  We introduce a circuit quantum electrodynamical setup for a "single-photon"
transistor. In our approach photons propagate in two open transmission lines
that are coupled via two interacting transmon qubits. The interaction is such
that no photons are exchanged between the two transmission lines but a single
photon in one line can completely block respectively enable the propagation of
photons in the other line. High on-off ratios can be achieved for feasible
experimental parameters. Our approach is inherently scalable as all photon
pulses can have the same pulse shape and carrier frequency such that output
signals of one transistor can be input signals for a consecutive transistor.
