论文标题
无环的嵌入环状信息理论结构：无限期因果关系的结果
Embedding cyclic information-theoretic structures in acyclic spacetimes: no-go results for indefinite causality
论文作者
论文摘要
量子信息和时空物理群落中采用的因果关系概念是不同的。尽管这两个概念在物理实验中都起着作用，但在理论上，他们的一般相互作用却一无所知。我们开发了一个将两个因果关系概念联系起来的理论框架，同时也清楚地区分了它们。该框架通过反馈回路描述了量子操作的组成，以及在无环时期结构中所得的，可能是环状信息理论结构的嵌入。相对论因果关系（禁止静脉通信）是两种结构之间的图理论兼容条件。在我们的框架中制定无限期因果秩序（ICO）过程，我们阐明了无限期和周期性因果关系之间的联系，以及有关其身体的问题。特别是，有几种实验声称在Minkowski时空实施ICO过程，表现出明显的悖论：不确定的信息理论因果结构如何与确定的时空因果结构一致？我们通过无需定理来解决这一问题，表明，相对论因果关系的结果，（a）ICO过程的实现必然涉及时空中系统的非定位，并且（b）不愿意以确定的和ac causal Causal Order序列的范围来承认在一个良好的级别上的解释。这完全解决了明显的悖论，并对ICO实验的物理解释产生了影响，并且可以通过引入细粒度的概念来实现，该概念允许因果结构以不同的细节进行分析。我们的工作还阐明了量子信息处理的限制以及无限期因果关系的操作含义，在固定时空的背景下和之外。
The notions of causality adopted within the quantum information and spacetime physics communities are distinct. Although both notions play a role in physical experiments, their general interplay is little understood in theory. We develop a theoretical framework that connects the two causality notions, while also clearly distinguishing them. The framework describes a composition of quantum operations through feedback loops, and the embedding of the resulting, possibly cyclic information-theoretic structure in an acyclic spacetime structure. Relativistic causality (which forbids superluminal communication) follows as a graph-theoretic compatibility condition between the two structures. Formulating indefinite causal order (ICO) processes in our framework, we shed light on the links between indefinite and cyclic causality, and on questions regarding their physicality. In particular, there are several experiments that claim to implement ICO processes in Minkowski spacetime, presenting an apparent paradox: how can an indefinite information-theoretic causal structure be consistent with a definite spacetime causal structure? We address this through no-go theorems, showing that as a consequence of relativistic causality, (a) realisations of ICO processes necessarily involve the non-localisation of systems in spacetime and (b) will nevertheless admit an explanation in terms of a definite and acyclic causal order process, at a fine-grained level. This fully resolves the apparent paradox and bears implications for the physical interpretation of ICO experiments, and is achieved by introducing the concept of fine-graining that allows causal structures to be analysed at different levels of detail. Our work also sheds light on the limits of quantum information processing in spacetime and on the operational meaning of indefinite causality, within and beyond the context of a fixed spacetime.
