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Oxygen vacancies in niobium pentoxide: a key source of loss in Nb superconducting qubits

SQMS researchers uncover loss mechanisms in the surface oxide, paving the way for mitigation strategies to enhance qubit performance.

The Science: By combining experiments on three-dimensional superconducting niobium electromagnetic resonators with detailed secondary ion mass spectrometry analysis, we have identified oxygen vacancies in the surface oxide as a critical defect limiting superconducting qubit performance. By systematically altering the oxide stoichiometry through controlled vacuum baking, we demonstrate a direct correlation between system losses and the number of oxygen vacancies, providing key insights into the mechanisms underlying quantum decoherence. 

The Impact: These results identify oxygen vacancies in the niobium surface oxide as a source of decoherence in superconducting quantum systems and further highlight the deleterious effects of sub-stoichiometric oxides. These findings further motivate efforts focused on developing mitigation strategies to prevent vacancy formation to unlock improved qubit performance.

(Left): Two-level system-driven RF loss characterized by the filling factor F and loss tangent δ0 at 1.4 K < T < 1.6 K of a three-dimensional superconducting radio-frequency niobium post sequential rounds of in-situ vacuum baking. (Right): Missing number of scaled oxygen atoms within the niobium pentoxide extracted from a representative niobium sample subjected to the same in-situ vacuum baking used in superconducting resonator measurements. Data was obtained using time-of-flight secondary ion mass spectrometry.

Summary: By investigating the role of vacuum baking on niobium SRF resonators, which contain an approximately 5-nm full wet-grown native amorphous oxide, we find that vacancies in the niobium pentoxide host two-level systems (TLS), which limit the performance of 3D Nb resonators and 2D transmon qubits, which utilize oxidized Nb. Modest in situ vacuum-baking treatments (150C– 200°C) for durations as short as 25 min aggravate TLS-like losses that are eventually suppressed with increasing bake time.

We recreate the in-situ vacuum-baking treatments using time-of-flight secondary ion mass spectrometry on a Nb SRF resonator cutout and confirm that the oxide reduces considerably, suggesting an increase in the number of oxygen vacancies in the Nb2O5 layer. Continued in situ baking shows that the total number of oxygen vacancies in the pentoxide decreases as the layer is gradually reduced into suboxides. As such, the nonmonotonic evolution of Q0 with successive baking is due to the interplay of oxygen-vacancy generation in the Nb2O5 layer, which host TLS, and subsequent pentoxide dissolution. 

Contact: Daniel Bafia – dbafia@fnal.gov 

Focus Area: Materials 

Institutions: Fermilab 

Citation: D. Bafia et al., Oxygen vacancies in niobium pentoxide as a source of two-level system losses in superconducting niobium, Phys. Rev. Applied. 22, 024035 (2024). https://doi.org/10.1103/PhysRevApplied.22.024035 

Acknowledgement: This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under Contract No. DE-AC02-07CH11359.