Flying microwave qubits with nearly perfect transfer efficiency

We propose a procedure for transferring the state a microwave qubit via a transmission line from one resonator to another resonator, with a theoretical efficiency arbitrarily close to 100%. The emission and capture of the microwave energy is performed using tunable couplers, whose transmission coefficients vary in time. Using the superconducting phase qubit technology and tunable couplers with maximum coupling of 100 MHz, the procedure with theoretical efficiency $\eta=0.999$ requires a duration of about 400 ns (excluding propagation time) and an ON/OFF ratio of 45. The procedure may also be used for a quantum state transfer with optical photons.