Number-Resolved Photocounter for Propagating Microwave Mode*

The first detectors of propagating microwave photons have been realized using superconducting circuits a decade ago. However a number-resolved photocounter is still missing. We demonstrate a single-shot counter for propagating microwave photons that can resolve up to 3 photons. It is based on a pumped Josephson Ring Modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of 0.96 ± 0.04 and a dark count probability of 0.030 ± 0.002. To maximize its performance, the device is first used as an in situ waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from (54 ± 2) % to 99%.[1]

[1] R. Dassonneville, et al., Phys. Rev. Applied 14, 044022 (2020)