Unidirectional emitter and receiver of an itinerant microwave photon in an open waveguide*

Quantum networks based on itinerant microwave photons can be an alternative approach toward a large-scale superconducting quantum computing. Recently, there have been several implementations of quantum state transfer between two localized superconducting qubits. Toward more complex networking, the control of the propagation direction of itinerant photons, such as routing, switching or circulating, is demanded. Here, we theoretically demonstrate unidirectional emission and absorption of an itinerant microwave photon in an open waveguide using a unit consisting of two superconducting qubits that are parametrically coupled to the waveguide via transfer resonators a quarter-wavelength apart. Upon preparing an appropriate entangled state of the two qubits, a photon is then unidirectionally and deterministically emitted to the open waveguide, as a result of the destructive interference — on the right or left of the device — of the radiation emitted from each qubit in both directions. We also show that this two-qubit system is able to deterministically receive a photon arriving from either direction of the waveguide, which cannot be realized with a single qubit.