{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"77f33e13d0ea1cc65c56a27cae1e3e6c17fcda0b36a365aca789d3f29b839dee","created":"2026-10-03T07:18:01Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"670646a2f6a90a19a8924f03c2b23370432d878758e3edf43a23f02ff802e83c","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.qubit-decoherence-defects.junction-tls-identity","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"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.","posed_since":"2005","precise":"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.","problem_ref":null,"references":"","settled_by":"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_note":"A 2026 ab initio study proposes hydrogen tunneling in Nb2O5 and Ta2O5 as the defects (Mendez and Arias, arXiv 2603.17194); experimental confirmation is lacking.","title":"Microscopic identity of two-level defects in qubit 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