Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
Department of Physics &Astronomy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
Nat Commun. 2017 May 9;8:15149. doi: 10.1038/ncomms15149.
Understanding the causal influences that hold among parts of a system is critical both to explaining that system's natural behaviour and to controlling it through targeted interventions. In a quantum world, understanding causal relations is equally important, but the set of possibilities is far richer. The two basic ways in which a pair of time-ordered quantum systems may be causally related are by a cause-effect mechanism or by a common-cause acting on both. Here we show a coherent mixture of these two possibilities. We realize this nonclassical causal relation in a quantum optics experiment and derive a set of criteria for witnessing the coherence based on a quantum version of Berkson's effect, whereby two independent causes can become correlated on observation of their common effect. The interplay of causality and quantum theory lies at the heart of challenging foundational puzzles, including Bell's theorem and the search for quantum gravity.
理解系统各部分之间的因果影响，对于解释系统的自然行为以及通过有针对性的干预来控制它都至关重要。在量子世界中，理解因果关系同样重要，但可能性的范围要丰富得多。一对按时间顺序排列的量子系统可能存在因果关系的两种基本方式是通过因果机制或共同原因同时作用于两者。在这里，我们展示了这两种可能性的一种连贯混合。我们在量子光学实验中实现了这种非经典因果关系，并基于 Berkson 效应的量子版本推导出了一组用于见证相干性的标准，根据该标准，两个独立的原因可以在观察到它们共同的影响时变得相关。因果关系和量子理论的相互作用是具有挑战性的基础难题的核心，包括贝尔定理和对量子引力的探索。
