However, a QIP system outperforming a classical system in current information processing tasks is extremely demanding. Still, R-based quantum memories seem much closer to reach the required operating parameters than quantum computers, which are clearly much more complex devices. Strong theoretical and experimental improvements are needed, including development of new materials. Here, bulk single crystals are the preferred choice because of their outstanding spectroscopic properties, although some parameters have still to be improved. Attractive alternatives, like nanostructured materials, may provide additional and important features. In particular, this could allow coupling R ions to other atomic quantum systems, nanoscale quantum mechanical oscillators, or nanocavities.

In such hybrid quantum systems, one could expect to combine efficient processing and storage functionalities, and interface quantum states of different physical nature. This approach may not only benefit QIP but also open the way to fundamental studies in quantum physics.