Optically Active Yb ³⁺ Spin Defects in Cerium Oxide Nanocrystals

Nanocrystalline CeO ₂ is a chemically tunable wide-bandgap host lattice of interest for quantum information technologies due to its ability to accommodate optically-and spin-active defects. Here, we investigate CeO ₂ nanocrystals as hosts for near-infrared-compatible Yb ³⁺ ions as optically active spin quantum bits (qubits). We characterize the as-synthesized Yb ³⁺ -doped CeO ₂ nanocrystals (0.01-10% Yb) by photoluminescence, which reveals short photoluminescence lifetimes dominated by non-radiative decay arising from surface and Ce ³⁺ -related defects. Thermal annealing at 700 °C increases crystalline domain sizes (from ~7 nm to ~20 nm) and reduces the concentration of Ce ³⁺ defects, yielding an enhancement in excited-state lifetimes. Low-temperature optical spectroscopy reveals at least two distinct Yb ³⁺ lattice sites with cubic and trigonal symmetries, and a ~5-fold difference in photoluminescence lifetimes between the two sites. Continuous-wave EPR measurements support these site assignments, while pulse EPR measurements further show that annealing the nanocrystals nearly doubles the spin-lattice relaxation time ( T ₁ ) from 33.9 to 62.7 µs at 0.08% Yb. Phase memory times, however, remain relatively constant, indicating that coherence is limited by dephasing mechanisms (e.g., slowly fluctuating local fields or spectral diffusion) rather than by spin-lattice relaxation. Collectively, these results establish defect mitigation, rather than dopant identity alone, as a bottleneck in realizing nanocrystalline CeO ₂ as a viable quantum host. This work provides quantitative benchmarks for optical and spin coherence in CeO ₂ :Yb ³⁺ nanocrystals and highlights defect engineering as a critical pathway toward scalable rare-earth-based qubits.