Mitigating Losses of Superconducting Qubits Strongly Coupled to Defect Modes

TL;DR

This study investigates how to reduce the impact of material defects that strongly couple to superconducting qubits, aiming to improve qubit coherence and operation fidelity by understanding defect properties and fabrication methods.

Contribution

It provides a systematic analysis of defect modes' spectral density and demonstrates that reducing Josephson junction size can mitigate strongly coupled defects.

Findings

Warming the sample redistributes defect frequencies without reducing total defect count.

Smaller Josephson junctions decrease the spectral density of strongly coupled defect modes.

Surface cleaning methods influence defect properties and coupling strengths.

Abstract

The dominant contribution to the energy relaxation of state-of-the-art superconducting qubits is often attributed to their coupling to an ensemble of material defects which behave as two-level systems. These defects have varying microscopic characteristics which result in a large range of observable defect properties such as resonant frequencies, coherence times and coupling rates to qubits $g$. Here, we investigate strategies to mitigate losses to the family of defects that strongly couple to qubits ($g/2\pi\ge$ 0.5 MHz). Such strongly coupled defects are particularly detrimental to the coherence of qubits and to the fidelities of operations relying on frequency excursions, such as flux-activated two-qubit gates. To assess their impact, we perform swap spectroscopy on 92 frequency-tunable qubits and quantify the spectral density of these strongly coupled modes. We show that the frequency configuration of the defects is rearranged by warming up the sample to room temperature, whereas the total number of defects on a processor tends to remain constant. We then explore methods for fabricating qubits with a reduced number of strongly coupled defect modes by systematically measuring their spectral density for decreasing Josephson junction dimensions and for various surface cleaning methods. Our results provide insights into the properties of strongly coupled defect modes and show the benefits of minimizing Josephson junction dimensions to improve qubit properties.