We describe a microwave photon counter based on the current-biased Josephsonjunction. The junction is tuned to absorb single microwave photons from theincident field, after which it tunnels into a classically observable voltagestate. Using two such detectors, we have performed a microwave version of theHanbury Brown and Twiss experiment at 4 GHz and demonstrated a clear signatureof photon bunching for a thermal source. The design is readily scalable to tensof parallelized junctions, a configuration that would allow number-resolvedcounting of microwave photons.
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