  We present a realistic scheme for how to construct a single-photon transistor
where the presence or absence of a single microwave photon controls the
propagation of a subsequent strong signal signal field. The proposal is
designed to work with existing superconducting artificial atoms coupled to
cavities. We study analytically and numerically the efficiency and the gain of
our proposal and show that current transmon qubits allow for error
probabilities ~1% and gains of the order of hundreds.
