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

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 Al₂O₃ 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 Al₂O₃ using first-principles calculations and ab initio molecular dynamics. Our results show that both the coordination environment and concentration of V_Os strongly influence electrical conductivity. In particular, two- and three-coordinated V_Os, 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.