Number-Resolved Photocounter for Propagating Microwave Mode

Detectors of propagating microwave photons have recently been realized using superconducting circuits. However, a number-resolved photocounter is still missing. In this article, we demonstrate a single-shot counter for propagating microwave photons that can resolve up to three 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 for an average dead time of 4.5 mu s. 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%.