Tm³⁺:Y₃Ga₅O₁₂ materials for spectrally multiplexed quantum memories

We investigate the relevant spectroscopic properties of the 795 nm ³H₆leftrightarrow³H₄ transition in 1% Tm³⁺:Y₃Ga₅O₁₂ at temperatures as low as 1.2 K for optical quantum memories based on persistent spectral tailoring of narrow absorption features. Our measurements reveal that this transition has uniform coherence properties over a 56 GHz bandwidth, and a simple hyperfine structure split by pm44 MHz/T with lifetimes of up to hours. Furthermore, we find a ³F₄ population lifetime of 64 ms - one of the longest lifetimes observed for an electronic level in a solid --, and an exceptionally long coherence lifetime of 490 μs - the longest ever observed for optical transitions of Tm³⁺ ions in a crystal. Our results suggest that this material allows realizing broadband quantum memories that enable spectrally multiplexed quantum repeaters.