Tunable inductive coupler for high fidelity gates between fluxonium qubits

Abstract

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 $\sim 50$MHz frequencies and $\sim 5$ GHz anharmonicities. The coupler enables the qubits to have a large tuning range of $\text{XX}$ coupling strengths ($-35$ to $75$ MHz). The $\text{ZZ}$ coupling strength is $<3$kHz across the entire coupler bias range, and $<100$Hz 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 $\sqrt{i\mathrm{SWAP}}$ gate in $258$ns with fidelity $99.72\%$, and by driving at the sum frequency of the two qubits, we achieve a $\sqrt{b\mathrm{SWAP}}$ gate in $102$ns 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 $\sqrt{b\mathrm{SWAP}}$ drift ($2\sigma$) $<0.02\%$ and $\sqrt{i\mathrm{SWAP}}$ drift $<0.08\%$.