  Fluxonium qubit is a promising building block for quantum information
processing due to its long coherence time and strong anharmonicity. In this
paper, we realize a 60 ns direct CNOT-gate on two inductively-coupled fluxonium
qubits using selective darkening approach, resulting in a gate fidelity as high
as 99.94%. The fidelity remains above 99.9% for 24 days without any
recalibration between randomized benchmarking measurements. Compared with the
99.96% fidelity of a 60 ns identity gate, our data brings the investigation of
the non-decoherence-related errors during gate operations down to $2 \times
10^{-4}$. The present result adds a simple and robust two-qubit gate into the
still relatively small family of "the beyond three nines" demonstrations on
superconducting qubits.
