7 Conclusion and Outlook
R-doped crystals fulfill many of the requirements for an efficient QIP system. Protocols exist that take advantage of their specific spectroscopic properties, which can be moreover enhanced and tailored by a number of techniques. In particular, R-doped crystals can provide interfaces between photonic quantum bits, in the optical and microwave ranges, and solid-state qubits. As these qubits can be further processed by optical control, a complete quantum network node could be obtained.

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.