Dielectric Loss and TLS in Josephson Junction Arrays and Fluxonium Qubits

Fluxonium is a promising superconducting qubit offering high coherence and excellent single- and two-qubit gate fidelities. However, the performance of the state-of-the-art qubits is hindered by dielectric loss and strongly coupled two-level systems (TLS) in Josephson junction arrays that form the fluxonium's inductance. In this talk, we present our progress in the characterization and understanding of dielectric loss in these inductances. We utilize multi-mode resonators built from similar junction arrays, performing frequency- and time-domain measurements to extract resonators' internal quality factors and lifetimes at frequencies below 4 GHz. Additionally, by tuning the resonators' frequencies in situ with an external magnetic field, we observe avoided crossings with strongly coupled TLS, which allow us to estimate their density and examine its frequency dependence. Our results provide new insights into dielectric loss mechanisms in fluxonium inductances and suggest potential pathways for mitigating its impact on qubit performance.