EXPLORING FLUXONIUM-BASED QUANTUM COMPUTING

Abstract

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 thesis, we first introduce a novel fluxonium qubit operating at zero magnetic field with high coherence. We implement and characterize single-qubit gates with an average gate fidelity of 99.93%, extracted from randomized benchmarking. This qubit serves as a ready-to-use superconducting qubit that operates in the frequency range of conventional transmons and exhibits stronger anharmonicity. Next, we implement a 60 ns direct CNOT gate on two inductively coupled fluxoniums, which behave almost exactly like a pair of transversely coupled spin-1/2 systems. Notably, the typically undesirable static ZZ term, arising from non-computational transitions, is nearly absent even in the presence of strong qubit-qubit hybridization. The CNOT gate fidelity, estimated via randomized benchmarking, reaches 99.94%. Furthermore, this fidelity remains above 99.9% over a span of 24 days without any recalibration between measurements. Compared with the 99.96% fidelity of a 60 ns identity gate, our results constrain non-decoherence-related errors during logical operations to as low as 2 × 10^−4. This work adds a simple and robust two-qubit gate to the still relatively small family of “beyond three nines” gates on superconducting qubits.