Tunable inductive coupler for high fidelity gates between fluxonium qubits

The fluxonium qubit is a promising candidate for quantum computation due to its long coherence times and large anharmonicity. We present a tunable coupler that realizes strong inductive coupling between two heavy-fluxonium qubits, each with sim50MHz frequencies and sim5 GHz anharmonicities. The coupler enables the qubits to have a large tuning range of textit{XX} coupling strengths (-35 to 75 MHz). The textit{ZZ} coupling strength is <3kHz across the entire coupler bias range, and <100Hz at the coupler off-position. These qualities lead to fast, high-fidelity single- and two-qubit gates. By driving at the difference frequency of the two qubits, we realize a sqrt{iSWAP} gate in 258ns with fidelity 99.72\%, and by driving at the sum frequency of the two qubits, we achieve a sqrt{bSWAP} gate in 102ns with fidelity 99.91\%. This latter gate is only 5 qubit Larmor periods in length. We run cross-entropy benchmarking for over 20 consecutive hours and measure stable gate fidelities, with sqrt{bSWAP} drift (2 σ) < 0.02\% and sqrt{iSWAP} drift < 0.08\%.