  Quantum plasmonics is a rapidly growing field of research that involves the
study of the quantum properties of light and its interaction with matter at the
nanoscale. Here, surface plasmons - electromagnetic excitations coupled to
electron charge density waves on metal-dielectric interfaces or localized on
metallic nanostructures - enable the confinement of light to scales far below
that of conventional optics. In this article we review recent progress in the
experimental and theoretical investigation of the quantum properties of surface
plasmons, their role in controlling light-matter interactions at the quantum
level and potential applications. Quantum plasmonics opens up a new frontier in
the study of the fundamental physics of surface plasmons and the realization of
quantum-controlled devices, including single-photon sources, transistors and
ultra-compact circuitry at the nanoscale.
