Oxygen vacancies in niobium pentoxide as a source of two-level system losses in superconducting niobium

We identify a major source of quantum decoherence in three-dimensional superconducting radio-frequency (SRF) resonators and two-dimensional transmon qubits composed of oxidized niobium: oxygen vacancies in the niobium pentoxide, which drive two-level system (TLS) losses. By probing the effect of sequential in situ vacuum-baking treatments on the rf performance of bulk $\mathrm{Nb}$ SRF resonators and on the oxide structure of a representative $\mathrm{Nb}$ sample using TOF SIMS, we find a nonmonotonic evolution of cavity quality factor $Q_0$, which correlates with the interplay of ${\mathrm{Nb}}_2{\mathrm{O}}_5$ vacancy generation and oxide-thickness reduction. We localize this effect to the oxide itself and present the insignificant role of diffused interstitial oxygen in the underlying $\mathrm{Nb}$ by regrowing the oxide via wet oxidation, which reveals a mitigation of aggravated TLS losses. We hypothesize that such vacancies in the pentoxide serve as magnetic impurities and are a source of TLS-driven rf loss.