Detection of 5 GHz photons using Al Josephson junctions at 0.7 K

We predict that threshold detectors based on Al Josephson junctions with critical currents below 100 nA exhibiting a phase diffusion regime can be exploited for microwave photon detection at both 17 mK and 700 mK. We demonstrate the detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count times of about 0.1 s and 0.01 s, respectively. The weak temperature dependence of the detector’s performance observed in the sub-kelvin range fully confirms its phase diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of detector noise. Consequently, there is still room to optimize the detector’s performance. These results are important for axion search experiments in the range of 5–25 GHz (20–100 μeV).