Several schemes for single microwave photon detection have been proposed and demonstrated lately in circuit quantum electrodynamics [1-4]. However, most 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 a 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.

1. K. Inomata*, Z. R. Lin*, et al. (2016). Single microwave-photon detector using an artificial Λ-type three-level system. Nature Commun., 7, 12303.
2. S. Kono et al. (2018). Quantum non-demolition detection of an itinerant microwave photon. Nature Physics, 14, 546.
3. Jean-Claude Besse et al. (2018). Single-Shot Quantum Nondemolition Detection of Individual Itinerant Microwave Photons. Phys. Rev. X, 8, 021003.
4. Raphaël Lescanne et al. (2019). Detecting itinerant microwave photons with engineered non-linear dissipation. arXiv:1902.05102

Keywords: single microwave photon detection, circuit quantum electrodynamics, superconducting qubit