  We propose a detector of microwave photons which can distinguish the vacuum
state, one-photon state, and the states with two or more photons. Its operation
is based on the two-photon transition in a biased Josephson junction and
detection occurs when it switches from a superconducting to a normal state. We
model the detector theoretically. The detector performs with more than 90%
success probability in several microseconds. It is sensitive for the 8.2GHz
photons. The working frequency could be set at the design stage in the range
from about 1GHz to 20GHz.
