In studying the radiation-matter interaction, one can take two different approaches. The first is typical of spectroscopy: one considers the interaction between radi- ation and a single atom, i. e., one studies those phenomena in which the presence of other atoms is irrelevant. The other attitude consists, in contrast, in studying those phenomena which arise just from the simultaneous presence of many atoms. In fact, all the atoms interact with the same electromagnetic field; under suitable conditions, this situation creates strong atom-atom correlations, which in turn give rise to a...

This volume contains the lectures and seminars presented at the NATO Advanced Study Institute on "Coherence in Spectroscopy and Modern Physics," the seventh course of the International School of Quantum Electronics, affiliated with the "Ettore Majorana" Centre for Scientific Culture, Erice, Sicily. The Institute was held at Villa LePianore (Lucca), Versilia, Italy, July 17-30, 1977. The International School of Quantum Electronics was started in 1970 with the aim of providing instruction for young researchers and advanced students already engaged in the area of quantum electronics or wishing...