Ancilla-Error-Transparent Controlled-SWAP Gate*

One of the big obstacles in quantum computing is engineering efficient and highly controllable quantum gates. A gate with a broad range of applications is the controlled-SWAP gate that can be used in universal quantum computing and various verification processes. This gate has been realized in circuit QED with a transmon-based ancilla to entangle two microwave fields (Gao et al. Nature 566, 509 (2019)) but its fidelity was limited by the ancilla errors, particularly transmon relaxation. To improve the gate performance, we propose a controlled beam splitter with a SNAIL-based Kerr-cat ancilla. Using the coherent states of the Kerr cat enables us to create a controlled-phase beam splitter operation on two microwave fields and biases the ancilla noise towards phase flips. This can then be combined with a deterministic beam splitter to generate a controlled beam splitter gate that is transparent to the dominant error channel preventing the propagation of the ancilla errors into the fields. This error transparency enables improved coherence compared to the transmon setup and allows improved performance of applications such as swap tests or quantum random access memory protocols.