Solid state atomic processors for light

This paper is devoted to optics in rare earth ion doped crystal at low temperature. In cryogenic conditions, interesting features come from absorption rather than from transparency. The optical transition linewidth is considerably reduced, which also corresponds to a strong increase of quantum state lifetime. Linewidth narrowing leads to signal processing applications. Specific use for RADAR warning receivers is considered here. Then the quantum lifetime extension is illustrated by coherent transient processes that represent necessary experimental steps on the way to quantum information research