PNNL

Abstract

Recent advances in superconducting qubit technology have led to remarkable progress in quantum computing, but the challenge of achieving long coherence time remains. Tantalum (Ta) based qubits have shown exceptional lifetime performance to-date, however, their uncontrolled surface oxidation, which is detrimental to Ta-based qubits as oxidized regions, are believed to be the primary sources of the two-level system loss in two-dimensional transmon qubits. In this study, the surface oxidation mechanism of Ta films and its potential impact on lifetime of superconducting qubit were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results show that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding 1:1 O:Ta ratio drives disordering and, eventually, the formation of an amorphous Ta2O5 phase. Furthermore, we unravel an unexpected non-insulating suboxide phase, TaO, whose extra conductive contributions may generate a local electric field that can interact with the superconducting thin film and cause dielectric loss. Our findings further show that this surface oxidation mechanism can produce a substantial Ta-Ta bond rearrangement, which lead to electronic charge density redistribution and, as a result, the development of surface dipole moments, which may give rise to losses in superconducting qubit coherence energy. The findings enhance our comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.