  The realization of a scalable architecture for quantum information processing
is a major challenge for quantum science. A promising approach is based on
emitters in nanostructures that are coupled by light. Here, we show that erbium
dopants can be reproducibly integrated at well-defined lattice sites by
implantation into pure silicon. We thus achieve a narrow inhomogeneous
broadening, less than 1 GHz, strong optical transitions, and an outstanding
optical coherence even at temperatures of 8 K, with an upper bound to the
homogeneous linewidth of around 10 kHz. Our study thus introduces a promising
materials platform for the implementation of on-chip quantum memories,
microwave-to-optical conversion, and distributed quantum information
processing.
