The idea that light is composed of discrete particles can be traced to Newton's Opticks (Newton, 1952), in which he introduced the term ‘corpuscles’ to describe what we now call ‘particles’. However, the overwhelming evidence in favor of the wave nature of light led to the abandonment of the corpuscular theory for almost two centuries. It was resurrected—in a new form—by Einstein's 1905 explanation of the photoelectric effect, which reconciled the two views by the assumption that the continuous electromagnetic fields of Maxwell's theory describe the average behavior of individual particles of light. At the same time, the early quantum theory and the principle of wave‐particle duality were introduced into optics by the Einstein equation, E = hν, which relates the energy E of the light corpuscle, the frequency ν of the associated electromagnetic wave, and Planck's constant h.
This combination of ideas marks the birth of the field now called quantum optics. This subject could be defined as the study of all phenomena involving the particulate nature of light in an essential way, but a book covering the entire field in this general sense would be too heavy to carry and certainly beyond our competence. Our more modest aim is to explore the current understanding of the interaction of individual quanta of light—in the range from infrared to ultraviolet wavelengths—with ordinary matter, e.g. atoms, molecules, conduction electrons, etc. Even in this restricted domain, it is not practical to cover everything; therefore, we have concentrated on a set of topics that we believe are likely to provide the basis for future research and applications.
One of the attractive aspects of this field is that it addresses both fundamental issues of quantum physics and some very promising applications. The most striking example is entanglement, which embodies the central mystery of quantum theory and also serves as a resource for communication and computation. This dual character makes the subject potentially interesting to a diverse set of readers, with backgrounds ranging from pure physics to engineering. In our attempt to deal with this situation, we have followed a maxim frequently attributed to Einstein: ‘Everything should be made as simple as possible, but not simpler’ (Calaprice, 2000, p. 314). This injunction, which we will call Einstein's rule, is a variant of Occam's razor: ‘it is vain to do with more what can be done with fewer’ (Russell, 1945, p. 472).
Our own grasp of this subject is largely the result of fruitful interactions with many colleagues over the years, in particular with our students. While these individuals are responsible for a great deal of our understanding, they are in no way to blame for the inevitable shortcomings in our presentation.
With regard to the book itself, we are particularly indebted to Dr Achilles Spe‐liotopoulos, who took on the onerous task of reading a large part of the manuscript, and made many useful suggestions for improvements. We would also like to express our thanks to Sonke Adlung, and the other members of the editorial staff at Oxford (p.viii) University Press, for their support and patience during the rather protracted time spent in writing the book.
J. C. Garrison and R. Y. Chiao