在时间非局域化子系统上违反因果不等式过程的存在性
Existence of processes violating causal inequalities on time-delocalised subsystems
AI总结：
本文证明了三方过程的所有幺正扩展均可在时间非局域化子系统上实现，该类包含违反因果不等式的过程，并论证了这种违反在表明变量间无明确因果顺序方面的意义。
摘要：
已有研究表明，在理论上存在这样的量子和经典过程：其中不同参与方执行的操作不以明确的因果顺序发生。一个核心问题是此类过程是否以及如何在实践中被实现。为了为某些此类过程在标准量子理论中存在实现这一观点提供严格论证，人们引入了时间非局域化量子子系统的概念。在本文中，我们证明了三方过程的所有幺正扩展都存在在时间非局域化子系统上的实现。值得注意的是，该类包含违反因果不等式的过程，即能够产生以设备无关方式见证与明确因果顺序不相容的关联。我们考虑了此类具有幺正扩展的三方经典过程的一个已知显著例子，并研究了其在时间非局域化子系统上的实现。随后，我们讨论了在此设定下违反因果不等式意味着什么的问题，并论证了这确实是一个有意义的概念，用以表明感兴趣变量之间不存在明确的因果顺序。
英文摘要：
It has been shown that it is theoretically possible for there to exist quantum and classical processes in which the operations performed by separate parties do not occur in a well-defined causal order. A central question is whether and how such processes can be realised in practice. In order to provide a rigorous argument for the notion that certain such processes have a realisation in standard quantum theory, the concept of time-delocalised quantum subsystem has been introduced. In this paper, we show that realisations on time-delocalised subsystems exist for all unitary extensions of tripartite processes. Remarkably, this class contains processes that violate causal inequalities, i.e., that can generate correlations that witness the incompatibility with definite causal order in a device-independent manner. We consider a known striking example of such a tripartite classical process that has a unitary extension, and study its realisation on time-delocalised subsystems. We then discuss the question of what a violation of causal inequalities implies in this setting, and argue that it is indeed a meaningful concept to show the absence of a definite causal order between the variables of interest.
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