Light can often be understood as a propagating wave, following the laws of classical electrodynamics. In some instances, however, the quantum mechanical nature of light takes centre stage and reveals its composition from individual light quanta — photons.

The field of quantum optics explores the quantum mechanical properties of photons and their interactions with matter. The ability to create and control such quantum states of light opens up many applications from ultra-precise sensing, and quantum information processing to quantum-secure communication. While many such applications are based on the strong interaction of light with a single quantum system (e.g. an atom or an ion), the optical coupling to controllable quantum many-body systems can also give rise to fascinating collective phenomena, that include the generation of strong effective interactions between single photons and the emergence of exotic matter-like quantum phases composed of light.