Decoherence, einselection, and the quantum origins of the classical
Wojciech Hubert Zurek Theory Division, LANL, Mail Stop B210, Los Alamos, New Mexico 87545
(Published 22 May 2003 Reviews of Modern Physics)
"The manner in which states of some quantum systems become effectively classical is of great significance for the foundations of quantum physics, as well as for problems of practical interest such as quantum engineering. In the past two decades it has become increasingly clear that many (perhaps all) of the symptoms of classicality can be induced in quantum systems by their environments. Thus decoherence is caused by the interaction in which the environment in effect monitors certain observables of the system, destroying coherence between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselected pointer states are stable. They can retain correlations with the rest of the universe in spite of the environment. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly nonlocal ‘‘Schrodinger-cat states.’’ The classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit. Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation. Only the preferred pointer observable of the apparatus can store information that has predictive power. When the measured quantum system is microscopic and isolated, this restriction on the predictive utility of its correlations with the macroscopic apparatus results in the effective ‘‘collapse of the wave packet.’’ The existential interpretation implied by einselection regards observers as open quantum systems, distinguished only by their ability to acquire, store, and process information. Spreading of the correlations with the effectively classical pointer states throughout the environment allows one to understand ‘‘classical reality’’ as a property based on the relatively objective existence of the einselected states. Effectively classical pointer states can be ‘‘found out’’ without being re-prepared, e.g, by intercepting the information already present in the environment. The redundancy of the records of pointer states in the environment (which can be thought of as their ‘‘fitness’’ in the Darwinian sense) is a measure of their classicality. A new symmetry appears in this setting. Environment-assisted invariance or envariance sheds new light on the nature of ignorance of the state of the system due to quantum correlations with the environment and leads to Born’s rules and to reduced density matrices, ultimately justifying basic principles of the program of decoherence and einselection."
Simple eh? ;-)
"How the classical world arises from an ultimately quantum substrate has been a question since the advent of quantum mechanics [1–7]. Decoherence is now commonly used to study this quantum-classical transition [8–10]. Its theory, however, treats the environment as a sink where information about the system gets lost forever. Yet the information deposited in the environment can be intercepted, and it is our primary source of information about the Universe. Indeed, decohering interactions with the environment can amplify and store an impression of the system. Amplification was invoked already by Bohr [11] in the context of measurements. Early [12], as well as more recent [9,13,14], discussions of decoherence note the importance of redundancy, and provide an information- theoretic framework for how the environment acts as an amplifier and as a source of information about the ‘‘system of interest’’ [15–19].Quantum Darwinism reflects this new focus on the environment as a communication channel [15–17]. When one receives a fragment of the environment by, for instance, intercepting with one’s eyes a portion of photons that are scattered off a system of interest (e.g., the text of this Letter), one acquires information about it. Previous studies found that, with an initially pure environment, one can acquire information about the preferred observables of the system even from small environment fragments [17]. This explains the emergence of objectivity, as it allows many initially ignorant observers to independently obtain nearly complete information and reach consensus about the state of the system by intercepting different fragments of the environment. Classicality of states can now be quantified in terms of the redundancy of information transferred to and recorded by the environment. However, it is unclear how well one can accumulate information starting with a mixed, or hazy, environment, such as one at finite temperature. Yet the photon environment that is responsible for the vast majority of the information we gain has precisely such a hazy character. This Letter shows that even hazy environments will, in the end, communicate a very clear image."PRL 103, 110402 (2009)PHYSICAL REVIEW LETTERS week ending 11 SEPTEMBER 2009Quantum Darwinism in a Mixed EnvironmentMichael Zwolak, H. T. Quan, and Wojciech H. ZurekTheoretical Division, MS-B213, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA(Received 29 April 2009; revised manuscript received 8 August 2009; published 8 September 2009)