Characterization and Comparison of Energy Relaxation in Fluxonium Qubits

TL;DR

This study investigates energy relaxation in fluxonium qubits, identifying dielectric loss as a key factor, and compares fabrication processes to improve qubit coherence times.

Contribution

It introduces a circuit-based model for dielectric loss, applies it to compare fabrication methods, and assesses the impact of fluorine treatment on qubit energy relaxation.

Findings

Dielectric loss model explains frequency dependence of T1

Fluorine treatment slightly improves capacitive quality factor

Primary loss source remains unaffected by the fabrication process

Abstract

Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities, making them a favorable building block for superconducting quantum processors. We investigate the dominant limitations to fluxonium qubit energy relaxation time $T_1$ using a set of eight planar, aluminum-on-silicon qubits. We find that a circuit-based model for capacitive dielectric loss best captures the frequency dependence of $T_1$, which we analyze within both a two-level and a six-level energy relaxation model. We convert the measured $T_1$ into an effective capacitive quality factor $Q_\mathrm{C}^{\mathrm{eff}}$ to compare qubits on equal footing, accounting for independently estimated contributions from $1/f$ flux noise and radiative loss to the control and readout circuitry. We apply this methodology to compare qubits from two fabrication processes: a baseline process and one that applies a fluorine-based wet treatment prior to Josephson junction deposition. We resolve a small improvement of (13.8 $\pm$ 8.4$)\%$ in the process mean $Q_\mathrm{C}^{\mathrm{eff}}$, indicating that the fluorine treatment may have reduced loss from the metal-substrate interface, but did not address the primary source of loss in these fluxonium qubits.