Analysis of Crosstalk in Larger Scale Fluxonium-Based Quantum Processors with Tunable Couplers

In recent years, several architectures have been proposed for implementing two-qubit operations on fluxonium superconducting qubits. A particularly promising approach, which was demonstrated experimentally by [1,2], employs a transmon superconducting qubit as a tunable coupler between the fluxonium qubits. These experiments have shown that the transmon coupler enables fast, high-fidelity two-qubit operations while suppressing unwanted ZZ crosstalk between the fluxonium qubits. In this work, we numerically study the scalability of this architecture. We find that the undesired coupling between computational fluxonium levels remains well suppressed, but that the transmon couplers induce strong hybridization between non-computational states. As a result, this architecture is prone to crosstalk and spectator errors which we quantify using a tensor network formalism. By investigating the impact of these errors on the two-qubit gate operation in different parameter regimes, we provide quantitative insights into the scalability of this multi-qubit architecture.

[1] L. Ding et al., Phys. Rev. X 13, 031035 (2023)

[2] S. Singh et al., arXiv:2504.13718 (2025)