Single-Photon Sources and Detectors Book: Chapter 1: Introduction

In the beginning there was light. And it was good. Not long thereafter people began to look for a comprehensive understanding of its nature. While the publication record starts off a little spotty, in the fifth century BC the Greek philosopher Empedocles concluded that light consists of rays that emanate from the eye. This point of view was questioned using what today might be characterized as a local realism argument by Euclid in his classic text on light propagation Optica. Euclid hypothesized that rays of light are emitted by external sources. But it was not until Ibn al-Haytham in 1000 AD that this view was put on a scientific footing. The character of light itself was described by Descartes in the 17th century as “pressure” that was transmitted through space from a source to an eye (a detector). This idea was later developed by Huygens and Hooke into the wave theory of light. At about the same time, Gassendi put forward the contravening notion that light was a particle, an idea embraced and developed further by Newton. The differing perspectives of light as a particle versus a wave were generally considered resolved in favor of the wave picture by Young’s double-slit experiment in 1803, and by Fresnel’s experiments in diffraction. In the 1860s, further confirmation of this conclusion was framed in an elegant and deeply satisfying manner by the Maxwell equations: the prediction of polarized electromagnetic waves that propagate at what was understood to be the speed of light. Problems with the waves-and-fluids view of electromagnetism arose in 1897, when J.J. Thomson discovered discrete particles carrying negative electric charge moving through vacuum. Then in 1900, in “an act of despair” Planck invoked quantized bundles of electromagnetic energy in the derivation of the blackbody radiation law [2, 3], a step that not only embraced prior conjectures by Boltzmann in statistical mechanics, but also flew in the face of conventional understanding. It was originally considered an artifact of the derivation to be corrected later, but Einstein took the theory of light quanta more seriously in his 1905 description of the photoelectric effect [4]. Then in 1913 Bohr invoked the quantization of both energy and angular momentum to explain the discrete spectral emission lines observed in the Hydrogen-Balmer series. The wheels came completely off the wagon in 1924 when de Broglie built on these ideas to hypothesize that not only light, but also particles of matter have wave-like properties. A flurry of subsequent discovery and advance that established the framework of quantum mechanics, most notably by Heisenberg, Born, Schrodinger, Pauli, and Dirac, culminated, for the purposes of this book, in 1927 when Dirac quantized the electromagnetic field, effectively developing a theory of light that encompassed the physical phenomenon that kicked off the entire revolution in the first place. The first direct detection of light at the single-photon level was achieved in the 1930s. The advent of the atomic-cascade photon-pair source [5] in the 1950s and their use in the 1970s and 1980s [6–9] brought the first single-photon sources to light. Then there was quantum light, and it was really good.