This thesis focuses on two aspects of the development of quantum memories for quantum communication applications, in two rare earth ion-doped solid state materials. The first part consists in fundamental investigations on Yb:YSO. After a spectroscopy phase, we showed how its anisotropic hyperfine interaction between nuclear and electronic spins enhances the coherence times of both microwave and optical transitions in two different regimes of magnetic field, opening new avenues for a broadband quantum memory with strong microwave coupling. The second part reports on improvements on existing storage protocols in Eu:YSO. A general upgrade of the components of the spin-wave atomic frequency comb protocol led to an improved performance at the single-photon level and to the storage of a time-bin qubit for 20 milliseconds. The same improvements enabled the generation of quantum-correlated pairs of photons delayed by 20 milliseconds, an important step towards quantum repeaters based on the DLCZ protocol.