Quantum Measurements And Decoherence

Quantum measurement (Le., a measurement which is sufficiently precise for quantum effects to be essential) was always one of the most impor tant points in quantum mechanics because it most evidently revealed the difference between quantum and classical physics. Now quantum measure ment is again under active investigation, first of all because of the practical necessity of dealing with highly precise and complicated measurements. The nature of quantum measurement has become understood much bet ter during this new period of activity, the understanding being expressed by the concept of decoherence. This term means a physical process lead ing from a pure quantum state (wave function) of the system prior to the measurement to its state after the measurement which includes classical elements. More concretely, decoherence occurs as a result of the entangle ment of the measured system with its environment and results in the loss of phase relations between components of the wave function of the measured system. Decoherence is essentially nothing else than quantum measurement, but considered from the point of view of its physical mechanism and resolved in time. The present book is devoted to the two concepts of quantum measure ment and decoherence and to their interrelation, especially in the context of continuous quantum measurement.

The quantum measurement problem is one of the most fascinating and challenging topics in physics both theoretically and experimentally. It involves deep questions and the use of very sophisticated and elegant techniques. After analyzing the fundamental principles of quantum mechanics and of the Copenhagen interpretation, this book reviews the most important approaches to the measurement problem and rigorously reformulates the “collapse of the wave function” by measurement, as a dephasing process quantitatively characterized by an order parameter (called the decoherence parameter), according to the many-Hilbert-space approach to the problem. The book deals not only with the measurement processes (including imperfect measurements) but also with related interference and mesoscopic phenomena — by means of general arguments — of solvable models and of numerical simulations. The quantum Zeno effect and the issue of irreversibility are also discussed. Contents:General and Historical SurveyElements of Quantum MechanicsCritical Review of Measurement TheoriesThe Many-Hilbert-Space TheorySolvable Detector ModelsNeutron InterferometryNumerical Simulations of Measurement ProcessesQuantum Zeno EffectQuantum Dephasing by Chaos Readership: Physicists interested in the foundations of quantum theory. keywords:Decoherence;Irreversibility;Quantum Zeno Effect;Quantum Measurements;Environment;Classical Properties;Mesoscopic Systems;Time Evolutions;Projection Postulate;Collapse of the Wave Function “… considerable background material is given, allowing the volume to serve general educational and reference purposes as well. I can recommend it to anyone wanting an orientation to quantum measurement theory and, in particular, wanting to focus on the Many Hilbert Space Theory.” Foundations of Physics “The book may be useful both for students and experts.” Zentralblatt MATH

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.

Modern quantum measurement for graduate students and researchers in quantum information, quantum metrology, quantum control and related fields.

This volume is the proceedings of the 5th International Conference on Quantum Communication, Computing , and Measurement, held in Capri in July 2000, and covers all areas of quantum information science, which has developed dramatically recently. An excellent overview of the current topics in the field, it discusses quantum information theory, quantum measurement theory, decoherence, and entanglement theory as the theoretical aspect of quantum information science. Also included are quantum communication systems, quantum computing, quantum cryptography, new quantum effects and their experiments, generation and detection of nonclassical light, and teleportation as applications of quantum information science. In addition, the result of the third international quantum communication award is introduced.

The amazing accuracy in verifying quantum effects experimentally has recently renewed interest in quantum mechanical measurement theory. In this book the authors give within the Hilbert space formulation of quantum mechanics a systematic exposition of the quantum theory of measurement. Their approach includes the concepts of unsharp objectification and of nonunitary transformations needed for a unifying description of various detailed investigations. The book addresses advanced students and researchers in physics and philosophy of science. In this second edition Chaps. II-IV have been substantially rewritten. In particular, an insolubility theorem for the objectification problem has been formulated in full generality, which includes unsharp object observables as well as unsharp pointers.