A Universal Quantum Gate Set Operating on Itinerant Microwave Photons*

Photons are ideal carriers of quantum information to communicate over large distances in quantum networks. Many applications, such as quantum routers and quantum repeaters, require interactions between two or more photonic qubits. Such photon-photon gates are typically realized by interactions with a non-linear medium. Probabilistic and deterministic photon-photon gates, based on reflection off or overlap with a non-linear medium, have previously been demonstrated in the optical regime. In this talk, we present absorption and re-emission-based, deterministic, photon-photon gates. We emit shaped itinerant photons in the microwave regime from a source chip, containing two transmon qubits, and send them to a second similar chip which implements a universal gate set by combining controlled absorption and re-emission with single qubit gates and qubit-photon C-PHASE gates. We evaluate the performance of the gate set by quantum process tomography and find internal process fidelities of 90 % for arbitrary single photon gates and photon-photon C-PHASE gates limited mainly by decoherence of the gate-assisting transmon qubits.