Delocalised oxygen as the origin of two-level defects in Josephson junctions

TL;DR

This paper proposes that delocalized oxygen atoms in the oxide barrier of Josephson junctions are the microscopic origin of two-level defects, affecting qubit coherence.

Contribution

It introduces a microscopic model showing delocalized oxygen as the source of two-level defects in Josephson junctions, linking atomic behavior to observable qubit parameters.

Findings

Defects are charge neutral but respond to electric field and strain.

Model explains long coherence times despite charge noise.

Defects originate from oxygen delocalization in the oxide barrier.

Abstract

One of the key problems facing superconducting qubits and other Josephson junction devices is the decohering effects of bi-stable material defects. Although a variety of phenomenological models exist, the true microscopic origin of these defects remains elusive. For the first time we show that these defects may arise from delocalisation of the atomic position of the oxygen in the oxide forming the Josephson junction barrier. Using a microscopic model, we compute experimentally observable parameters for phase qubits. Such defects are charge neutral but have non-zero response to both applied electric field and strain. This may explain the observed long coherence time of two-level defects in the presence of charge noise, while still coupling to the junction electric field and substrate phonons.