Introduction

Abstract

The theory of open systems has been a theme in quantum optics since the birth of the subject some thirty years ago. The principal reason for this is that quantum optics was formed as a discipline around the invention of a new source of light — the laser. Sources of light are open systems. Thus, those working on the quantum theory of the laser found that they needed a way to treat dissipation in a quantum mechanical way [1]. The central ideas of a dissipative process axe embodied in Fermi’s golden rule and were used in quantum mechanics for many years before the invention of the laser. A complete theory of the laser needed more than this, however. Fermi’s golden rule gives us a picture of quantum dynamics expressed in terms of transitions between discrete states. Needless to say, the fundamental equations of quantum mechanics axe not expressed in these terms; they describe the continuous evolution of complex amplitudes, the coefficients in a superposition of states. With its emphasis on coherence, laser dynamics involves the complex amplitudes; to understand the properties of laser light something more than a calculation of rates for the incoherent emission and absorption of photons is required; the quantum theory of the laser must be formulated in a way that reveals the roles of both coherence and incoherence in open system dynamics.