  We have constructed a microwave detector based on the voltage switching of an
underdamped Josephson junction, that is positioned at a current antinode of a
{\lambda}/4 coplanar waveguide resonator. By measuring the switching current
and the transmission through a waveguide capacitively coupled to the resonator
at different drive frequencies and temperatures we are able to fully
characterize the system and assess its detection efficiency and sensitivity.
Testing the detector by applying a classical microwave field with the strength
of a single photon yielded a sensitivity parameter of 0.5 in qualitative
agreement with theoretical calculations.
