Demonstration of SWAP gate between superconducting and microwave-photon qubits

We demonstrate a deterministic SWAP gate between a superconducting atom and a single microwave photon propagating in a waveguide, proposed in Phys. Rev. Appl. 7, 064006 (2017). In this scheme, the superconducting qubit is encoded on its ground/excited state and the photon qubit is encoded on its carrier frequency of a single photon, and the SWAP gate is completed by bouncing the photon qubit at a resonator coupled dispersively to the atom. By using a weak coherent-state photon instead of a single photon, we demonstrated bidirectional quantum-state transfer between the atom and the photon. The average fidelity of the photon-to-atom (atom-to-photon) state transfer is 0.829 (0.793) with infidelities due mainly to short lifetime of the superconducting qubit. The present scheme has the advantages of simple setup, in-situ tunability of the gate type, passive atom-photon interaction and dual-rail encoding of photon qubit, and is therefore suited to entangling remote superconducting qubits on different processors.