Sub-kilohertz optical homogeneous linewidth in transparent Er³⁺:Y₂O₃ ceramics

Global quantum networks require quantum memories with long coherence times to act as quantum repeaters connected by optical fibers. Erbium doped solid-state systems are good candidates for such memories, as they exhibit long coherence lifetimes and an optical transition in the low-loss telecom band. We measure an optical homogeneous linewidth of 580 Hz in transparent Er³⁺:Y₂O₃ ceramics at millikelvin temperatures, the narrowest so far in rare-earth doped ceramics, and suppressed spectral diffusion at 300 Hz/decade. Temperature, field, and time dependence studies of homogeneous linewidth reveal the limiting dephasing mechanisms as tunneling two-level systems and superhyperfine interactions between the electronic spins of erbium and nuclear spins of yttrium. These spectroscopic results put Er³⁺:Y₂O₃ ceramics as a promising candidate for telecommunication quantum memories.