3. Materials

The realization of solid optical quantum systems requires a light-matter interface that can coherently absorb photons. Among the various possible systems, crystals doped with rare-earth ions appear to be good candidates. Indeed, these crystalline materials are robust, easy to use and feature a high density of optically active ions (typically 10 ¹⁸ ions/cm ³ ) compared to atomic vapors (10 ⁷ atoms/cm ³ ).