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.
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.
The development of quantum mechanics and quantum physics in general is one of the greatest intellectual achievements of the 20th century. Initially, quantum physicists focused on mathematically describing and understanding the laws of a micro-world, comprised of atoms, molecules and photons. Nuclei and sub-atomic particles were discovered, particle physics evolved, and complex quantum systems in chemistry and solid-state physics are now described in terms of quantum physics. Within a century, quantum physics became the framework, which allows us to describe and predict, albeit with probabilities, the behavior of the microscopic world as we know it. The powerful quantum framework led to revolutionary technical advances for a century, such as semiconductor technology and the entire electronics industry based on it, enabling computers and the information age. The development of lasers found applications in many areas of our current technologies; we use them for printers, storing and retrieving information using CDs, industrial welding, medical surgery, and for range finding and communication purposes. Medical imaging using MRI is now a routine instrument, as are precision measurements using tunneling and atomic force microscopes. Superconducting technologies are used for high magnetic fields and many more current technologies are based on and enabled by the laws of quantum physics. While the framework of quantum physics has thus proven to be extremely successful and powerful, throughout the last century there were many conceptual, and to some extent philosophical, discussions on our understanding of quantum physics. As early as the 1930s, Einstein and Schrödinger realized that quantum physics allows for some peculiar states, which Schrödinger called “entangled”, which have a non-local character and thus do find an easy interpretation in our classical world. It took a few more decades until John Bell conceived a way to quantify such states, which thus became amenable to measurements. Only in the eighties and nineties of the last century did the generation of entangled states, their characterization and their subsequent application become feasible and accessible in laboratories. Early on, Bell states were proposed for applications in quantum communication, employing protocols based on teleportation. Soon, entanglement was recognized to be useful also for sensing, for precision measurements and eventually for quantum information processing. Starting with the early concept of a trapped-ion quantum computer introduced by Ignacio Cirac and Peter Zoller in 1995, many other platforms were proposed for quantum information processing, based on superconducting quantum bits, photonic processes, engineered quantum dots and many more. The first decade of the 21st century saw an explosive development of quantum information physics, which is by now a mature field and increasingly focuses on applications. Worldwide efforts are ongoing to realize quantum networks for secure communication and to implement quantum computers. Enhanced sensing has been demonstrated using advanced quantum states, and quantum simulators have been developed, already tackling problems that are hard to compute with classical computers. While architectures for full-fledged quantum computers are already available, the realization of such computers still requires the routine implementation of error correction, which is not yet available, for full scalability. Currently, noisy intermediate-scale quantum (NISQ) processors are already used for computations and are even accessible via cloud services. Since about 2000, quantum technologies have been developed for many platforms with the aim to surpass the capablities of their classical counterparts. Many such devices now make use of entangled states, which are seen as the new resource for advanced quantum technologies. For this, describing, understanding and generally mastering the generation and manipulation of entanglement is a key element. Moreover, both hardware and software technologies have to be improved in order to better control quantum devices. Mastering quantum phenomena for communication, metrology, simulation and computation purposes is currently one of the hottest topics in science since the wide applicability of quantum technologies will revolutionize these fields. The development of advanced quantum technologies is thus seen worldwide as a strategic area for generating technical knowledge and industrial evolution. This special issue intends to showcase the breadth and impact of the ongoing research in advancing quantum technologies. We have invited a selection of renowned scientists to present their work, and we are confident that this issue will find a broad readership and attract much interest within the scientific community.
Introduction It is believed that the basic physical building blocks forming the world we live in may be categorized into particles of matter and carriers of force between matter. All known elementary constituents of matter and transmitters of force are quantized. For example, energy, momentum, and angular momentum take on discrete quantized values. The electron is an example of an elementary particle of matter, and the photon is an example of a transmitter of force. Neutrons, protons, and atoms are composite particles made up of elementary particles of matter and transmitters of force. These composite particles are also quantized. Because classical mechanics is unable to explain quantization, we must learn quantum mechanics in order to understand the microscopic properties of atoms – which, for example, make up solids such as crystalline semiconductors. Historically, the laws of quantum mechanics have been established by experiment. The most important early experiments involved light. Long before it was realized that light waves are quantized into particles called photons, key experiments on the wave properties of light were performed. For example, it was established that the color of visible light is associated with different wavelengths of light. Table 2.1 shows the range of wavelengths corresponding to different colors. The connection between optical and electrical phenomena was established by Maxwell in 1864. This extended the concept of light to include the complete electromagnetic spectrum. A great deal of effort was, and continues to be, spent gathering information on the behavior of light.
Photons, the individual quanta of the light field, are what the science of quantum photonics is dedicatedly investigating. The manipulation and coherent control of photons in quantum photonics enables the exploration of various quantum phenomena of high fundamental interest. In the meantime, due to the fast speed and a lack of the interaction with the environment, photons are now regarded as a promising platform for the emerging quantum information processing (QIP) studies. Therefore, there is a growing number of works on quantum computing, quantum communication, and quantum metrology that are solidly based on the techniques of quantum photonics.
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.