Continuous wave single photon transistor based on a superconducting   circuit
  We propose a microwave frequency single photon transistor which can operate under continuous wave probing, and represents an efficient single microwave photon detector. It can be realized using an impedance matched system of a three level artificial ladder-type atom coupled to two microwave cavities connected to input/output waveguides. Using a classical drive on the upper transition, we find parameter space where a single photon control pulse incident on one of cavities can be fully absorbed into hybridized excited states. This subsequently leads to series of quantum jumps in the upper manifold and the appearance of a photon flux leaving the second cavity through a separate input/output port. The proposal does not require time variation of the probe signals, thus corresponding to a passive version of single photon transistor. The resulting device is robust to qubit dephasing processes, possesses low dark count rate for large anharmonicity, and can be readily implemented using current technology. 
Oleksandr Kyriienko, Anders S. S{\o}rensen
quant-ph cond-mat.mes-hall cond-mat.supr-con