A Reconfigurable Buffer for Two-Photon Dissipation and High Fidelity Cat-Transmon Gates (Part 2/2)

Hybrid cat-transmon architectures, which leverage dissipative cat qubits as exponentially noise-biased data qubits and transmons for high-fidelity syndrome measurements, promise substantially reduced hardware overhead for quantum error correction. Current schemes for cat-transmon gates either directly couple a transmon to the cat qubit or introduce additional nonlinear couplers, both of which compromise the exponential noise-bias of the cat qubit when idling. To remedy this, we propose a reconfigurable coupling scheme where the existing buffer hardware (usually used to stabilize cat states) is tuned, in situ, to mediate high-fidelity cat-transmon gates. Our proposal relies on a detailed consideration of parametric interactions and a properly colored loss environment to ensure that the buffer can operate in both regimes, without degrading the transmon or cat qubit performance. In part 2 of this talk, we present a practical implementation of this proposal, including circuit design, optimization, and fabrication.