Verifying the analogy between transversely coupled spin-1/2 systems and inductively-coupled fluxoniums

TL;DR

This paper demonstrates that two inductively coupled fluxonium qubits closely mimic transversely coupled spin-1/2 systems, enabling high-fidelity gates with minimal unwanted interactions, and provides detailed characterization and insights for future device optimization.

Contribution

It shows that fluxonium qubits can behave like transversely coupled spins, with nearly absent static ZZ interaction, and identifies a spurious LC mode affecting coupling.

Findings

Minimal static ZZ-term despite strong hybridization

Presence of a spurious LC-mode from coupling inductance and capacitance

Fluxonium qubits can emulate spin-1/2 systems for quantum gates

Abstract

We report 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 LC-mode arising from the combination of the coupling inductance and the capacitive links between the terminals of the two qubit circuits. Such a mode has a minor effect on our specific device, but it must be carefully considered for optimizing future designs.