Number-Resolved Photocounter for Propagating Microwave Mode

Detecting the presence of photons in a propagating microwave mode has been demonstrated only recently, and an important tool still missing is a photocounter able to determine in a single shot the number of photons in an incoming mode. The authors create such a photocounter by catching an incoming wave packet in a stationary mode, and then measuring the photon number of that mode bit by bit, using an ancillary qubit. Additionally, this device can measure the envelope of the incoming wave packet $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$. Beyond its direct applications in quantum sensing, this photocounter allows development of quantum information protocols that benefit from real-time feedback based on photon number.