## Description

Collecting a series of reprints and publishing them in the form of a book may seem a little pretentious. Actually, when this was suggested to me by World Scientific, my response was first negative. The first thought which caused me to change my mind came when I surveyed my own bookcase in which were contained a few sets of reprints bound together given by colleagues working in similar fields; I find them quite convenient. When I need to consult one particular paper, I find it immediately without having to spend hours looking for it in piles of reprints and preprints, most of the time without success.

Also, when I started to think about how I could organise the chapters of such a book, I found it worthwhile to look back over my research field and to try to find a few guidelines for understanding its evolution. During the last forty years, the field of atomic physics has experienced a number of spectacular developments, but a close inspection shows that the atom-photon interactions at the heart of these developments can always be analyzed with a small number of general ideas. Conservation laws are an obvious example. Conservation of angular momentum plays an essential role in optical pumping, whereas laser cooling is based on the conservation of linear momentum. A more subtle example is the influence of the correlation time of the electromagnetic field on the atomic dynamics. If this correlation time is very short compared to all the other characteristic times, which is the case of the vacuum field or of the broadband fields emitted by thermal sources, the atomic evolution can be described by rate equations or, equivalently, by a random sequence of quantum jumps associated with the absorption or emission processes. On the contrary, if the field correlation time is very long, the atomic evolution is described by optical Bloch equations or, equivalently, by a Rabi nutation between the two states of the atomic transition driven by the applied field. This explains why so many concepts developed for magnetic resonance turned out to be useful for the analysis of experiments performed with monochromatic laser sources as well. Another example of a useful guideline is the distinction between the reactive and the dissipative responses of an atom to electromagnetic excitation. For the internal degrees of freedom, they give rise to a shift and to a broadening of the atomic levels, respectively. This is true not only for the interaction with the vacuum field (Lamb shift and natural width), but also for the interaction with an applied field (light shift and power broadening). For external degrees of freedom, one also finds a reactive force (dipole force) and a dissipative force (radiation pressure force). One of the most recent developments in laser cooling, the so-called Sisyphus cooling, can also be interpreted as resulting from a correlation between the spatial modulations of the two responses of the atom to a laser excitation with a spatially modulated polarization. More precisely, the light shifts of two Zeeman sublevels (reactive response) and the optical pumping rates from one sublevel to the other (dissipative response) are spatially modulated and correlated in such a way that the moving atom is running up the potential hills more often than down. As a final example, one may mention the importance of linear superpositions of atomic sublevels. They give rise to well-known effects, such as level crossing resonances (Hanle effect), and they also play an essential role in the new laser cooling mechanisms, such as velocity selective coherent population trapping, allowing one to cool atoms below the recoil limit associated with the kinetic energy of an atom absorbing or emitting a single photon.

The reprints I have selected here consist of review papers and lectures given at international conferences or summer schools, as well as original theoretical or experimental papers. Some of them are not easily available and I hope it will be useful to find them in this book. They all deal with the physical effects which can be observed on atoms interacting with various types of electromagnetic fields (broadband fields, radiofrequency fields, laser fields, vacuum fields, etc.). The problems which are addressed in these papers concern not only the effect of the electromagnetic field on atoms, i.e. the dynamics of their internal degrees of freedom and the motion of their center of mass, but also the new features of the light which is absorbed or emitted by these atoms, such as the spectral distribution of the fluorescence light and photon correlations. I have tried to select papers which put emphasis on the physical mechanisms and general approaches, such as the dressed-atom approach, having a wide range of applications. I thus hope that they could be useful to a wide audience, and not only to specialists.

A short introduction has been written for each paper. It gives the historical context of the paper, explains how it fits into the general evolution of the research field, and points out connections with other ideas or other work done at different periods. A few references are given in these introductory notes, but they are not intended to be exhaustive.

I am very much indebted to Alfred Kastler and Jean Brossel for their constant interest in my work. They supervised my thesis and initiated me to this branch of atomic physics which has so greatly benefited from their inspiration. I would also like to express my gratitude to all the coauthors of the papers presented in this volume. Their contribution has been essential and the work described here could not have been done without their enthusiasm.

All the editors who have been contacted have given permission to reprint the papers for which I would like to thank them. I am also very grateful to Jean Dalibard, Jacques Dupont-Roc, and John Lawall for their help in the preparation of the introductory remarks and to Michele Sanchez and Beatrice Cardon for the typing of these notes.

In this second edition, 14 papers written after the publication of the first edition have been added. In order to keep a reasonable size for the book, paper 1.1 has been removed. The interested reader can find it in the first edition of the book.