The link between light, matter, and vacuum has been a subject of discussion since antiquity. The conceptual revolution that started in the 1920s with quantum mechanics and led to the development of quantum electrodynamics (QED) changed our understanding of light–matter interactions—particularly the role of vacuum, the space through which light propagates.
In the 1970s a series of milestone experiments by atomic physicists showed that optical cavities could fundamentally modify the spontaneous emission of photons from excited atoms by either enhancing or suppressing it. The effect had already been predicted by Edward Purcell in 1947 and is best understood by considering that a cavity modifies the density of optical states of vacuum. Atoms can emit light only into available optical states. In a cavity the density of those states is altered, and therefore so is the probability of photon emission. Such a system can even enter a regime in which a...
