An optical cavity is an arrangement of optical elements (such as mirrors) that allows to confine photons inside the thereby formed resonator. This tight confinement of the electromagnetic modes of the resonator makes it possible to enhance their coupling to other quantum systems (like atoms or ions) placed inside the cavity.

Cavity quantum electrodynamics (cavity-QED) opens up regimes of such light-matter interactions that reveal the quantum nature of the optical field. The strong coupling of photons to atoms or ions can, for example, be used to store optical quantum information, to produce quantum states of light or to generate large optical nonlinearities that act like effective interactions between individual photons.