In a quantum measurement process, classical information about the measured system spreads throughout the environment. Meanwhile, quantum information about the system becomes inaccessible to local observers. Here we prove a result about quantum channels indicating that an aspect of this phenomenon is completely general. We show that for any evolution of the system and environment, for everywhere in the environment excluding an $O(1)$-sized region we call the "quantum Markov blanket," any locally accessible information about the system must be approximately classical, i.e. obtainable from some fixed measurement. The result strengthens the earlier result of Brandão et al. ($\textit{Nat. comm. 6:7908}$) in which the excluded region was allowed to grow with total environment size. It may also be seen as a new consequence of the principles of no-cloning or monogamy of entanglement. Our proof offers a constructive optimization procedure for determining the "quantum Markov blanket" region, as well as the effective measurement induced by the evolution. Alternatively, under channel-state duality, our result characterizes the marginals of multipartite states.

中文翻译：

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一般多方状态和通道中的涌现经典性

在量子测量过程中，关于被测系统的经典信息会传播到整个环境。与此同时，本地观察者无法获得有关该系统的量子信息。在这里，我们证明了一个关于量子通道的结果，表明这种现象的一个方面是完全普遍的。我们表明，对于系统和环境的任何演化，对于环境中除 $O(1)$ 大小的区域之外的任何地方，我们称之为“量子马尔可夫毯”，关于系统的任何局部可访问信息都必须近似经典，即可以从一些固定的测量中获得。结果加强了 Brandão 等人的早期结果。($\textit{Nat. comm. 6:7908}$) 其中允许排除的区域随着总环境大小而增长。它也可以被视为非克隆或纠缠一夫一妻制原则的新结果。我们的证明提供了一个建设性的优化程序，用于确定“量子马尔可夫毯”区域，以及由演化引起的有效测量。或者，在通道状态二元性下，我们的结果表征了多部分状态的边缘。