Impact of Oxygen Vacancies in Josephson Junction on Decoherence of Superconducting Qubits

TL;DR

This study explores how oxygen vacancies in amorphous Al$_2$O$_3$ Josephson junctions affect electrical noise and qubit decoherence, offering insights for designing more radiation-resistant superconducting quantum devices.

Contribution

It provides a detailed first-principles analysis of oxygen vacancy effects on conductivity and noise in amorphous Al$_2$O$_3$, linking microscopic defects to qubit decoherence.

Findings

Oxygen vacancies increase conductivity fluctuations in amorphous Al$_2$O$_3$.

Higher V$_O$ concentrations lead to shorter qubit coherence times.

Two- and three-coordinated V$_O$s enhance conductivity more than four-coordinated ones.

Abstract

Superconducting quantum circuits are promising platforms for scalable quantum computing, where qubit coherence is critically determined by microscopic defects in the oxide tunneling barrier of Josephson junctions. Amorphous Al$_2$O$_3$ is widely used as a barrier material, but under irradiation, oxygen vacancy (V$_O$) defects are readily generated, introducing noise sources that accelerate qubit decoherence. We systematically investigate the structural characteristics and electronic impact of V$_O$ defects in amorphous Al$_2$O$_3$ using first-principles calculations and \textit{ab initio} molecular dynamics. Our results show that both the coordination environment and concentration of V$_O$s strongly influence electrical conductivity. In particular, two- and three-coordinated V$_O$s, unique to the amorphous structure, enhance conductivity more than conventional four-coordinated vacancies. Increasing V$_O$ concentration amplifies conductivity fluctuations, which we link to critical current noise in Josephson junctions. Using a noise model, we estimate that higher V$_O$ densities lead to shorter qubit coherence times. These findings provide insights for radiation-hard design of superconducting quantum devices.