A two-mode 3D cQED device with a Fluxonium ancilla

The fluxonium has emerged as a promising superconducting qubit, with coherence times and gate fidelities now comparable to the best transmons, and with room for further improvement. While transmons have been successfully used as ancillas in 3D circuit QED (cQED) systems, their shorter coherence times relative to the cavity modes limit the fidelity of bosonic operations and error correction. The Fluxonium therefore offers a promising path toward realizing more fault-tolerant ancilla circuits for bosonic control [1]. However, integrating fluxonium into 3D cQED architectures remains largely unexplored due to challenges in modeling its behavior in arbitrary 3D electromagnetic environments and incorporating flux biasing into high-Q 3D superconducting packages.

In this talk, we present a 3D cQED device consisting of two coaxial cavities coupled to a fluxonium ancilla with high-Q compatible flux delivery using a transformer loop. We model the device using a modified black-box-quantization–based approach that harnesses classical electromagnetic simulations of the cavity modes to simulate the fluxonium-cQED Hamiltonian without treating the junction nonlinearity perturbatively. We compare the simulations with experimental measurements and present our progress toward  fluxonium-mediated cavity gates using dispersive control.

[1] Ke Nie et al, arXiv:2505.23641 (2025).