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

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. The fluxonium qubit, which can be made with transition frequencies as low as several MHz and widely tunable matrix elements, serves as a natural probe of charge- and flux-coupled low-frequency noise. Here we investigate energy relaxation in a low-frequency fluxonium qubit (approximately 50 MHz), whose sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux [1]. We explore the dependence of T1 on temperature and in-plane magnetic field and offer a comparison of flux-coupled loss models near half-flux. 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. Our results aim to offer insight for fluxonium coherence modeling and inform microscopic theories of intrinsic noise in superconducting circuits.

[1] Ateshian, Lamia, et al., arXiv:2507.01175 (2025).