Ab Initio Modeling of Two-Level Systems in Niobium Oxide

Microwave loss from atomic two-level systems (TLS) at surfaces and interfaces limits the coherence of superconducting qubits and the quality factor of SRF cavities. Mitigating these losses is fundamental to next-generation quantum devices, yet their physical origin—especially in niobium oxides—remains poorly understood. Because it is light and ubiquitous, hydrogen in Nb oxides is a prime suspect. We present a high-throughput, first-principles–guided workflow that links atomistic H defects in Nb2​O5​ (and impurity-stabilized Nb) to device-scale frequencies. We enumerate interstitial configurations by relaxing H on uniform 3D grids with machine-learning interatomic potentials, cluster unique minima, and compute hop barriers using the nudged elastic band (NEB) method. A calibration subset is validated with DFT to confirm ML accuracy. For each hop, we map the barrier B and site separation r to a tunnel splitting via a WKB model augmented by a barrier-shape factor calibrated from NEB profiles, yielding predicted TLS frequencies. We find oxygen-vacancy–decorated H in Nb2​O5​ naturally produces GHz splittings, whereas analogous O or N interstitials lie far outside the microwave band. This framework identifies concrete defect motifs and processing levers to reduce TLS density in Nb-based quantum hardware.