  Quantum metamaterials generalize the concept of metamaterials (artificial
optical media) to the case when their optical properties are determined by the
interplay of quantum effects in the constituent 'artificial atoms' with the
electromagnetic field modes in the system. The theoretical investigation of
these structures demonstrated that a number of new effects (such as quantum
birefringence, strongly nonclassical states of light, etc) are to be expected,
prompting the efforts on their fabrication and experimental investigation. Here
we provide a summary of the principal features of quantum metamaterials and
review the current state of research in this quickly developing field, which
bridges quantum optics, quantum condensed matter theory and quantum information
processing.
