Characterization of loss mechanisms in a fluxonium qubit

TL;DR

This study investigates the energy relaxation mechanisms in a fluxonium qubit, revealing how dielectric loss and flux noise contribute to relaxation rates and how qubit design influences coupling to defects.

Contribution

It provides a detailed quantitative analysis of loss mechanisms in fluxonium qubits, including flux noise and TLS interactions, with insights into optimizing qubit coherence.

Findings

Relaxation rate varies with flux bias and Josephson energy.

Dielectric loss and flux noise explain relaxation behavior.

Increasing Josephson energy reduces coupling to TLS defects.

Abstract

Using a fluxonium qubit with in situ tunability of its Josephson energy, we characterize its energy relaxation at different flux biases as well as different Josephson energy values. The relaxation rate at qubit energy values, ranging more than one order of magnitude around the thermal energy $k_B T$, can be quantitatively explained by a combination of dielectric loss and $1/f$ flux noise with a crossover point. The amplitude of the $1/f$ flux noise is consistent with that extracted from the qubit dephasing measurements at the flux sensitive points. In the dielectric loss dominant regime, the loss is consistent with that arises from the electric dipole interaction with two-level-system (TLS) defects. In particular, as increasing Josephson energy thus decreasing qubit frequency at the flux insensitive spot, we find that the qubit exhibits increasingly weaker coupling to TLS defects thus desirable for high-fidelity quantum operations.