Optical coherence and quantum optics, by Leonard Mandel and Emil Wolf

Indianara Lima Silva, Olival Freire

The history of the concept of the photon in twentieth-century physics is far from a simple story opening with Einstein’s vision of light as a collection of indivisible particles whose energy and momentum are conserved during its interaction with matter, and reaching closure with the wave-particle duality as an accomplishment of quantum mechanics. Since then there has been an intermittent debate on the need for and adequacy of such a concept, even if this debate has been absent from the literature on the history of physics and from physics teaching. This paper analyzes a major event which led to the revival of this debate, namely, the experiment carried out by Robert Hanbury Brown and Richard Quentin Twiss (HBT) in 1956 in the context of low-intensity interferometry. As part of their work to build a new kind of interferometer to measure the diameter of optical stars, their results seemed to suggest that photons split through two different channels and detectors. These results stirred up a debate involving Edward Purcell, Eric Brannen, Harry Ferguson, Peter Fellgett, Richard Sillitto, Lajos Jánossy, Leonard Mandel, and Emil Wolf, in addition to Hanbury Brown and Twiss themselves. The building of this device in astronomy thus renewed the old controversy about the nature of light. Later on, with the invention of lasers, the HBT experimental results played a role in developments leading to the creation of quantum optics and currently play a role in various fields in physics.

Chris J. Bednar

Harry J. Kimble

I thank Rodger Walser and his colleagues for organizing this conference to celebrate George \nSudarshan’s birthday and landmark achievements in science. It is an honor for me to be here and \nalso a great pleasure to be back in Austin to join the chorus of the FOGS. \nI would like begin with a brief historical introduction and then turn to topics that we \nhaven’t heard so much about thus far in the symposium, namely experiments in Quantum Optics. \nLet me remind you that blackbody radiation has had a profound impact on physics since the time \nof Planck. However, even in more modern times after the development of quantum mechanics \nand quantum electrodynamics, one really did not know how to describe the underlying field \nfluctuations as required for diverse interference phenomena. Emil Wolf laid critical foundations \nfor this enterprise with his theory of classical coherence developed in collaboration with Max \nBorn in their classic book Principles of Optics. \nA great challenge to the physics community was the experiment by R. H. Brown and R. \nQ. Twiss in 1956 [1], which led to a confusing and controversial time for photons and fields. In \n1958 Leonard Mandel helped to resolve the controversy by explaining how light fluctuations are \nconverted to photocurrents in the detection process, albeit in with a classical description of the \nfield [2]. The confusion arose from the effort to understand, on the one hand, interference \nphenomena that are expressed in terms of the complex amplitude of the electromagnetic field, for \nwhich only a classical description was available. On the other hand, quantum electrodynamics \nhad been developed based upon energy eigenstates (i.e., Fock states) with photons as quanta. It \nwas really difficult to understand how the electric and magnetic fields behave in terms of these \nquantum states. For example, the expectation value of any electromagnetic field is zero in a Fock \nstate. The confusion was compounded by the historic invention of the maser and laser and by the \npressing need for quantum theories of the dynamical processes of these and other laboratory \nadvances.

Richard M. Sillitto

T. Stewart McKechnie

Helmut Rauch, Samuel A. Werner

Abstract Coherence phenomena play an important role in any kind of interferometry (Gabor 1956; Glauber 1963; Mandel and Wolf 1965, 1995; Born and Wolf 1975; Walls and Milburn 1994; Mandel and Wolf 1995). In this chapter we summarize some known results, add some new ones obtained with neutrons, and analyse them in terms of general quantum optics, which can be applied to photon and matter waves as well. Indeed, neutrons have many well-known particle properties, but in interference experiments they behave like wave fields, which provides the connection to the quantum optical terminology. Coherence appears as a system property (neutron + apparatus) which persists as long as it does not become destroyed by statistical or dissipative effects.