Microwave Photon Counter Based on Josephson Junctions

We describe a microwave photon counter based on current-biased Josephson junctions. The absorption of a single microwave photon causes a junction to switch to the voltage state, producing a large and easily measured classical signal. With a two-junction circuit, we have performed a microwave version of the Hanbury Brown and Twiss experiment at 4 GHz, and demonstrated a clear signature of photon bunching for a thermal source. The design is readily scalable to tens of parallelized junctions, a configuration that would allow number-resolved counting of microwave photons. We discuss possible applications to cavity state readout and to measurement of the counting statistics of microwave photons emitted by mesoscopic conductors.