Decoherence is the physical process by which the classical world - the world of common sense - emerges from its quantum underpinnings. This physical process refers to the loss of phase coherence between the parts of a quantum system, because of the interaction of the system with the environment. This text with contributions from the Advanced Research Workshop (ARW) provides an in-depth discussion of this physical process and of its relevance in understanding and interpreting quantum mechanics, with special emphasis given to its implications in implementing the practical aspects of quantum computation and information transfer. The ARW brings together scientists in diverse fields such as mathematical physics, quantum logic, and quantum computing for an intense, interdisciplinary discussion of decoherence and its practical implications in quantum computing. Quantum computing represents possibly the epitome of the technological impact that quantum mechanics can have on technological developments in the foreseeable future. Its realization would allow the performance of tasks that are practically unthinkable in terms of classical physics. Such exotic occurrences as breaking hitherto unbreakable codes, searching enormous lists for specific entries, and communicating with messages that betray the presence of eavesdropping would fall within reach. These wonders are possible when one exploits the logical consequences of the laws that govern natural phenomena at the most fundamental level, namely, the laws of quantum mechanics standing squarely in the way of these developments is decoherence. The aim of the workshop was to provide an in-depth discussion of the state of affairs regarding the progress that is being made in understanding the effects of decoherence in quantum computation and information transfer and in getting around it. The present is a particularly fitting time for discussions of an interdisciplinary nature, encompassing quantum logic, mathematics and physics, addressing these issues. The invited speakers have been chosen among those who continue to make significant contributions in this domain of science and we expect lively and informative presentations and discussions. Also invited were those who are expected to benefit greatly from exposure to the issues under consideration and who will carry developments further as their own scientific career unfolds.
Decoherence is the physical process by which the classical world - the world of common sense - emerges from its quantum underpinnings. This physical process refers to the loss of phase coherence between the parts of a quantum system, because of the interaction of the system with the environment. This text with contributions from the Advanced Research Workshop (ARW) provides an in-depth discussion of this physical process and of its relevance in understanding and interpreting quantum mechanics, with special emphasis given to its implications in implementing the practical aspects of quantum computation and information transfer. The ARW brings together scientists in diverse fields such as mathematical physics, quantum logic, and quantum computing for an intense, interdisciplinary discussion of decoherence and its practical implications in quantum computing. Quantum computing represents possibly the epitome of the technological impact that quantum mechanics can have on technological developments in the foreseeable future. Its realization would allow the performance of tasks that are practically unthinkable in terms of classical physics. Such exotic occurrences as breaking hitherto unbreakable codes, searching enormous lists for specific entries, and communicating with messages that betray the presence of eavesdropping would fall within reach. These wonders are possible when one exploits the logical consequences of the laws that govern natural phenomena at the most fundamental level, namely, the laws of quantum mechanics standing squarely in the way of these developments is decoherence. The aim of the workshop was to provide an in-depth discussion of the state of affairs regarding the progress that is being made in understanding the effects of decoherence in quantum computation and information transfer and in getting around it. The present is a particularly fitting time for discussions of an interdisciplinary nature, encompassing quantum logic, mathematics and physics, addressing these issues. The invited speakers have been chosen among those who continue to make significant contributions in this domain of science and we expect lively and informative presentations and discussions. Also invited were those who are expected to benefit greatly from exposure to the issues under consideration and who will carry developments further as their own scientific career unfolds.
We are in the midst of a renaissance in the study and interpretation of fundamental quantum mechanics, from quantum coherence and quantum entanglement to the quantum measurement process. This stems partly from the impressive advances in experimental techniques that are making yesterday's gedanken experiment today's reality. Another reason quantum coherence and entanglement have come to the forefront is the emerging and tantalizing field of quantum information science. The ability to store information in quantum systems may someday yield devices that process quantum superpositions of inputs in parallel or improve communication through the `wiring' implicit in quantum entanglement. Indeed, quantum computing has the remote prospect of revolutionizing the way we store and process information. This has become particularly important in the face of impending limits on the miniaturization of conventional computers due to quantum effects. Of course, all real systems face limits imposed by decoherence, or the environmentally-induced destruction of quantum coherence. Here, superpositions of quantum states suddenly become probabilistic mixtures of states when they interact with the environment or when the system parameters are fluctuating. Decoherence theory is commonly interpreted as a way to quantify the elusive boundary between quantum and classical worlds and almost always precludes the existence of complex quantum superpositions, except at extremely short time scales. In practice, it is very important to study how decoherence appears in particular quantum systems and try to fight against it. In this special issue of Journal of Optics B: Quantum and Semiclassical Optics , we highlight recent results in the exciting area of quantum coherence and entanglement, including generating interesting and complex quantum states, characterizing coherence and entanglement in these systems, and triumphing over relevant decoherence processes. We hope this issue will not only act as a bookmark in the ongoing study of fundamental quantum mechanics, but will also help guide the rapid developments in quantum information science.
Recent work in quantum information science has produced a revolution in our understanding of quantum entanglement. Scientists now view entanglement as a physical resource with many important applications. These range from quantum computers, which would be able to compute exponentially faster than classical computers, to quantum cryptographic techniques, which could provide unbreakable codes for the transfer of secret information over public channels. These important advances in the study of quantum entanglement and information touch on deep foundational issues in both physics and philosophy. This interdisciplinary volume brings together fourteen of the world's leading physicists and philosophers of physics to address the most important developments and debates in this exciting area of research. It offers a broad spectrum of approaches to resolving deep foundational challenges - philosophical, mathematical, and physical - raised by quantum information, quantum processing, and entanglement. This book is ideal for historians, philosophers of science and physicists.
In the last decade decoherence has become a very popular topic mainly due to the progress in experimental techniques which allow monitoring of the process of decoherence for single microscopic or mesoscopic systems. The other motivation is the rapid development of quantum information and quantum computation theory where decoherence is the main obstacle in the implementation of bold theoretical ideas. All that makes the second improved and extended edition of this book very timely. Despite the enormous efforts of many authors decoherence with its consequences still remains a rather controversial subject. It touches on, namely, the notoriously confusing issues of quantum measurement theory and interpretation of quantum mechanics. The existence of different points of view is reflected by the structure and content of the book. The first three authors (Joos, Zeh and Kiefer) accept the standard formalism of quantum mechanics but seem to reject orthodox Copenhagen interpretation, Giulini and Kupsch stick to both while Stamatescu discusses models which go beyond the standard quantum theory. Fortunately, most of the presented results are independent of the interpretation and the mathematical formalism is common for the (meta)physically different approaches. After a short introduction by Joos followed by a more detailed review of the basic concepts by Zeh, chapter 3 (the longest chapter) by Joos is devoted to the environmental decoherence. Here the author considers mostly rather `down to earth' and well-motivated mechanisms of decoherence through collisions with atoms or molecules and the processes of emission, absorption and scattering of photons. The issues of decoherence induced superselection rules and localization of objects including the possible explanation of the molecular structure are discussed in details. Many other topics are also reviewed in this chapter, e.g., the so-called Zeno effect, relationships between quantum chaos and decoherence, the role of decoherence in quantum information processing and even decoherence in the brain. The next chapter, written by Kiefer, is devoted to decoherence in quantum field theory and quantum gravity which is a much more speculative and less explored topic. Two complementary aspects are studied in this approach: decoherence of particle states by the quantum fields and decoherence of field states by the particles. Cosmological issues related to decoherence are discussed, not only within the standard Friedmann cosmology, but also using the elements of the theory of black holes, wormholes and strings. The relations between the formalism of consistent histories defined in terms of decoherence functionals and the environmental decoherence are discussed in chapter 5, also written by Kiefer. The Feynman--Vernon influence functional for the quantum open system is presented in detail as the first example of decoherence functional. Then the general theory is outlined together with possible interpretations including cosmological aspects. The next chapter by Giulini presents an overview of the superselection rules arising from physical symmetries and gauge transformations both for nonrelativistic quantum mechanics and quantum field theory. Critical discussion of kinematical superselection rules versus dynamical ones is illustrated by numerous examples like Galilei invariant quantum mechanics, quantum electrodynamics and quantum gravity. The introduction to the theory of quantum open systems and its applications to decoherence models is given in chapter 7 by Kupsch. Generalized master equations, Markovian approximation and a few Hamiltonian models relevant for decoherence are discussed. Some mathematical tools, e.g., complete positivity and entropy inequalities are also presented. The last chapter by Stamatescu is devoted to stochastic collapse models which can be interpreted either as certain representations of the dynamics of open quantum systems or as fundamental modifications of the Schr\"odinger equation. The final part of the book consists of remarks by Zeh on related concepts and methods and seven appendices. The broad spectrum, mathematically-friendly presentation, inclusion of the very recent developments and the extensive bibliography (about 550 references) make this book a valuable reference for all researchers, graduate and PhD students interested in the foundations of quantum mechanics, quantum open systems and quantum information. The relative independence of the chapters and numerous redundancies allow for selective reading, which is very helpful for newcomers to this field.
This book provides a comprehensive and organized study of quantum technology, focusing on principles, mathematical foundations, and practical realizations of quantum teleportation and other processes in the field of quantum information. It starts with the basic principles of quantum mechanics (wave-particle duality, for instance) superposition and measurement theory all helping as the theoretical backbone of current quantum science. In addition to these basics, we provide a concise introduction and discussion of quantum energy or the quantization of energy levels, energy eigenstates and Planck’s constant, which determines what happens in microscopic systems allowing quantum theory an overlap with data from physical observables. Building on these principles, the book introduces quantum states and quantum entanglement, highlighting their non-classical correlations and their crucial role in facilitating quantum communication protocols. This book also discusses quantum gates and quantum circuits as the units of quantum computation and how quantum information is computed and manipulated by using controlled unitary operations. Also, it discusses the impact of both quantum decoherence and environmental noise which pose significant difficulties to coherent quantum systems and dependable information transmission. It discusses methods of error correction and coherence . This book further expanded to quantum cryptography and secure communications with more importance on protocols like quantum key distribution which make use of quantum principles to provide provable security. Quantum computing architectures and algorithms are also measured and various physical implementations and computational models that strive to produce scalable quantum processors are outlined. Problems and limitations of quantum teleportation are critically examined and these are, reliability issues, channel losses, scaling, and resource limitations. Lastly, the book ends by outlining the future of quantum technology, investigating the new technologies and opportunities, the possible advances, and the revolutionary use of quantum technologies in the fields of computation, communication and secure information processing.
Quantum theory and quantum computing in recent years, has raised more and more increasing interest within the scientific community because of various reasons. First, the highly interdisciplinary nature of the subject which involves diverse topics such as, on the theoretical side, information theory and foundational issues of quantum mechanics and on the experimental side, the physics of quantum, optical, atomic and solid state systems. Secondly, the field is extremely promising for revolutionary technological implications in the near future. It is now perfectly clear that one can use quantum mechanics to process and transmit information in a way which does not have a classical analogue, and that using quantum techniques one can implement fully secure cryptographic systems. Recent breakthroughs, such as quantum teleportation, have led to extremely rapid progresses in a fascinating new field and, as a consequence, have attracted the interest of many scientists from varied disciplines. The purpose of this article is to provide a comprehensive overview about the deep involvement of quantum theory in quantum computation to young researchers and newcomers in the field.
A fundamental concept in quantum mechanics, quantum entanglement describes a one-of-a-kind occurrence in which two particles, separated by physical distance, become physically connected so that the states of one particle can instantly affect the states of the other. Quantum entanglement has both theoretical and practical consequences for communication and information processing, which are explored in this study. This article explores the latest developments in quantum cryptography, quantum teleportation, and quantum computing, focusing on how these technologies have the ability to transform communication networks, computational speed, and data security. While outlining potential future research directions, the report delves into issues including scalability and decoherence. This manuscript uses entanglement to highlight its revolutionary impact on the development of scientific and technological frontiers.