The interaction of coherent radiation with matter has been and remains one of the major themes of modern physics. The conventional quantum theory of the laser and its realization in experimental physics and technology is but the most obvious example of such interactions in nonrelativistic systems. One of the active horizons of laser physics continues to be the drive toward producing efficient sources of coherent radiation at ever shorter wavelengths. Indeed the drive to shorter wavelengths has led to the extension of the physics of such interactions to relativistic systems, most notable the free-electron laser (FEL).

The FEL represents one of the few areas of overlap between the physics of relativistic beams and conventional laser theory; between accelerator and laser technology. A second area of overalp has been the use of lasers as one of the means to reduce the emittance to ion beams in low-energy storage rings. In recent experiments in Heidelberg and Aarhus accelerator physicists have produced ion beams with unprecedented, ultra-low longitudinal energy spread. One goal of this work has been to produce space-charge-dominated beams with a high degree of order: i.e. a crystalline structure. Such beams have not yet been produced. At the same time the same techniques have been used by researchers in quantum optics to produce atomic and ionic systems at extremely low temperatures and with a very high degree of order. Both complex crystalline structures and Bose condensates have been seen in the laboratory.

Despite the similarity of both the underlying physics and even the techniques used in the atomic systems and in the relativistc systems, the communication between the research communities pursuing these areas has been minimal. To the end of enhancing this cross-fertilization in the youngest members of our research community we organized the CXXXI Course of the Enrico Fermi School, bringing together as instructors prominent researchers in both areas. The topics of the lectures ranged from the fundamentals of laser trapping and cooling of atoms to free-electron lasers and to the characteristics of storage rings in which ion beams can be confined. Particular attention has been given to recent theory and experimental effects devoted to investigate the relation between FEL physics and recoil-induced gain in two-level systems. We sought for a balance of theory and experiment, emphasizing the fundamental aspects so as to provide a firm basis for further research by the students.

The intellectual atmosphere of the school was greatly enhanced by the beauties of the Villa Monastero in Varenna and the excellent organizational and administrative support provided by the Italian Physical Society. On behalf of all the lecturers and students we wish to express our deepest appreciation to SIF for its support and encouragement. We owe a particular note of thanks to the secretarial staff and to E. Mazzi for her total and generous commitment to the school.

A. Aspect, W. Barletta and R. Bonifacio