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arXiv quant-ph/9601004quant-phgr-qchep-th
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流体动力学历史的退相干:一个简单的自旋模型

Decoherence of Hydrodynamic Histories: A Simple Spin Model

T. A. Brun, J. J. Halliwell

摘要:

在封闭系统的量子力学的退相干历史方法的背景下,Gell-Mann和Hartle论证了表征大型复杂系统的准经典域的典型变量是局部守恒密度的小体积上的积分-流体动力学变量。本文的目的是展示一些简单的模型,其中近似退相干产生的局部守恒的结果。我们导出了一个公式,它表明了局部守恒和近似退相干之间的明确联系。然后,我们考虑一类模型,由大量的弱相互作用的组件,其中的投影到局部密度可以分解成投影到两个替代品的个别组件。我们考虑的主要例子是一个一维的局部耦合自旋链,并且投影是在链的一个子部分中的总自旋上。我们计算的退相干功能的局部密度的历史,在极限时的组件的数量是非常大的。我们发现,消相干需要两件事:涂抹体积必须足够大,以确保近似守恒,局部密度必须划分到足够大的范围内,以确保免受量子涨落。

英文摘要:

In the context of the decoherent histories approach to the quantum mechanics of closed systems, Gell-Mann and Hartle have argued that the variables typically characterizing the quasiclassical domain of a large complex system are the integrals over small volumes of locally conserved densities -- hydrodynamic variables. The aim of this paper is to exhibit some simple models in which approximate decoherence arises as a result of local conservation. We derive a formula which shows the explicit connection between local conservation and approximate decoherence. We then consider a class of models consisting of a large number of weakly interacting components, in which the projections onto local densities may be decomposed into projections onto one of two alternatives of the individual components. The main example we consider is a one-dimensional chain of locally coupled spins, and the projections are onto the total spin in a subsection of the chain. We compute the decoherence functional for histories of local densities, in the limit when the number of components is very large. We find that decoherence requires two things: the smearing volumes must be sufficiently large to ensure approximate conservation, and the local densities must be partitioned into sufficiently large ranges to ensure protection against quantum fluctuations.

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