Hyperfine Transitions: A New Class of TLS Defect in Amorphous Nb Oxides

Removal of the amorphous Nb oxide layer of qubits and resonators with Nb capacitor pads has been shown to be correlated with improved qubit coherence times and resonator quality factors. A component of this loss has been linked to an intrinsic density of two-level system (TLS) defects. These TLS have been traditionally associated with configurational changes in the glass structure which are mediated by electric dipole couplings with the electric field of the qubit/resonators. We show that (1) Nb₂O_5-x (0 <x ≤0.25) possesses an intrinsic magnetization density across a wide range of oxygen deficiencies, (2) that hyperfine couplings between Nb nuclei and local magnetic moments can form a multi-level structure with transition frequencies of 100-1000 MHz or higher, and (3) such moments and their corresponding hyperfine structure are quenched or highly suppressed in Ta₂O_5-x (0 <x ≤0.25) [1]. To the best of our knowledge, such hyperfine transitions represent a newly identified class of TLS defect in amorphous Nb oxides which may impair qubit coherence.

[1] P. G. Pritchard and J. M. Rondinelli. (2024). Suppressed paramagnetism in amorphous Ta₂O_5−x oxides and its link to superconducting qubit performance. http://arxiv.org/abs/2410.13160

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.