Erbium Spin Defects in Colloidal Ceria Nanocrystals with Near Microsecond Spin Coherence

We experimentally demonstrate spin coherence times approaching a microsecond in erbium doped ceria nanocrystals with an ensemble average level of a single erbium per nanocrystal. To achieve these spin coherence times, ceria was chosen as a host material with a low natural abundance of nuclear spins. As a result, we observe population lifetimes that approach a millisecond, suggesting substantial improvements to the spin coherence should be possible. 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. Quantum beats are observed which correspond to the Larmor frequency of hydrogen, which further suggests the erbium ions are sensitive to the nearby hydrogen nuclei. Scanning transmission electron microscopy measurements 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 spin coherence already demonstrated suggests that spin defects in nanocrystals are a promising materials platform for quantum information processing.