From experiments with single atoms to sources of quantum light

When quantum physics was developed, it was absolutely inconceivable that quantum processes of single atoms could ever be observed. Discussion of the subject was thus always confined to gedanken experiments and quantum mechanics simply made do with the conception that probabilities can only be stated for an ensemble. In recent years, however, the methods used in laser spectroscopy have made it possible to observe single free atoms and also single molecules in solids and, in particular, to trace their quantum behaviour. The ability to manipulate the vacuum field that comes into play in quantisation of the radiation field has afforded further interesting experiments. It is thus possible nowadays to trace the quantum processes and their external influence in single atoms and make phenomena visible that to not occur in the observation of several atoms or that would be averaged out. The fact that the vacuum field can be manipulated has yielded a new field of research named resonator quantum electrodynamics. The experiments conducted at MPQ in this area laid the foundation. They have led to the observation of a series of new results, which are summarised int he following