Advances in technology often rely on a world of photons as the basic units of light. Increasingly one reads of photons as essential to enterprises in Photonics and Quantum Technology, with career and investment opportunities. Notions of photons have evolved from the energy-packet crowds of Planck and Einstein, the later field modes of Dirac, the seeming conflict of wave and particle photons, to the ubiquitous laser photons of today.
Advances in technology often rely on a world of photons as the basic units of light. Increasingly one reads of photons as essential to enterprises in Photonics and Quantum Technology, with career and investment opportunities. Notions of photons have evolved from the energy-packet crowds of Planck and Einstein, the later field modes of Dirac, the seeming conflict of wave and particle photons, to the ubiquitous laser photons of today.
This book describes the changing views of the physics community toward photons, and how photons are viewed today in several contexts. The first portion, a ninechapter Memoir with few equations and many definitions, explains the changing view of physicists toward radiation and its wave-particle photons, written for those with interest but possibly without technical background. It gives operational definitions that have been used for photons and their association with quantum-state manipulations that include Quantum Information, astronomical sources and crowds of photons, the boxed fields of cavity Quantum Electrodynamics It defines, qualitatively, the historical photons of Planck, Einstein, Compton, and Bohr, the later photons of Dirac, Feynman, and Glauber, and the photon constituents of the Standard Model of Particle Physics. It points to contemporary photons as causers of change to atoms, as carriers of messages, and as subject to controllable creation and alteration. A second portion, of three tutorial appendices, explains the mathematical background of quantum theory and radiation needed by those whose profession involves photonics and who therefore want more detailed understanding of the Memoir portion: quantum theory and the Schrodinger equation for quantum-state manipulation; Maxwell equations for electromagnetism with wave modes that become photons through a quantization postulate, possibly exhibiting quantum entanglement; and the coupling of atoms and fields to create quasiparticles that are seen as slow and stored light pulses. As with other Memoirs, the present book has idiosyncrasies of the author. Most notably, on the opening page of each chapter, and at the end of the book, is a cartoon drawn by the author, as a grad student, that reflects the changing views of a PhD aspirant toward the grad school experience as he progressed through the graduate school of MIT in the 1950s.
It was the photon that started the world of quantum in 1905, when Einstein took seriously a concept introduced by Planck and such proposed the laser. Soon, photons seen as exchange particles for the electromagnetic force triggered QED and Feynman to postulate quantum machines. Discovery of photon’s quantum statistical properties led to the understanding and practical technical use of quantum correlations, entanglement and quantum coherence. The photon is the most accessible handle for quantum information. It is used to set and read-out qubits, quantum sensors, etc. It is the only carrier to allow large distance transport of quantum information. Tamed photons are the key enabling tool for Quantum 2.0 and today’s laser are an incarnation thereof, as basis for QT applications we can only start to anticipate.
Some two centuries before the quantum revolution, Newton (1) suggested that corpuscles of light generate waves in an aethereal medium like skipping stones generate waves in water, with their motion then being affected by these aether waves. Times have changed. Light corpuscles are now known as photons, and the majority of physicists have dispensed with the notion of aether. Nevertheless, certain features of Newton's metaphor live on in one particular version of quantum mechanics. According to pilot wave theory, first proposed by de Broglie (2) and later developed by Bohm (3) with Einstein's encouragement, microscopic elements such as photons and electrons consist of both particle and wave, the former being guided by the latter. Although this physical picture has not been widely accepted, it has had some notable proponents, including Bell (4). Its principal appeal is that it restores realism and determinism to quantum mechanics, its weakness that the physical nature of the guiding wave field remains unclear. At the time that pilot wave theory was developed and then overtaken by the Copenhagen interpretation as the standard view of quantum mechanics, there was no macroscopic pilot wave analog to draw upon. Now there is.
In 2005 we celebrate the hundredth anniversary of the publication of five seminal papers by Albert Einstein establishing the basis of three fundamental fields of physics: the theory of relativity, quantum theory and the theory of Brownian motion. This happened at a time when the reality of atoms and molecules was still far from certain. The microscopic view Einstein took of molecular motion led to the calculation of Avogadro's number and the size of molecules by studying the motion of small particles in liquids. Combining kinetic theory and classical thermodynamics finally led Einstein to the conclusion that the displacement of a microparticle under Brownian motion varies as the square root of time. The experimental demonstration of this law three years later was considered as the striking proof that atoms and molecules are physically real. Today single atoms are probed directly in experiments, and we are able to observe the individual paths of colliding or dissociating particles. These new techniques are described in some of the papers of this issue in a very impressive way. An even more exciting development was initiated with the demonstration of Bose–Einstein condensation of dilute gases of ultracold atoms, a phenomenon first predicted by Einstein in papers published between 1916 and 1924. The first paper of Einstein's famous series of 1905, 'On an heuristic point of view about the creation and conversion of light', was an explanation of the photoelectric effect, demonstrated in several experiments over the previous few years. The explanation was based on the quantum hypothesis introduced by Max Planck five years earlier, and was considered as an additional and important proof of Planck's hypothesis. Today quantum theory and photons are the basis for much of modern science and technology. We have learned to experiment with single photons, and we have demonstrated the advantages of information transfer by single photons. Photons permit new and incredibly precise time and frequency standards. These are not only technically important, but may also lead to even more stringent tests of relativity and other fundamental laws and concepts of physics. Examples of these applications and many other uses of modern optics are discussed in this issue. Special relativity, the third of Einstein's 1905 topics, has always been closely connected with atomic, molecular and optical physics. Today atomic physics provides some of the most stringent tests of special relativity. Furthermore, fast electron beams are used in advanced light sources such as synchrotron radiation generators and free electron lasers. Those aspects and related applications are also discussed here. We hope that this special issue will be of great interest to the reader by highlighting recent advances in atomic, molecular and optical physics. This field continues to provide one of the most fertile areas for research one hundred years after it first emerged from the work of Einstein published in 1905, his 'miraculous year'.
We are told that our present understanding of physical law was ushered in by the Quantum Revolution, which began around 1900 and was brought to fruition around 1930 with the formulation of modern Quantum Mechanics. The photon" was supposed to be the centerpiece of this revolution, conveying much of its conceptual avor. What happened during that period was a rather violent redirection of the prevailing world view in and around physics|a process that has still not settled. In this paper I critically review the evolution of the concepts involved, from the time of Maxwell up to the present day. At any given time, discussions in and around any given topic take place using a language that presupposes a world view or zeitgeist. The world view itself limits what ideas are expressible. We are all prisoners of the language we have created to develop our understanding to its present state. Thus the very concepts and ways of thinking that have led to progress in the past are often the source of blind spots that prevent progress into the future. The most insidious property of the world view at any point in time is that it involves assumptions that are not stated. In what follows we will have a number of occasions to point out the assumptions in the current world view, and to develop a new world view based on a quite di erent set of assumptions.
A photon-like wavepacket based on novel solutions of Maxwell's equations is proposed. It is believed to be the first 'classical' model that contains so many of the accepted quantum features. In this new work, novel solutions to Maxwell's classical equations in dispersive guides are considered where local helical twists with an arbitrary angular frequency W modulate a classical mode (angular frequency w, group velocity vg). The modal field patterns are unchanged, apart from the twist, provided that the helical velocity vh equals vg. Pairs of resonating retarded and advanced waves with modal and helical frequencies (w,W) and (w,-W)respectively, trap one temporal period of the underlying classical mode forming a photon-like packet provided W = (M+1/2)w: 'Schrodinger' frequencies. This theory supports experimental evidence that the photon velocity does not change with M in dispersive systems. Promotion and demotion increase or decrease the helical frequencies in units of w. An energy of interaction between retarded and advanced waves in the wave-packet is also proportional to these helical frequencies W = (M+1/2)w similar to Planck's law. Group velocity and polarisation are unaffected by the value of M. Advanced waves enable phase and polarisation to be predicted along all future paths and may help to explain the outcomes of experiments on delayed-choice interference and entanglement, without causality being violated.