At the Langevin Institute, we leverage rare-earth-doped crystals (e.g., Tm:YAG, Er:YSO) as powerful platforms for quantum sensing, memories, and analog signal processing. These materials, operated at cryogenic temperatures (~3K), offer exceptional optical linewidths, long population lifetimes, and strong light-matter interactions, key for quantum coherence. However, commercial cryogenics introduce vibrations and thermal loads, degrading performance. This internship explores alternative matrices (YGG, co-doped crystals, molecular crystals, ceramics, waveguides) to extend lifetimes, damp vibrations, and optimize light guiding, enabling lower ion concentrations and reduced decoherence. The intern will use advanced spectroscopy (photon echo, spectral hole-burning) to characterize these materials. Ideal for M1/M2 or engineering students in optics/physics with experimental skills. Collaborative project with IRCP and Thales.