Monthly Archives: December 2014

Reply to Physics Today article and post re ‘Quantum Darwinism’

Comment on “Quantum Darwinism, Decoherence, and the Randomness of Quantum Jumps,” arxiv:1412.5206

R. E. Kastner
University of Maryland, College Park
December 18, 2014

This brief note points out an extant rebuttal in the literature to the claims made in a recent publication by Zurek [1].

In [1], it is stated that

Decoherence selects preferred pointer states that survive interaction with the environment. They are    localized and effectively classical. They persist while their superpositions decohere. Decoherence marks the border between quantum and classical, alleviating concern about flagrant…manifestations of quantumness in the macroscopic domain. Here we consider emergence of `the classical’ starting at a more fundamental pre-decoherence level, tracing theorigin of preferred pointer states…”[1], p. 1.

However, the idea that preferred pointer states naturally ‘emerge’ from the quantum level has been refuted in a publication [2] apparently overlooked in [1].[3] It is shown in [2] that ‘classical’ pointer states do not emerge unless a key aspect of classicality has been tacitly assumed from the beginning. In other words, the ‘Quantum Darwinism’ program is fatally circular.

The assumption generating the circularity generally takes the form of a pre-designated ‘system’ as separable from its ‘environment’. The presupposed ‘system’ acquires ‘decohered’ observable states (or ‘stable states’) because it has been presumed to be distinguishable from a designated set of environmental subsystems, all of which are assumed to be distinguishable (i.e., phases randomized) from each other as well as from the system. The total system comprising the observed ‘system’ and each of the environmental subsystems is assumed to be representable by a product state. The only correlations between the system and the environmental subsystems are assumed to be established via the designated Hamiltonians.

However, in the absence of this pre-designated partitioning of all the degrees of freedom into system and its surroundings (i.e. measurement apparatus and/or environment), where the initial phase of each subsystem is random, the desired ‘decoherence’ and ‘einselection’ does not follow. The partitioning is inevitably based on what a human observer would be able to identify and measure (such as distinguishable atoms and molecules), making the account dependent on assuming the classical realm of the observer. This fatally conflicts with the claim that classicality emerges naturally from the quantum realm: absent a pre-partitioning of all degrees of freedom into uncorrelated, randomized systems of interest, there is no ‘emergence’ of classical pointer states. The quantum realm does not have any a priori preference for the assumed uncorrelated degrees of freedom in a unitary-only dynamics (i.e, without collapse). On the contrary, a unitary-only evolution would typically begin with a maximally entangled Universal state.

As noted in [2], we can of course observe the decoherence process experimentally, but this does not demonstrate that ‘einselection’ occurs in a unitary-only dynamics nor that classicality emerges in such a dynamics. A key missing ingredient in the Quantum Darwinism program is some real physical randomization process, such as collapse, that could create an observer-independent, physical partitioning at the quantum level. Absent such a process, Quantum Darwinism contains the same kind of logical circularity as Boltzmann’s H-Theorem, which attempted to ‘derive’ irreversible thermodynamical laws from reversible laws. Boltzmann inadvertently ‘smuggled in’ irreversibility via his assumption of ‘molecular chaos’, while Quantum Darwinists smuggle in classicality via their partitioning of the Universe into distinguishable systems of interest interacting with mutually randomized environmental subsystems.

This published refutation of the ‘Quantum Darwinism’ program is not addressed in [1]. To date, there has been no rebuttal to that refutation.


[1] Zurek, W. H. “Quantum Darwinism, Decoherence, and the Randomness of Quantum Jumps,” Physics Today, vol. 67, pp. 44-50 (2014). arxiv:1412.5206

[2] Kastner, R. E. “Einselection of pointer observables: The new H-theorem?” Studies in History and Philosophy of Modern Physics 48, 56-58 (2014). arxiv:1406.4126

[3] Additional references for the proposals termed ‘decoherence,’ ‘einselection,’ and ‘Quantum Darwinism’ are given in [2].