Characterization and comparison of energy relaxation in planar fluxonium qubits

Fluxonium superconducting qubits have demonstrated high coherence times and high single and two qubit gate fidelities, making them a promising building block for superconducting quantum processors. In this work, we characterize the energy relaxation times T1 of multiple fluxonium qubits across their tunable frequency range in order to assess the dominant contributors to decoherence. Of the mechanisms considered, a circuit-based model for capacitive loss best captures the trends in the data over the majority of the tuning range. Motivated by this, we also consider modifications to this model accounting for a frequency-dependent effective capacitive quality factor. Furthermore, we use this analysis to bound the contributions of various other loss mechanisms. Finally, we utilize a composite model to compare these loss mechanisms on equal footing across individual qubits, and compare qubits fabricated with different materials processing techniques.