Tunable coupler for high-fidelity two-qubit gates in fluxonium*

The superconducting fluxonium qubit has emerged as a promising alternative to the widely-studied transmon qubit due to increased coherence times at the half-flux quantum sweet-spot, large anharmonicity, and robust charge-noise insensitivity. Scaling to multi-qubit fluxonium systems requires implementation of fast, high-fidelity, and highly expressive quantum gates, with small residual coupling when the gate is off. In this work we present the design of and experimental progress towards realizing a 2D tunable coupler composed of a tunable fluxonium element and a direct coupling path achieving these requirements. We study the family of gates realizable with charge and flux control, and investigate their limits with regard to gate time, leakage, and drive-induced decoherence.