# A Quantum Single Photon Transistor in Circuit Quantum Electrodynamics

We introduce a circuit quantum electrodynamical setup for a quantum single
photon transistor. In our approach single photons propagate in two open
transmission lines that are coupled via two interacting transmon qubits. The
interaction is such that photons are not exchanged between the two transmission
lines but a 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.