24 Days-Stable CNOT Gate on Fluxonium Qubits with Over 99.9% Fidelity

Fluxonium qubit is a promising elementary building block for quantum information processing due to its long coherence time combined with a strong anharmonicity. In this paper, we realize a 60-ns direct gate on two inductively coupled fluxoniums, which behave almost exactly as a pair of transversely coupled spin-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>1</a:mn><a:mo>/</a:mo><a:mn>2</a:mn></a:math> systems. The -gate fidelity, estimated using randomized benchmarking, was as high as 99.94%. Furthermore, the fidelity remains above 99.9% for 24 days without any recalibration between measurements. Compared with the 99.96% fidelity of a 60-ns identity gate, our data brings the investigation of the nondecoherence-related errors during logical operations down to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mn>2</d:mn><d:mo>×</d:mo><d:msup><d:mn>10</d:mn><d:mrow><d:mo>−</d:mo><d:mn>4</d:mn></d:mrow></d:msup></d:math>. The present result adds a simple and robust two-qubit gate into the still relatively small family of the “beyond three nines” gates on superconducting qubits.