This chapter begins with a survey of early experimental results — the Planck spectrum, the photoelectric effect, and Compton scattering — that are usually presented as evidence for the existence of photons. The photon concept is indeed sufficient to explain these results, but it is not necessary. There are semi-classical models that explain the same data without introducing photons. The crucial experiment, in which light consisting of a single photon reflects from a beam splitter, depends on the essential indivisibility of photons, and it excludes all semi-classical descriptions of light. This discussion is followed by a preview of modern methods of production and detection of individual photons, e.g., spontaneous down conversion and Silicon avalanche-photodiode counters, together with an introduction to the quantum theory of light based on the correspondence principle.
This chapter begins with a survey of early experimental results — the Planck spectrum, the photoelectric effect, and Compton scattering — that are usually presented as evidence for the existence of photons. The photon concept is indeed sufficient to explain these results, but it is not necessary. There are semi-classical models that explain the same data without introducing photons. The crucial experiment, in which light consisting of a single photon reflects from a beam splitter, depends on the essential indivisibility of photons, and it excludes all semi-classical descriptions of light. This discussion is followed by a preview of modern methods of production and detection of individual photons, e.g., spontaneous down conversion and Silicon avalanche-photodiode counters, together with an introduction to the quantum theory of light based on the correspondence principle.
Experiments with single photons Central to the entire discipline of quantum optics, as should be evident from the preceding chapters, is the concept of the photon. Yet it is perhaps worthwhile to pause and ask: what is the evidence for the existence of photons? Most of us first encounter the photon concept in the context of the photo-electric effect. As we showed in Chapter 5, the photo-electric effect is, in fact, used to indirectly detect the presence of photons – the photo-electrons being the entities counted. But it turns out that some aspects of the photo-electric effect can be explained without introducing the concept of the photon. In fact, one can go quite far with a semiclassical theory in which only the atoms are quantized with the field treated classically. But we hasten to say that, for a satisfactory explanation of all aspects of the photo-electric effect, the field must be quantized. As it happens, the other venerable “proof” of the existence of photons, the Compton effect, can also be explained without quantized fields. In an attempt to obtain quantum effects with light, Taylor, in 1909, obtained interference fringes in an experiment with an extremely weak source of light. His source was a gas flame and the emitted light was attenuated by means of screens made of smoked glass.
This chapter is centered upon the concepts of the quantum nature of light. It begins with a discussion of the photoelectric effect, which provides historical background and a vehicle to discuss the properties of photons. In addition, the photoelectric effect provides the starting point for the explanation of the production of X-rays. After a study of the X-ray spectrum, Compton scattering is introduced as another example of the photon nature of light. As an appendix, the second synthesis opportunity of the volume is presented about lasers. These quantum-electronic devices provide an opportunity to synthesize almost all the topics covered in the volume from the quantum nature of light and matter, to optics, electronics, and thermodynamics. This section is optional, but provides a way to bring the entire text together in the explanation of well-known devices.
Quantum optics is the field of physics describing the interaction of individual photons with matter, but in recent years it has expanded beyond pure physics to become an important driving force for technological innovation. This book starts with an elementary description of the underlying physics and then builds up a more advanced treatment. The theory begins with the quantum description of the simple harmonic oscillator, and is subsequently extended to provide the tools required to discuss coherent states, the interaction of light with atoms, entangled states, quantum noise and dissipation, linear optical amplifiers, and the fundamental issues associated with Bell's theorem. There is an equally strong emphasis on experimental methods. A quantum description of lenses, mirrors, beam splitters, Y-junctions, circulators, and stops is applied to a collection of important experiments in linear optics. A description of the most important methods of primary photon detection is followed by an explanation of heterodyne and homodyne techniques. Spontaneous down conversion and quantum tomography are discussed, together with important experimental applications. These experimental and theoretical considerations come together in a chapter briefly discussing quantum noise and its suppression in telecommunications; the limitations and possibilities for quantum cloning; the principles and techniques of quantum cryptography; and the physical basis for quantum computing.
The idea of the photon has stirred the imaginations of physicists ever since 1905 when Einstein originally proposed the use of light quanta to explain the photoelectric effect. This concept is formalized in the quantum theory of radiation, which has had unfailing success in explaining the interaction of electromagnetic radiation with matter, seemingly limited only by the ability of physicists to perform the indicated calculations. Nevertheless, it has its conceptual problems—various infinities and frequent misinterpretations. Consequently an increasing number of workers are asking, “to what extent is the quantized field really necessary and useful?” In fact the experimental results of the photoelectric effect were explained by G. Wentzel in 1927 without the quantum theory of radiation. Similarly most electro-optic phenomena such as stimulated emission, reaction of the emitted field on the emitting atom, resonance fluorescence, and so on, do not require the quantization of the field for their explanation. As we will see, these processes can all be quantitatively explained and physically understood in terms of the semiclassical theory of the matter–field interaction in which the electric field is treated classically while the atoms obey the laws of quantum mechanics. The quantized field is fundamentally required for accurate descriptions of certain processes involving fluctuations in the electromagnetic field: for example, spontaneous emission, the Lamb shift, the anomalous magnetic moment of the electron, and certain aspects of blackbody radiation. (The Compton effect also fits here, but see later under references 8b and c.) Here we will outline how the photon concept originated and developed, where it is not required and is often misused, and finally where it plays an essential role in the understanding of physical phenomena. In our discussion we will attempt to give a logically consistent definition of the word “photon”—a statement far more necessary than one might think, for so many contradictory uses exist of this elusive beast. In particular consider the original coining of the word by G. N. Lewis: “[because it appears to spend] only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains an important structural element within the atom…, I therefore take the liberty of proposing for this hypothetical new atom which is not light but plays an essential part in every process of radiation, the name photon!” (our exclamation point). Clearly the present usage of the word is very different.
While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0)=0.0177±0.0026, which violates the classical inequality g(2)(0)⩾1 by 377 standard deviations.
Light occupies a special position in our attempts to understand nature both classically and quantum mechanically. We recall that Newton, who made so many fundamental contributions to optics, championed a particle description of light and was not favorably disposed to the wave picture of light. However, the beautiful unification of electricity and magnetism achieved by Maxwell clearly showed that light was properly understood as the wave-like undulations of electric and magnetic fields propagating through space. The central role of light in marking the frontiers of physics continues on into the twentieth century with the ultraviolet catastrophe associated with black-body radiation on the one hand and the photoelectric effect on the other. Indeed, it was here that the era of quantum mechanics was initiated with Planck's introduction of the quantum of action that was necessary to explain the black-body radiation spectrum. The extension of these ideas led Einstein to explain the photoelectric effect, and to introduce the photon concept. It was, however, left to Dirac to combine the wave-and particlelike aspects of light so that the radiation field is capable of explaining all interference phenomena and yet shows the excitation of a specific atom located along a wave front absorbing one photon of energy. In this chapter, following Dirac, we associate each mode of the radiation field with a quantized simple harmonic oscillator, this is the essence of the quantum theory of radiation.