Quantum measurement of itinerant microwave photons using superconducting circuits

• Speaker: Kono, Shingo (U of Tokyo, Japan)

Itinerant microwave photons are indispensable for realizing a quantum network between localized superconducting qubits. However, it remains challenging to measure itinerant microwave photons due to the lack of high-efficiency photodetectors in the microwave domain, in contrast to the optical frequency domain.

In this talk, we show the results on quantum measurements of itinerant microwave photons by using a circuit quantum electrodynamical system, where a microwave cavity plays a crucial role in facilitating the interaction between itinerant photons and a superconducting qubit. First, we characterize the photon-number distribution of a microwave squeezed vacuum by measuring the photon-number-resolved excitation spectrum of the qubit in a cavity that is driven externally and continuously with the squeezed vacuum. We confirm that the photon-number distribution reveals an even-odd photon number oscillation and quantitatively constitutes the nonclassicality. Second, we implement a deterministic entangling gate between a superconducting qubit and an itinerant microwave photon reflected by a cavity containing the qubit. Using entanglement and high-fidelity qubit readout, we demonstrate a quantum non-demolition detection of a single photon. The existence of the detected microwave photon is confirmed by using Wigner quantum state tomography.

These results on the fundamental characterizations of itinerant microwave photons have promising applications for quantum sensing and metrology in the microwave regime. Furthermore, the efficient entangling gate between itinerant photons and a superconducting qubit can be a building block for quantum networks connecting distant qubit modules.