A hybrid controlled-SWAP gate between two bosonic modes

The controlled-SWAP (cSWAP) gate, which exchanges the states of two qubits conditioned on the state of an ancilla qubit, is at the heart of the SWAP-test sequence for quantum state comparison, as well as proposals for quantum random access memory. We implement this gate in an architecture that combines a tunable beamsplitter interaction between two bosonic modes in superconducting microwave cavities with universal single cavity control provided by a dispersively coupled transmon ancilla. Until now, the fidelity of cSWAP has been limited by transmon errors during the long SWAP time (~10us). By using a purpose-built SNAIL coupler to mediate cavity-cavity interaction, we achieve a 10x faster beamsplitter rate (on the order of the dispersive shift) while preserving cavity coherence, thereby reducing the cSWAP gate time to 1.3us. We then show how this tool can be used to generate entanglement by preparing a Bell state with measurement-corrected fidelity of 95%. Finally, we show how SWAP tests can be used to purify a quantum state from two imperfect copies.