Approaching microwave photon sensitivity with Al Josephson junction

Here we experimentally test the applicability of aluminum Josephson junction of few microns size as a single photon counter in microwave frequency range. We have measured the switching from the superconducting to the resistive state due to absorption of 10 GHz signal. The dependence of the switching probability on the signal power suggests that the switching is initiated by simultaneous absorption of three and more photons with the dark count time above 0.01 s.