  Defects in solids are in many ways analogous to trapped atoms or molecules.
They can serve as long-lived quantum memories and efficient light-matter
interfaces. As such, they are leading building blocks for long-distance quantum
networks and distributed quantum computers. This chapter describes the
quantum-mechanical coupling between atom-like spin states and light, using the
diamond nitrogen-vacancy (NV) center as a paradigm. We present an overview of
the NV center's electronic structure, derive a general picture of coherent
light-matter interactions, and describe several methods that can be used to
achieve all-optical initialization, quantum-coherent control, and readout of
solid-state spins. These techniques can be readily generalized to other defect
systems, and they serve as the basis for advanced protocols at the heart of
many emerging quantum technologies.
