Quantum non-demolition detection of an itinerant microwave photon

Microwave quantum optics in superconducting circuits enables us to investigate unprecedented regimes of quantum optics. The strong nonlinearity brought by Josephson junctions together with the strong coupling of the qubits with microwave modes reveals rich physics not seen in the optical domain before. However, single-photon detection in the microwave domain is still a challenging task because of the photon energy four to five orders of magnitude smaller than in optics.
Here, we demonstrate a quantum non-demolition detection of an itinerant microwave photon using a circuit QED architecture with a transmon qubit in a largely detuned 3D cavity. When an itinerant photon is reflected by the cavity, it interacts dispersively with the qubit. With an appropriate adjustment of the cavity bandwidth, the qubit deterministically acquires a phase flip upon the reflection. With a single-shot measurement of the phase flip, we detect the existence of the single photon without destroying it. We evaluate the quantum efficiency of the detection and characterize the reflected photon by using conditional quantum state tomography based on the outcome of the qubit readout.