Er-doped rare earth oxides for optical quantum memory on Si substrate

Quantum memory is a key device for quantum repeaters that are a prerequisite for building global quantum networks. In particular for on-chip quantum memory operating at the telecommunication wavelength, Er-doped $\mathrm{CeO}_{2}\left(\mathrm{Er}: \mathrm{CeO}_{2}\right)$ is a promising material platform, in which a long coherence time is expected. This is because the Er ions are dilutely embedded in the nuclear-spin-free CeO2 matrix (crystal). Although we have prepared high-quality Er: CeO2 crystals on Si substrates by molecular beam epitaxy, further improvement of their optical properties is desired. Here, we introduced a post-growth annealing process and explored the optimum annealing conditions using Bayesian optimization with a view to increasing the intensity and narrowing the optical absorption linewidth. The post-growth annealing process turned out to be effective, and the resultant improvement in the optical properties can be attributed to improved crystallinity of the host material CeO2 that contributes to providing a uniform environment around the Er ions.