Title: Observation of thermal microwave photons with a Josephson junction detector

Abstract: When measuring electromagnetic radiation of frequency $f$, the most sensitive detector is the one that counts the single quanta of energy $h f$. Single photon detectors (SPDs) were demonstrated from $\gamma$-rays to infrared wavelengths, and extending this range down to the microwaves is the focus of intense research. The energy of $10\,\mathrm{GHz}$ microwave photon, about $40\,\mathrm{\mu eV}$ or $7\, \mathrm{yJ},$ is enough to force a superconducting Josephson junction into its resistive state, making it suitable to be used as a sensor. In this work, we use an underdamped Josephson junction to detect single thermal photons stochastically emitted by a microwave copper cavity at millikelvin temperatures. After characterizing the source and detector, we vary the temperature of the resonant cavity and measure the increased photon rate. The device shows an efficiency up to 40% and a dark count rate of $0.1\,\mathrm{Hz}$ in a bandwidth of several gigahertz. To confirm the thermal nature of the emitted photons we verify their super-Poissonian statistics, which is also a signature of quantum chaos. We discuss detector application in the scope of Dark Matter Axion searches, and note its importance for quantum information, metrology and fundamental physics.