# Efficient optical pumping using hyperfine levels in $^{145}$Nd$^{3+}$:Y$_2$SiO$_5$

In the rare-earth ion community, non-Kramers ions are traditionally thought to achieve larger quantum memory efficiencies. This is due to the fact that the best storage efficiencies (around 30-35% in bulk) were achieved in non-Kramers ion-doped solids, while Kramers ions have been limited to less than 30% until now. In our recent paper, published in NJP, we show that the secret to improve the efficiency of Kramers ions relies on exploiting the optical pumping with nuclear spin states. Thanks to a detailed study of the relaxation processes limiting optical pumping in $^{145}$Nd$^{3+}$:Y$_2$SiO$_5$ , we achieve efficiencies comparable to the best state-of-the-art bulk non-Kramers quantum memories. This paves the way towards high efficiency optical quantum memories for quantum repeaters, taking advantage of the many interesting properties of Kramers ions, such as the frequency of the relevant optical transition and broadband capabilities.