Filled with optical fibre and joy

Optical Coherence and Quantum Optics
September 29, 1995

Optics is an old field, rich in phenomena and with a wide impact on pure and applied science. On the practical side, we have a subject everyone depends on in everyday life, in the form of spectacles, cameras, laser scanners at supermarkets and even endoscopes used to image the inside of the body. This range of applications has been growing rapidly since the invention of the laser. Optics are used in communications in the fibre-optic links of the information superhighway. When a mistake was made in the design of the optics in the space telescope it cost billions of dollars to correct.

So much for the practical aspects, what about fundamentals? Are these all worked out? Many of the basic theories were developed a good while ago. In particular, the link between optics and electricity and magnetism was made by James Clerk Maxwell in the last century.

The theory of optical phenomena, especially in the areas addressed in this book, is the subject of intense research activity. Trying to separate practical issues from fundamental ones in optics is a process full of pitfalls. I have been amazed in my career to see arcane issues in quantum optics - one of the main areas treated in this book - become an issue for banks, security agencies and telephone companies. Optics also continues to be fruitful ground for the study of issues in the theories of radiation and matter.

Many scientists need a working knowledge of the basics of optics for their research because so many pieces of apparatus have optical elements in them. This means that an optics course is part of most engineers' and scientists' training, as indeed it was of mine. As a student, the fact that one could start with the famous equations of Maxwell and predict what one would see in a simple experiment was a great joy. I was attracted by the fact that the effects of the wavelike nature of light can be seen with very simple laboratory apparatus. This is an important reason for experiments in optics still being a favourite of undergraduates - at least it is at Oxford. As an undergraduate the book that struck me as being truly authoritative, and still does today, is the one by Max Born and Emil Wolf. Though comprehensive and therefore long, it is relatively easy to enter at any point. It is also authoritative, setting out the "canon" of classical optics.

Optics is, however, a rapidly changing subject, particularly in the areas treated by Leonard Mandel and Emil Wolf in their book. This describes the important material that has come out of the subject in the areas of coherence and quantum optics - which is a great deal. It is therefore, like Born and Wolf's, a long text, and also a superb monograph in the same spirit. For a researcher it takes one from the edges of the field into the deepest jungles. The scholarship of the authors is truly impressive. The technical issues are presented in a balanced and clear way and the discussion is easily accessible.

The book deals with the basic areas of the statistical and quantum theory of light. The statistical issues arise because light is often produced in what could be called impure, or partially coherent, forms. Even if one starts with such pure light it does not remain in that form for very long if it interacts with matter. The description of coherence in its various forms, and the way it evolves as light moves around, is a very complex and important matter. Mandel and Wolf have played a leading role in the elucidation of this theory for many years and so are well placed to present this masterly account of the field.

The authors are strong on the mathematical techniques needed to handle the physics they treat. A good example is their exposition of probability and stochastic processes. The mathematics here is essential in the description of physical phenomena that depend on the coherence of the light used, a good example being holography.

The other important area addressed concerns the quantum nature of the radiation field. In classical optics light is thought of in straightforward terms, with its constituent waves similar to waves on a pond. The quantum theory of matter has, however, taught us that the energy in a system must come in lumps - photons - and that light cannot be like a simple classical wave. One of the mysteries of quantum theory is that these photons can exhibit both wave- and particle-like properties, depending on which phenomena we look at.

In fact, the quantum nature of light was invoked very early on in the history of quantum theory to explain the photoelectric effect. A classical theory of light is sufficient to explain very many phenomena, but the quantum theory is essential for many of the most interesting. Quantum optics has become such an important field that there is a community of scientists who call themselves quantum opticians. One system where quantum theory is essential is the laser. Simpler theories can handle some aspects of laser behaviour but quantum theory is needed to account for it all. The book describes how in recent years it has been possible to examine the quantised nature of the radiation field in increasing detail. This has led to insight not only into optics but also into the nature of the behaviour and response of more general quantum systems.

Quantum optics has also become one of the main areas in which the fundamental issues of quantum mechanics are tested. The latter set the intrinsic limit to what can be achieved in experiments on microscopic systems. These advances have in great part come through the development of new experimental techniques and this brings me to a truly wonderful aspect of this book. This is the fact that experiments are given a prominent place in the exposition of the physics. Many texts present theory as though it came from the wind. While physical theory does indeed come from the human imagination, it is driven by the need to understand real things, not imaginary ones. Mandel and Wolf show very clearly the critical role of experiment in driving, motivating and confronting theory.

Overall, they have given a very thorough and balanced guide to the highways and some of the byways of the subject. In doing so they have done a great service to the community in which they work, and have constructed something of lasting value.

Keith Burnett is a lecturer in physics and fellow, St John's College, Oxford.

Optical Coherence and Quantum Optics

Author - Leonard Mandel and Emil Wolf
ISBN - 0 521 41711 2
Publisher - Cambridge University Press
Price - £30.00
Pages - 1,139

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