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arXiv:1811.09062

Decoherence, quantum Darwinism, and the generic emergence of our objective classical reality

Paul A. Knott

Published Nov 22, 2018• 1 citation
arXivHTMLPDF

Abstract

In this article I aim to provide an intuitive and non-technical introduction to decoherence and quantum Darwinism. Together these theories explain how our classical reality emerges from an underlying quantum mechanical description. Here I focus on two aspects of this and explain, firstly, how decoherence can tell us why we never see macroscopic superpositions, such as dead-and-alive cats, in our classical surroundings; and secondly I describe and then provide a resolution to the so-called preferred basis problem. I then introduce recent results demonstrating that certain aspects of classicality are generic phenomena that emerge from the basic mathematical structure of quantum mechanics. This is in stark contrast to previous work in this field that focused on specific models that cannot realistically be scaled up to explicitly answer questions about the macroscopic world. Finally, I demonstrate how decoherence and quantum Darwinism can shed significant light on the measurement problem, and I discuss the implications for how we should interpret quantum mechanics.

DOI
10.48550/arxiv.1811.09062

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In this article I aim to provide an intuitive and non-technical introduction to decoherence and quantum Darwinism. Together these theories explain how our classical reality emerges from an underlying quantum mechanical description. Here I focus on two aspects of this and explain, firstly, how decoherence can tell us why we never see macroscopic superpositions, such as dead-and-alive cats, in our classical surroundings; and secondly I describe and then provide a resolution to the so-called preferred basis problem. I then introduce recent results demonstrating that certain aspects of classicality are generic phenomena that emerge from the basic mathematical structure of quantum mechanics. This is in stark contrast to previous work in this field that focused on specific models that cannot realistically be scaled up to explicitly answer questions about the macroscopic world. Finally, I demonstrate how decoherence and quantum Darwinism can shed significant light on the measurement problem, and I discuss the implications for how we should interpret quantum mechanics.
The measurement problem has been a central puzzle of quantum theory since its inception, and understanding how the classical world emerges from our fundamentally quantum universe is key to its resolution. While the 'Copenhagen' and 'Many Worlds' interpretations have dominated discussion of this philosophically charged question, Zurek builds on the physics of decoherence and introduces the theory of 'Quantum Darwinism' to provide a novel account of the emergence of classical reality. Opening with a modern view of quantum theory, the book reconsiders the customary textbook account of quantum foundations, showing how the controversial axioms (including Born's rule) follow from the consistent core postulates. Part II discusses decoherence and explores its role in the quantum-to-classical transition. Part III introduces Quantum Darwinism, explaining how an information-theoretic perspective complements, elucidates, and reconciles the 'Copenhagen' and 'Many Worlds' interpretations. This insightful book is essential reading for any student or researcher interested in quantum physics.
This study offers a comprehensive technical analysis of the mechanisms through which classical behavior emerges from the dynamics of quantum measurement. Beginning with unitary evolution and the principles of measurement, we construct the framework of environment-induced decoherence, the selection of pointer states, and the reduction of interference. Our examination focuses on how effective classicality emerges through reduced density matrices, quantum Darwinism, and consistent histories, highlighting the importance of system-environment interactions in determining stable observables. The review investigates into mathematical constructs such as spectral theory, completely positive maps, and quantum trajectories, linking them to experimental observations in mesoscopic systems, optomechanics, and superconducting circuits. We explore the constraints of decoherence in addressing the outcome problem and review interpretational models that tackle the remaining issues. The discussion emphasizes applications in quantum information, metrology, and tests of macroscopic superpositions. The article favors physical insights and mathematical guidance over exhaustive proofs, making it suitable for researchers in quantum foundations and quantum technologies.
In our daily life experiences we face localized objects which are "here or there" not "here and there". The state of a cat could be "dead and alive" at the same time from a quantum mechanical point of view, which is not in agreement with our classical life observations. If we assume that quantum theory can explain the large scale events, how is it possible that quantumness disappears in the classical world? The answer lies in the interaction between the environment and the system of interest. Quantum Darwinism describes the proliferation of the quantum information of each system in the environment. We will show how entanglement plays a key role in coding and decoding the information of each system in its surrounding environment.
Quantum Darwinism posits that only specific information about a quantum system that is redundantly proliferated to many parts of its environment becomes accessible and objective, leading to the emergence of classical reality. However, it is not clear under what conditions this mechanism holds true. Here we prove that the emergence of classical features along the lines of quantum Darwinism is a general feature of any quantum dynamics: observers who acquire information indirectly through the environment have effective access at most to classical information about one and the same measurement of the quantum system. Our analysis does not rely on a strict conceptual splitting between a system-of-interest and its environment, and allows one to interpret any system as part of the environment of any other system. Finally, our approach leads to a full operational characterization of quantum discord in terms of local redistribution of correlations. Quantum Darwinism provides an explanation for the emergence of classical reality from the underlying quantum world, as information proliferates in the environment and becomes objective. Here, the authors show how some aspects of this mechanism is generic and classicality emerges for any quantum dynamics.
This article surveys a selection of key conceptual and interpretational developments in quantum mechanics, tracing the theory from its foundational postulates to contemporary discussions of measurement, nonlocality, and the emergence of classicality. Beginning with the structure of Hilbert space and the postulates governing state evolution and measurement, the epistemic stance of the Copenhagen interpretation and its modern reformulations are examined. The Einstein-Podolsky-Rosen argument, Bell's theorem, and Hardy's paradox are then discussed as probes of locality and realism, alongside the deterministic but explicitly nonlocal de Broglie-Bohm theory. The measurement problem and the implications of contextuality are analyzed in relation to objective collapse models, which introduce new physical dynamics to account for definite outcomes. Finally, the role of decoherence in the suppression of interference and the emergence of classical behavior is explored, together with the interpretational frameworks of many-worlds and consistent histories. This material aims to provide a coherent introductory overview of how several of the most prominent interpretations address the central concern of what quantum mechanics tells us about the nature of physical reality.
Introduction Quantum measurement problem is a technical euphemism for a much deeper and less well-defined question: How do we, “the observers,” fit within the physical universe? The problem is especially apparent in quantum physics because, for the first time in the history of science a majority of (but not all) physicists seriously entertains the possibility that the framework for the ultimate universal physical theory provided by quantum mechanics is here to stay. Quantum physics relevant for this discussion is (contrary to the common prejudice) relatively simple. By this I mean that some of the key features of its predictions can be arrived at on the basis of overarching principles of quantum theory and without reference to the minutiae of other specific ingredients (such as the details of the forces). Quantum superposition principle is such an overarching principle of quantum theory. It leads to predictions that seem difficult to reconcile with our perception of the familiar classical universe of everyday experience. The aim of this paper is to show that appearance of the classical reality can be viewed as a result of the emergence of the preferred states from within the quantum substrate through the Darwinian paradigm, once the survival of the fittest quantum states and selective proliferation of the information about them throughout the universe are properly taken into account.