Optical cavities, such as the Fabry-Perot cavity, are among other things known for its use in lasers to amplify the stimulated emission from atoms. However, when the cavity is made sufficiently small and highly reflective, the theory of quantum electrodynamics (QED) describes how the cavity can also amplify the spontaneous emission of atoms. This can occur when the mode volume of the cavity is of the same size as the wavelength of the light associated with the atomic transition to be enhanced, i.e. V~λ3. Such a small cavity with highly reflective mirrors alters the density of allowed vacuum states, which in turn causes changes to the spontaneous decay. This effect is called Purcell enhancement and can amplify the spontaneous emission of up to four orders of magnitude in rare-earth materials, for realistic cavities.
This strong coupling between an atom and the cavity can be exploited in several ways:
i) the high enhancement causes the radiative lifetime of the atoms to be reduced by several orders of magnitude, greatly increasing the fluorescence.
ii) Since all the enhanced spontaneous fluorescence occurs into the cavity mode rather than in an arbitrary direction, the collection efficiency can be close to unity. Both these two things contribute to enabling single atoms to be detected.
iii) With such a strong coupling, the presence of a single atom in the cavity can shift the cavity resonance, deciding whether incoming photons are reflected or transmitted. Thus, if the atom is in a superposition on a transition in resonance with the cavity, an incoming photon will be put into a superposition of being reflected and not. This allows a single-atom single-photon switch to be built, that can distribute entanglement over large distances as a node in a quantum network.