量子鸡-蛋困境：延迟选择因果顺序和因果不可分离性的现实
Quantum chicken-egg dilemmas: Delayed-choice causal order and the reality of causal non-separability
摘要：
最近的框架描述量子力学在缺乏一个整体的因果顺序承认因果不确定的过程的存在，它是不可能的因果顺序的事件A和B。这些框架甚至允许违反所谓的因果不等式的过程，这类似于贝尔不等式。然而，在一般情况下，这些奇异过程的物理性仍在辩论中，使人们对其基础相关性产生疑问。虽然已知因果不确定的过程可以通过量子线路以及附加的条件事件C来概率地实现，但是迄今为止对这种实现方案的本体论意义的具体见解是有限的。在这里，我们表明，因果关系不确定的过程可以实现与计划，其中C只作为一个经典的标志预示着因果关系不确定的过程实现。然后，我们表明，有过程中，任何纯粹的条件测量C导致一个因果不定的过程A和B，从而建立因果不确定性作为一个独立的基础数量。最后，我们证明，量子力学允许的现象，C可以确定性地决定是否A之前B或反之亦然，没有信号。这类似于惠勒著名的延迟选择实验，该实验在量子力学中建立了确定的因果顺序作为工具- \textit{dependent}属性。
英文摘要：
Recent frameworks describing quantum mechanics in the absence of a global causal order admit the existence of causally indefinite processes, where it is impossible to ascribe causal order for events A and B. These frameworks even allow for processes that violate the so-called causal inequalities, which are analogous to Bell's inequalities. However, the physicality of these exotic processes is, in the general case, still under debate, bringing into question their foundational relevance. While it is known that causally indefinite processes can be probabilistically realised by means of a quantum circuit, along with an additional conditioning event C, concrete insights into the ontological meaning of such implementation schemes have heretofore been limited. Here, we show that causally indefinite processes can be realised with schemes where C serves only as a classical flag heralding which causally indefinite process was realised. We then show that there are processes where any pure conditioning measurement of C leads to a causally indefinite process for A and B, thus establishing causal indefiniteness as a basis-independent quantity. Finally, we demonstrate that quantum mechanics allows for phenomena where C can deterministically decide whether A comes before B or vice versa, without signalling to either. This is akin to Wheeler's famous delayed-choice experiment establishing definite causal order in quantum mechanics as instrument-\textit{dependent} property.
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