  We propose a nonabsorbing microwave single-photon detector that uses an
artificial atom as a coherent interaction mediator between a traveling photon
and a high-Q resonator, fully exploiting the knowledge of the photon's arrival
time. Our proposal can be implemented with the current level of technology and
achieves distinguishability (probability of distinguishing between zero and one
photon) in excess of 98% for realistic parameters. This is better than any of
the similar detector proposals, even the ones using several artificial atoms.
