First Principles Investigation of Er-Doped CeO₂ as a Solid-State Qubit Platform

Recent predictions¹ based on cluster correlation expansion methods suggest that multiple simple binary oxides like CeO₂, CaO, and MgO may possess long coherence times due to dilute nuclear spin baths. These findings point at the exciting prospect of designing new defects and hosts that are compatible with present telecommunication and manufacturing infrastructure and that can give rise to novel quantum applications. In fact, a model electron spin defect in CeO₂ has been predicted to have a coherence time of 47 ms¹; however, the specific defect is unknown. In this work, we investigate the Er³⁺ dopant in CeO₂ as a candidate spin defect. The sharp emission line of Er³⁺ in the telecom-C band and the possibility of data storage in ¹⁶⁷Er³⁺ nuclear spins, combined with the low nuclear spin concentration of the CeO₂ host, could make Er³⁺:CeO₂ a promising platform for quantum applications, such as quantum communications and quantum memories. Using advanced electronic structure methods, including hybrid density functional theory, time-dependent density functional theory, many-body perturbation theory, and embedding methods, we report the electronic structure and excited state properties of the Er³⁺ dopant in CeO₂. We also compute the spin coherence time and compare with experimentally reported values and discuss the role of oxygen vacancies and the Ce³⁺ polarons on the electronic, optical, and coherence properties of the defect. Our results can aid in identifying sources of decoherence in experiments on Er³⁺-doped CeO₂, which report a spin coherence time of 0.66 μs². More broadly, our study establishes an ab initio protocol to investigate rare earth dopants in rare earth oxides, which can be extended to fitting accurate crystal field parameters and studying charge transfer mechanisms and dopant-dopant interactions.

¹Kanai, Shun, et al. "Generalized scaling of spin qubit coherence in over 12,000 host materials." Proceedings of the National Academy of Sciences 119.15 (2022): e2121808119.
²Zhang, Jiefei, et al. "Optical and spin coherence of Er³⁺ in epitaxial CeO₂ on silicon." arXiv preprint arXiv:2309.16785 (2023).