Single-Shot Quantum Non-Demolition Detection of Itinerant Microwave Photons

We realize a quantum non-demolition detector for propagating single photons in the microwave domain. By using a cavity-assisted conditional phase flip gate between a photon and a superconducting artificial atom, the information of the presence of an itinerant photon is mapped into a definite state of the qubit [1]. We test the detector with a single photon source and perform single-shot readout of the state of the atom, whose first to second excited state transition is tuned to be resonant with the photon source. We demonstrate the quantum non-demolition nature of the detector by characterizing the field of the reflected photon, which is never absorbed. The internal detection fidelity is limited by the coherence properties of the qubit, but is independent of the temporal shape of the photon as long as its frequency bandwidth is smaller than the coupling of the atom to the assisting cavity.
[1] Reiserer et al., Nondestructive Detection of an Optical Photon, Science 342, 1349 (2013).