Josephson microwave photon detector operating at 0.7 K

TL;DR

This paper predicts and demonstrates a Josephson junction-based microwave photon detector operating effectively at 0.7 K, with high efficiency at 17 mK and stable performance across sub-kelvin temperatures, relevant for axion searches.

Contribution

It introduces a phase diffusion regime-based Josephson detector capable of detecting microwave photons at higher temperatures than traditional cryogenic limits.

Findings

Achieved 90% efficiency for two-photon detection at 5 GHz

Demonstrated stable detector performance at 0.7 K

Weak temperature dependence confirms phase diffusion operation

Abstract

We predict that the threshold detectors based on Al Josephson junctions, with critical currents below 100 nA, exhibiting a phase diffusion regime, can be exploited for the microwave photon detection both at 17 mK and 700 mK. We demonstrate a detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count time of about 0.1 s and 0.01 s, respectively. The observed weak temperature dependence of the detector's performance 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 the detector noise. Consequently, there is still a room to optimize the detector's performance. These results are important for axion search experiments in the range of 5-25 GHz (20-100 $\mu$eV).