Decoherence is a relatively recent concept in the field of quantum mechanics. Although the pioneers of the field must have understood that loss of phase coherence in quantum superpositions is the reason underlying the appearance of definite outcomes in the quantum measurement problem, the latter was not treated in terms of decoherence until sixty years after the formulation of quantum mechanics as described in a report by Joos and Zeh in 1983 [1]. However, soon after, the theory was developed in further detail, and experiments to measure the actual decoherence rates in various systems commenced. Today, decoherence is the main concern for another reason, i.e., the fact that superposition states must be maintained undisturbed in the quantum communication systems and decoherence presents a strong limitation for their practical application. Decoherence appears in open quantum systems, where the basic system under consideration interacts relatively strongly with an environment. Decoherence times may be as long as seconds for small atomic systems in extreme vacua, although below what is presently measurable (i.e., less than a fraction of a femtosecond) in most liquids and solids, where coupling to the surrounding molecules or atomic arrangements is strong. The relatively slow decoherence in well-isolated particle systems has been described in several recent articles on quantum optics, while the rapid decoherence in liquids and solids has not received this kind of attention. It is the purpose of the present review to fill this gap.
Decoherence is a relatively recent concept in the field of quantum mechanics. Although the pioneers of the field must have understood that loss of phase coherence in quantum superpositions is the reason underlying the appearance of definite outcomes in the quantum measurement problem, the latter was not treated in terms of decoherence until sixty years after the formulation of quantum mechanics as described in a report by Joos and Zeh in 1983 [1]. However, soon after, the theory was developed in further detail, and experiments to measure the actual decoherence rates in various systems commenced. Today, decoherence is the main concern for another reason, i.e., the fact that superposition states must be maintained undisturbed in the quantum communication systems and decoherence presents a strong limitation for their practical application. Decoherence appears in open quantum systems, where the basic system under consideration interacts relatively strongly with an environment. Decoherence times may be as long as seconds for small atomic systems in extreme vacua, although below what is presently measurable (i.e., less than a fraction of a femtosecond) in most liquids and solids, where coupling to the surrounding molecules or atomic arrangements is strong. The relatively slow decoherence in well-isolated particle systems has been described in several recent articles on quantum optics, while the rapid decoherence in liquids and solids has not received this kind of attention. It is the purpose of the present review to fill this gap.
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.
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.
Abstract Quantum decoherence is receiving a great deal of attention today not only in theoretical and experimental physics but also in branches of science as diverse as molecular biology, biochemistry, and even neuropsychology. It is no surprise that it is also beginning to appear in various philosophical debates concerning the fundamental structure of the world. The purpose of this article is primarily to acquaint non‐specialists with quantum decoherence and clarify related concepts, and secondly to sketch its possible implications – independent of particular interpretations of quantum mechanics – for broader philosophical debates. For example, decoherence shows that any method of parsing nature into levels or parts cannot be in principle but instead derives from our perception of the world as classical, a perception that is itself sustained by the process of decoherence.
Ultrafast spectroscopy is a valuable tool for monitoring the timescales of interactions between systems and their environments, resulting in pure dephasing. The superposition of ground and excited states of a molecule in a condensed phase, created by field–matter interactions, loses its coherence due to fluctuations from surrounding molecules that interact differently with the ground and excited states. Recently, quantum decoherence has become an intense area of research due to its relevance to the quantum-to-classical transition and its critical role in developing quantum technologies, such as quantum computers and cryptography. Although both pure dephasing and quantum decoherence result from the same process of environmental monitoring of systems through quantum entanglement between the system and its environment, they have been studied and discussed in very different contexts with seemingly disparate terminologies. In this work, we present a detailed theoretical description of pure dephasing and quantum decoherence in bosonic environments coupled to a two-level system, compare them directly, and demonstrate their connections to the wave–particle duality of isolated systems and the wave-particle-entanglement triality of composite systems consisting of systems and their environments. It is believed that the present review will be helpful for gaining a deeper understanding of ultrafast spectroscopy from a quantum mechanical perspective and the wave–particle duality of quantum objects interacting with their surrounding environments.
Theory and experiment have not fully resolved the apparent dichotomy , which has agonized physics for the past eighty years: on the one hand, the description of microsystems by quantum mechanics and, on the other, the description of macrosystems by classical dynamics or statistical mechanics. Derivations of the time-irreversible Liouville equation for an open quantum system, based on projecting out its environment, have narrowed the gap between the quantum and classical descriptions. Yet our `classical' intuition continues to be confronted by quantum-mechanical results like the Einstein--Podolsky--Rosen paradox that challenges the classical notion of locality, or the quantum Zeno effect which suggests that the isolation of a system is not the only way to preserve its quantum state. There are two key concepts in any discussion of such issues. The first, which is responsible for the most salient nonclassical properties, is entanglement , that is partial or complete correlation or, more generally, inseparability of the elements comprising a quantum ensemble . Even after their interaction has ceased, this inseparability, originating from their past interaction, can affect the state of one element when another element is subject to a nonunitary action, such as its measurement, tracing- out, or thermalization. The second key concept is decoherence of open quantum systems, which is the consequence of their entanglement with their environment, a `meter' or a thermal `reservoir', followed by the tracing-out of the latter. Despite new insights into entanglement and decoherence, there are still no complete, unequivocal answers to the fundamental questions of the transition from quantal to classical behaviour: how do irreversibility and classicality emerge from unitarity as systems and their environments become increasingly complex ? At what stage does system--meter entanglement give rise to a classical readout of the meter? Is there an upper limit on the size or complexity of systems displaying entanglement? Major developments have opened new vistas into controlled entanglement , which is the resource of quantum information processing. However, these developments have mainly focused on ensembles of simple two- and three-level systems that are thoroughly isolated from their environments. Treatments of coherence in quantum computing have mostly assumed that only a single or a few the elements of the quantum ensemble may simultaneously undergo an uncontrolled intervention---a quantum error. Decoherence-control protocols for more general types of errors are still incomplete. In order to resolve the outstanding issues of the quantum--classical transition, and study the control of entanglement and decoherence without the foregoing restrictions, we must venture into the domain of Quantum Complex Systems (QUACS), either consisting of a large number of inseparable elements or having many coupled degrees of freedom. Our conviction is that fundamental understanding and manipulation of entanglement within QUACS or their entanglement with the environment or a meter, call for the creation of a new conceptual framework or paradigm, that would encompass phenomena common to cold atoms in laser fields, large molecules, Josephson junctions, quantum gases and solids, with the view of employing these systems for quantum information processing and computing. Progress within this paradigm should allow us to answer the questions: does entanglement play an essential role in the evolution of large collections of complex systems? What are the size and complexity limits of systems and ensembles still controllable by an external intervention? What are the most appropriate decoherence protection schemes and control algorithms? This special issue addresses the challenge of understanding in depth and manipulating the basic quantum properties of optical, atomic, molecular and condensed-matter QUACS, and large ensembles thereof. In view of the interdisciplinary character of this issue, we find it expedient to look at the presented articles not only according to the physical objects they describe, but also from a unifying standpoint. Several unifying themes may be discerned: memory effects in dephasing and relaxation; protection of quantum information by its distribution throughout the ensemble; control of decoherence and entanglement by time-dependent interventions or measurements; manifestations of entanglement in scattering off statistical ensembles; measurment-induced dynamics. The concerted discussion of the topics outlined above should help us advance the new paradigm that addresses our abilities to diagnose and manipulate the entangled states of complex quantum objects and their robustness against decoherence. These abilities are required for quantum information (QI) applications in matter--wave interferometry in molecular, semiconducting or superconducting systems. On the fundamental level, this book may help establish the notion of dynamical information exchange between quantum systems and chart in detail the route from unitarity to classicality. Further developments within the outlined paradigm should yield novel, advantageous QI processing schemes and high-sensitivity interferometers, owing to decoherence suppression and effective control of many degrees of freedom. In the long run, these strategies may prove to be the first step towards the (hitherto unattempted) use of entanglement in nanotechnology, metrology and chemistry, with potentially remarkable novel applications.
In connection with the recent experiments on quantum measurement, such as the “which-way”detection using the Bragg scattering of cold atoms and quantum interference using a small molecular C 60 quantum system, we introduce the quantum measurement problem and its relevant fundamental concepts and ideas, such as quantum coherence, quantum decoherence and quantum entanglement. In order to understand the loss of quantum coherence for a quantum system due to its interaction with another system behaving classically or macroscopically, we discuss some quantum dynamical models to describe the quantum decoherence and quantum dissipation induced by couplings to the external environment or inner motions. In the classical or macroscopic limit, it is shown that the wave function of the total system formed by the quantum system plus another system interacting with it is factorized as an entangled state with correlation between the measured quantum states and the configurations of the external large system or the inner degrees of freedom. In association with a novel viewpoint about quantum measurement, which has been directly verified by recent experiments (e.g.,S. Durr et al., Nature,1998,33:359), it is shown that quantum entanglement is indeed responsible for the quantum decoherence, and thus it can be regarded as a “clean” quantum measurement when the large system behaves as a classical object. These arguments give us a possible solution to the “Schrdinger cat paradox” and to whether the coherent superposition of macroscopic states exists. These discussions are also applied to the quantum decoherence problems in quantum information including quantum computing and quantum communication.