Tunable inductive coupler with >99.9% two-qubit gate fidelity between fluxonia

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 ∼ 50MHz frequencies and ∼ 5 GHz anharmonicities. The coupler enables the qubits to have a large tuning range of XX coupling strengths (−35 to 75 MHz). The 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 √iSWAP gate in 258ns with fidelity 99.72%, and by driving at the sum frequency of the two qubits, we achieve a √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 √bSWAP drift (2σ) < 0.02% and √iSWAP drift < 0.08%.