Deterministic generation of shaped single microwave photons using a parametrically driven coupler*

A distributed quantum computing system requires to share information between spatially separated quantum processors by creating a quantum channel, which can be implemented by emitting a microwave photon from a sender processor and absorbing it by a receiver processor. For this purpose, deterministically encoding the state of a stationary qubit into a travelling photon is of great interest. Here we introduce a superconducting circuit consisting of a data qubit and an emitter qubit coupled to a quantum channel, with a coupler between them to parametrically exchange quantum state. With this structure, we demonstrate the deterministic transfer of a qubit state into a propagating microwave photon with a single-rail encoding, with a process fidelity in excess of 90%. We use a time-dependent parametric drive to shape the temporal profile of the propagating mode to be time-symmetric and with a constant phase so that the absorption process by the receiver can be implemented as a time-reversed version of the emission. Compared with previous works which utilize a second-order transition to control the emission, the presented technique uses a first-order parametric process, which requires no strong pump and can therefore help eliminate AC Stark shifts and heating of the data qubit. It offers ease of calibration and flexibility in fine-tuning photon emission and absorption, and can therefore facilitate high-fidelity quantum state transfer and remote entanglement operations in a distributed quantum computing system.