One Hundred Years of Light Quanta

It can’t have escaped you, after so many recent reminders, that this year marks the one hundredth birthday of the light quantum. I thought I would tell you this morning a few things about its century long biography. Of course we have had light quanta on Earth for eons, in fact ever since the good Lord said “let there be quantum electrodynamics”—which is a modern translation, of course, from the biblical Aramaic. So in this talk I’ll try to tell you what quantum optics is about, but there will hardly be enough time to tell you of the many new directions in which it has led us. Several of those are directions that we would scarcely have anticipated as all of this work started. My own involvement in this subject began somewhere around the middle of the last century, but I would like to describe some of the background of the scene I entered at that point as a student. Let’s begin, for a moment, even before the quantum theory was set in motion by Planck. It is important to recall some of the remarkable things that were found in the 19th century, thanks principally to the work of Thomas Young and Augustin Fresnel. They established within the first 20 years of the 19th century that light is a wave phenomenon, and that these waves, of whatever sort they might be, interpenetrate one another like waves on the surface of a pond. The wave displacements, in other words, add up algebraically. That’s called superposition, and it was found thus that if you have two waves that remain lastingly in step with one another, they can add up constructively, and thereby reinforce one another in some places, or they can even oscillate oppositely to one another, and thereby cancel one another out locally. That would be what we call destructive interference. Interference phenomena were very well understood by about 1820. On the other hand, it wasn’t at all understood what made up the underlying waves until the fundamental laws of electricity and magnetism were gathered together and augmented in a remarkable way by James Clerk Maxwell, who developed thereby the electrodynamics we know today. Maxwell’s theory showed that light waves consist of oscillating electric and magnetic fields. The theory has been so perfect in describing the dynamics of electricity and magnetism over laboratory scale distances, that it has remained precisely intact. It has needed no fundamental additions in the years since the 1860s, apart from those concerning the quantum theory. It serves still, in fact, as the basis for the discussion and analysis of virtually all the optical instrumentation we have ever developed. That overwhelming and continuing success may eventually have led to a certain complacency. It seemed to imply that the field of optics, by the middle of the 20th century, scarcely needed to take any notice of the granular nature of light. Studying the behavior of light quanta was then left to the atomic and elementary particle physicists—whose interests were largely directed toward other phenomena. The story of the quantum theory, of course, really begins with Max Planck. Planck in 1900 was confronted with many measurements of the spectral distribution of the thermal radiation that is given off by a hot object. It was known that under ideally defined conditions, that is, for complete or black absorbers and correspondingly perfect emitters this is a unique radiation distribution. The intensities of its color distribution, under such ideal conditions, should depend only on temperature and not at all on the character of the materials that are doing the radiating. That defines the so-called blackbody distribution. Planck, following others, tried finding a formula that expresses the shape of that blackbody color spectrum. Something of its shape was known at low frequencies, and there was a good guess present for its shape at high frequencies. The remarkable thing that Planck did first was simply to devise an empirical formula that interpolates between those two extremes. It was a relatively simple formula and it involved one constant which he had to adjust in order to fit the data at a single temperature. Then having done that, he found his formula worked at other temperatures. He presented the formula to the German Physical Society on October 19, 1900 Planck, 1900a and it turned out to be successful in describing still newer data. Within a few weeks the formula seemed to be established as a uniquely correct expression for the spectral distribution of thermal radiation. The next question obviously was: Did this formula have a logical derivation of any sort? There Planck, who was a sophisticated theorist, ran into a bit of trouble. First of all he understood from his thermodynamic background that he could base his discussion on nearly any model of matter, however oversimplified it might be, as long as it absorbed and emitted light efficiently. So he based his model on the mechanical system he understood best, a collection of one-dimensional harmonic os*The 2005 Nobel Prize for Physics was shared by Roy J. Glauber, John L. Hall, and Theodor W. Hänsch. This lecture is the text of Dr. Glauber’s address on the occasion of the award. REVIEWS OF MODERN PHYSICS, VOLUME 78, OCTOBER–DECEMBER 2006