Title: Stable CNOT-gate on inductively-coupled fluxoniums with over 99.9% fidelity
Abstract: In this talk, we present a detailed characterization of two inductively coupled superconducting fluxonium qubits for implementing high-fidelity cross-resonance gates. Our circuit stands out because it behaves very closely to the case of two transversely coupled spin-1/2 systems. In particular, the generally unwanted static ZZ-term due to the non-computational transitions is nearly absent despite a strong qubit-qubit hybridization. Spectroscopy of the non-computational transitions reveals a spurious L C-mode arising from the combination of the coupling inductance and the capacitive links between the terminals of the two-qubit circuit. Such a mode has a minor effect on the present device, but it must be carefully considered for optimizing future multi-qubit designs. Fluxonium qubits offer a promising foundation for quantum information processing due to their long coherence times combined with strong anharmonicity. We demonstrate a 60 ns direct CNOT-gate on two inductively coupled fluxoniums, behaving nearly identically to a pair of transversely coupled spin-1/2 systems. The CNOT-gate fidelity, estimated via randomized benchmarking, reached 99.94%. Remarkably, this fidelity remains above 99.9% for 24 days without recalibration. In comparison with the 99.96% fidelity of a 60 ns identity gate, our data narrows the investigation of non-decoherence-related errors during logical operations down to 2 \times 10^{-4}. This result introduces a robust, high-fidelity two-qubit gate into the “beyond three nines” category for superconducting qubits, further advancing the field.