Microscopic identity of two-level defects in qubit oxides
In plain words
Superconducting qubits lose energy to defects in the thin oxide layers in and around the circuit, which behave like the tunneling defects of glasses. Which atoms or trapped charges form these defects is debated.
Precise statement
In Al/AlOx/Al Josephson junctions and in Nb, Ta and Al surface oxides, individual two-level defects resonant at 4-8 GHz couple to qubits through electric dipoles of order 1 e Angstrom and shift under applied strain and electric field. Identify the dominant atomic configuration (hydrogen or hydroxyl tunneling, oxygen vacancies, dangling bonds, trapped electrons) and predict its density, dipole moment, strain coupling and isotope dependence.
What would settle it
An ab initio defect model whose parameters match strain- and field-tuning spectroscopy of individual defects, confirmed by a controlled change (isotope, hydrogen removal, oxide chemistry) that shifts the defect spectrum as predicted.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
A 2026 ab initio study proposes hydrogen tunneling in $\mathrm{Nb_2O_5}$ and $\mathrm{Ta_2O_5}$ as the defects (Mendez and Arias, arXiv 2603.17194); experimental confirmation is lacking.
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