Coherent Erbium Spin Defects in Colloidal Ceria Nanocrystals

Nanocrystals (NC), with its direct bottom-up synthesis capability and flexibility in device integration, have gained attention as viable quantum hosts for spin defects. Here, we explore erbium (Er) doped ceria NC with an ensemble average level of a single erbium per NC and experimentally demonstrate spin coherence times approaching a microsecond. Ceria was chosen as a host material due to a low natural abundance of nuclear spins. As a result, we observe population lifetimes that approach a millisecond, suggesting the possibility of substantial improvements on the spin coherence. We postulate that the spin coherence is limited by the nuclear spin bath of hydrogen atoms from the oleic acid ligands on the surface of the nanocrystal that are used during synthesis. The observation of the Larmor frequency of hydrogen further suggests that the Er ions are sensitive to the nearby hydrogen nuclei. Scanning transmission electron microscopy combined with electron energy loss spectroscopy further show that Ce³⁺ and O vacancies are prevalent on the surface of the nanocrystal, likely playing a role in the observed spin coherence and lifetime. Nonetheless, the initial spin coherence times demonstrated suggests that spin defects in nanocrystals are a promising materials platform for quantum information processing.