  Non-classical concerns light whose properties cannot be explained by
classical electrodynamics and which requires invoking quantum principles to be
understood. Its existence is a direct consequence of field quantization; its
study is a source of our understanding of many quantum phenomena. Non-classical
light also has properties that may be of technological significance. We start
this chapter by discussing the definition of non-classical light and basic
examples. Then some of the most prominent applications of non-classical light
are reviewed. After that, as the principal part of our discourse, we review the
most common sources of non-classical light. We will find them surprisingly
diverse, including physical systems of various sizes and complexity, ranging
from single atoms to optical crystals and to semiconductor lasers. Putting all
these dissimilar optical devices in the common perspective we attempt to
establish a trend in the field and to foresee the new cross-disciplinary
approaches and techniques of generating non-classical light.
