Real-time detection of an itinerant microwave photon using dressed-state engineering

Several schemes for single microwave photon detection have been proposed and demonstrated lately in circuit quantum electrodynamics. However, all experimental demonstration to date are performed in the time-gated mode. In this presentation, we demonstrate a real-time detection of itinerant microwave photons. In our setup, a superconducting flux qubit is coupled to two resonators, which have substantial difference in the dispersive shifts. Under an adequate choice of the frequency and the power of the qubit drive, one resonator is used to form an impedance-matched Λ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. We observe quantum jump produced by an itinerant microwave photon and attain a single-photon-detection efficiency of ~0.35. The detection efficiency of this detector is limited by the relatively short qubit relaxation time.