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arXiv:quant-ph/0405080 · Journal of Mathematical Physics

Initial states and decoherence of histories

Artur Scherer, Andrei N. Soklakov

Published Mar 24, 2005• 1 citation
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Abstract

We study decoherence properties of arbitrarily long histories constructed from a fixed projective partition of a finite dimensional Hilbert space. We show that decoherence of such histories for all initial states that are naturally induced by the projective partition implies decoherence for arbitrary initial states. In addition we generalize the simple necessary decoherence condition [Scherer et al., Phys. Lett. A 326, 307 (2004)] for such histories to the case of arbitrary coarse graining.

DOI
10.1063/1.1888030

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We study decoherence properties of arbitrarily long histories constructed from a fixed projective partition of a finite dimensional Hilbert space. We show that decoherence of such histories for all initial states that are naturally induced by the projective partition implies decoherence for arbitrary initial states. In addition we generalize the simple necessary decoherence condition [Scherer et al., Phys. Lett. A 326, 307 (2004)] for such histories to the case of arbitrary coarse graining.
Within the decoherent histories formulation of quantum mechanics, we consider arbitrarily long histories constructed from a fixed projective partition of a finite‐dimensional Hilbert space. We review some of the decoherence properties of such histories including simple necessary decoherence conditions and the dependence of decoherence on the initial state. Here we make a first step towards generalization of our earlier results [Scherer and Soklakov, e‐print: quant‐ph/0405080, (2004) and Scherer et al., Phys. Lett. A 326, 307, (2004)] to the case of approximate decoherence.
We study a formulation of quantum mechanics in which the central notion is that of a quantum-mechanical history---a sequence of events at a succession of times. The primary aim is to identify sets of ``decoherent'' (or ``consistent'') histories for the system. These are quantum-mechanical histories suffering negligible interference with each other, and, therefore, to which probabilities may be assigned. These histories may be found for a given system using the so-called decoherence functional. When the decoherence functional is exactly diagonal, probabilities may be assigned to the histories, and all probability sum rules are satisfied exactly. We propose a condition for approximate decoherence, and argue that it implies that most probability sum rules will be satisfied to approximately the same degree. We also derive an inequality bounding the size of the off-diagonal terms of the decoherence functional. We calculate the decoherence functional for some simple one-dimensional systems, with a variety of initial states. For these systems, we explore the extent to which decoherence is produced using two different types of coarse graining. The first type of coarse graining involves imprecise specification of the particle's position. The second involves coupling the particle to a thermal bath of harmonic oscillators and ignoring the details of the bath (the Caldeira-Leggett model). We argue that both types of coarse graining are necessary in general. We explicitly exhibit the degree of decoherence as a function of the temperature of the bath, and of the width to within which the particle's position is specified. We study the diagonal elements of the decoherence functional, representing the probabilities for the possible histories of the system. To the extent that the histories decohere, we show that the probability distributions are peaked about the classical histories of the system, with the distribution of their initial positions and momenta given by a smeared version of the Wigner function. We discuss this result in connection with earlier uses of the Wigner function in this context. We find that there is a certain amount of tension between the demands of decoherence and peaking about classical paths.
Discrete maps play an important role in the investigation \nof dynamical features of complex classical systems, especially \nwithin the theory of chaos. Similarly, quantum maps have proven \nto be a very useful mathematical tool within the study of complex \nquantum dynamical systems. Using the decoherent histories formulation \nof quantum mechanics we consider a particular framework for studying \nquantum maps which is motivated by the method of classical symbolic \ndynamics. Symbolic dynamics is known to be a very powerful method \nspecifically invented for the purpose of representing classical dynamical \nsystems by a discrete model that is suitable for information theoretic studies. \nOur framework uses the decoherent histories formalism which, \nsimilarly to classical symbolic dynamics, allows one to introduce information \ntheoretic quantities with respect to system dynamics. \nOur research within this framework can be viewed as a contribution \ntowards the development of a general theory of "quantum \nsymbolic dynamics". \n\nWe start by considering a special but very important example \nfor a quantum map: the quantum baker's map, invented for \nthe theoretical investigation of quantum chaos. \nHere we use the decoherent histories formalism to examine \nthe coarse-grained evolution of this map \nwith regard to the question of how classical predictability of \nthe evolution depends on the character of coarse-graining. \n%is affected by the choice of coarse-graining. \nWe demonstrate that hierarchical coarse-grainings display \ninteresting features with respect to this question. \n\nWe proceed with the investigation of decoherence properties \nof arbitrary unitary quantum maps. A number of interesting results \nis obtained within the framework of arbitrarily long histories \nconstructed from a fixed projective partition of a finite \ndimensional Hilbert space. In particular, we derive simple \nnecessary decoherence conditions, which employ only a single \niteration of a given unitary quantum map. Furthermore, \na surprising result is obtained with regard to the \nfundamental question of how the choice of the \ninitial state affects decoherence of histories. \nWithin the considered framework we show that if decoherence \nis established for arbitrary history lengths and all initial \nstates from the smallest natural set of states that \ncan be associated with the framework, then we get decoherence \nof such histories for arbitrary initial states. \nFinally, we make first steps towards proving analogous\nresults for approximate decoherence and suggest \nvarious questions for future research.\n
We examine the decoherence properties of a quantum open system as modeled by a quantum optical system in the Markov regime. We look for decoherence in both the environment-induced superselection (EIS) and consistent histories frameworks. We propose a general measure of the coherence of the reduced density matrix and find that EIS decoherence occurs in a number of bases for this model. The degree of "diagonality" achieved increases with bath temperature. We evaluate the decoherence functional of consistent histories for coarse-grained phase-space two-time projected histories. Using the measures proposed by Dowker and Halliwell we find that the consistency of the histories improves with increasing bath temperature, time, and final grain size and decreases with initial grain size. The peaking increases with increasing grain size and decreases with increasing bath temperature. Adopting the above-proposed measure of "coherence" to the decoherence functional gives similar results. The results agree in general with expectations while the anomalous dependence of the consistency on the initial grain size is discussed.
We study the decoherence properties of a certain class of Markovian quantum open systems from both the Decohering Histories and Environment Induced Superselection paradigms. The class studied includes many familiar quantum optical cases. For this class, we show that there always exists a basis which leads to {\em exactly} consistent histories for any coarse graining {\em irrespective} of the initial conditions. The magnitude of the off--diagonal elements of the reduced density matrix ρρρ in this basis however, depends on the initial conditions. Necessary requirements for classicality as advanced by the two paradigms are thus in direct conflict in these systems.