We report an optical homogeneous linewidth of 580 $\pm$ 20 Hz of ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{O}}_3$ ceramics at millikelvin temperatures, narrowest so far in rare-earth doped ceramics, as well as a slow spectral diffusion of $\sim 2$ kHz over a millisecond time scale. Detailed investigations of temperature and field dependence of optical coherence and spectral diffusions reveal the remaining weak dephasing mechanisms as coupling to elastic two-level systems and superhyperfine interactions of ${\mathrm{Er}}^{3+}$ with nuclear spins. The measured transient coherence properties can be understood in part from frozen ${\mathrm{Er}}^{3+}$ spins at dilution temperatures. In addition to informing possible refinement in material synthesis to further suppress dephasings, these spectroscopic results put ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{O}}_3$ ceramics as a promising candidate for quantum systems.

• Received 12 November 2019
• Accepted 18 May 2020

DOI:https://doi.org/10.1103/PhysRevB.101.214202