Quantum Optics is a branch of physics which has developed recently in different directions relevant to fundamental physics as well as to highly sophisticated technological applications. The scientific roots of quantum optics, however, originate from the broader subject of Quantum Electrodynamics and, more generally, from quantum field theory. Thus the boundary between quantum optics and quantum field theory is a particularly delicate conceptual ground which should be properly mastered by any prospective quantum optician, theorist or experimentalist alike. This book is intended to foster understanding and knowledge of this boundary region by presenting in a pedagogical fashion the basic theory of dressed atoms, which has been established as a concept of central importance in quantum optics, since it is capable of shedding light on such diverse physical phenomena as resonance fluorescence, the Lamb shift and van der Waals forces.

Coherently with the aims outlined above, the first part of this book, consisting of the first four chapters, is dedicated to the foundations of atom-field interactions. Both radiation and matter are treated from the quantum field theory point of view, and the coupling between matter and the electromagnetic field is derived using the principle of gauge invariance. The atom-photon Hamiltonian is obtained by specializing this general treatment to a nonrelativistic electron field describing the electrons around an atomic nucleus.