Native approach to controlled-Z gates in inductively coupled fluxonium qubits

TL;DR

This paper introduces an inductive coupling method for fluxonium qubits enabling native ZZ interactions, achieving high-fidelity controlled-Z gates with noise mitigation, advancing fluxonium-based quantum computing.

Contribution

The authors propose and demonstrate a flux-controlled inductive coupling scheme that provides native ZZ interactions for fluxonium qubits, enabling high-fidelity CZ gates.

Findings

Achieved a 20 ns CZ gate with 99.53% fidelity.

Demonstrated flux-controlled ZZ interaction for entanglement.

Implemented continuous dynamical decoupling to reduce flux noise.

Abstract

The fluxonium qubits have emerged as a promising platform for gate-based quantum information processing. However, their extraordinary protection against charge fluctuations comes at a cost: when coupled capacitively, the qubit-qubit interactions are restricted to XX-interactions. Consequently, effective XX- or XZ-interactions are only constructed either by temporarily populating higher-energy states, or by exploiting perturbative effects under microwave driving. Instead, we propose and demonstrate an inductive coupling scheme, which offers a wide selection of native qubit-qubit interactions for fluxonium. In particular, we leverage a built-in, flux-controlled ZZ-interaction to perform qubit entanglement. To combat the increased flux-noise-induced dephasing away from the flux-insensitive position, we use a continuous version of the dynamical decoupling scheme to perform noise filtering. Combining these, we demonstrate a 20 ns controlled-Z (CZ) gate with a mean fidelity of 99.53%. More than confirming the efficacy of our gate scheme, this high-fidelity result also reveals a promising but rarely explored parameter space uniquely suitable for gate operations between fluxonium qubits.