Concentration-dependent optical and spin inhomogeneous linewidth of europium-doped yttrium orthosilicate crystals

The transportable quantum memory is a feasible solution for realizing the long-distance quantum communication, which requires a storage lifetime of the order of hours. The isotope-enriched ¹⁵¹Eu³⁺:Y₂SiO₅ crystal is a promising candidate for this application. However, its optical storage efficiency and spin storage lifetime are limited by the wide inhomogeneous linewidth. In this work, we successfully grow isotope-enriched ¹⁵¹Eu³⁺:Y₂SiO₅ crystals with varying doping concentrations by utilizing the Czochralski method. The optical inhomogeneous broadening and spin inhomogeneous broadening are measured by the optical absorption spectroscopy and optically detected magnetic resonance tests, respectively. Notably, in the undoped samples, we identify a baseline level of inhomogeneous linewidths, happening at (390 ± 15) MHz for optical inhomogeneous broadening and (4.6 ± 0.2) kHz for spin inhomogeneous broadening. Our findings reveal that the point defects, induced by the doping ions, significantly contribute to the inhomogeneous broadening. For every increase of 10^–6 in doping concentration, the optical inhomogeneous broadening increases by 0.97 MHz, and the spin inhomogeneous broadening increases by 0.014 kHz. Furthermore, we discuss the influence of dislocations on inhomogeneous broadening and propose potential strategies to further mitigate these effects. These advancements are expected to promote the development of ultra-long-lifetime transportable quantum memory applications.