Cross-resonant gates in hybrid fluxonium-transmon systems

We propose a scalable dual-species fluxonium-transmon system that utilizes a central transmon to mediate high-fidelity gates and parity checks between two fluxonium qubits without the need for strong nonlocal interactions. This approach suppresses unwanted long-range interactions, which is critical for developing larger quantum processors. First, we analyze the performance of cross-resonance (CR) cnot gates between a fluxonium and a transmon. We show that even in the presence of a spectator qubit, these gates maintain high fidelity with coherent errors on the order of 10−5. We then demonstrate that these gates, when applied sequentially, enable high-fidelity parity checks and logical fluxonium-fluxonium cnot gates. In addition, the central transmon can facilitate the readout of the neighboring fluxoniums, consolidating multiple critical functions into a single ancilla. Our work establishes the viability of a dual-species architecture as a promising path toward fault-tolerant quantum computation.