Temperature and Magnetic-Field Dependence of Energy Relaxation in a Fluxonium Qubit

TL;DR

This study explores how temperature and magnetic fields affect energy relaxation in fluxonium qubits, revealing linear flux noise scaling with temperature and a magnetic field response of dielectric loss, informing noise mitigation strategies.

Contribution

It provides new experimental insights into temperature and magnetic field effects on fluxonium qubits, including a multi-level decoherence model and observations of defect-related noise behaviors.

Findings

Flux noise scales linearly with temperature

Dielectric loss follows a T^3 dependence up to 100 mK

Dielectric-loss-limited T1 decreases with weak magnetic fields

Abstract

Noise from material defects at device interfaces is known to limit the coherence of superconducting circuits, yet our understanding of the defect origins and noise mechanisms remains incomplete. Here we investigate the temperature and in-plane magnetic-field dependence of energy relaxation in a low-frequency fluxonium qubit, where the sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux. We observe an approximately linear scaling of flux noise with temperature $T$ and a power-law dependence of dielectric loss $T^3$ up to 100 mK. Additionally, we find that the dielectric-loss-limited $T_1$ decreases with weak in-plane magnetic fields, suggesting a potential magnetic-field response of the underlying charge-coupled defects. We implement a multi-level decoherence model in our analysis, motivated by the widely tunable matrix elements and transition energies approaching the thermal energy scale in our system. These findings offer insight for fluxonium coherence modeling and should inform microscopic theories of intrinsic noise in superconducting circuits.