Towards Atomic Precision in Superconducting Qubits: Mechanisms of Ta Oxidation and Strategies for Oxidation Suppression

Coherence times of transmon devices can be affected by oxidation of the components made of superconducting metals, such as Nb and Ta. Spontaneous oxidation results in the formation of suboxide phases and surface amorphization that contribute to dielectric losses that are primarily attributed to two-level systems within such native oxide layers. Mitigating undesirable effects of surface oxidation requires understanding the mechanisms of interfacial interactions at the atomic scale. <br/> <br/>We review recent experimental studies that provide new insights into the atomic structure and composition of the native oxide layer and focus on ab initio simulations of the mechanisms of Ta interaction with oxygen. In particular, we consider energetics and pathways of the early stages of the Ta(110) surface oxidation, including propagation of the oxidation front into the Ta subsurface and corresponding electronic structure changes. We also explore strategies for suppressing Ta oxidation with the help of reactive metal coating and discuss atomic-scale models of candidate two-level systems.<br/><br/>C. Zhou, J. Mun, J. Yao, A. K. Anbalagan, M. D. Hossain, R. A. McLellan, R. Li, K. Kisslinger, X. Tong, G. Li, A. R. Head, C. Weiland, A. L. Walter, Q. Li, Y. Zhu, P. V. Sushko, M. Liu <i>Ultrathin Magnesium-based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials</i>, arXiv preprint arXiv:2309.12603