statistical properties of microwave photons without amplification of noise or added quantum noise. We apply our studies to thermal states generated by a blackbody radiation source, operating in the regime of quantum electrodynamics. We aim to observe photon bunching and n(n+1) photon number variance in agreement with the Bose-Einstein distribution. These results pave the way to the full tomography of propagating microwave photons at cryogenic temperatures.