Gachechiladze Mariami, Miklin Nikolai, Chaves Rafael
Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany.
International Centre for Theory of Quantum Technologies (ICTQT), University of Gdansk, 80-308 Gdańsk, Poland.
Phys Rev Lett. 2020 Dec 4;125(23):230401. doi: 10.1103/PhysRevLett.125.230401.
Quantum mechanics challenges our intuition on the cause-effect relations in nature. Some fundamental concepts, including Reichenbach's common cause principle or the notion of local realism, have to be reconsidered. Traditionally, this is witnessed by the violation of a Bell inequality. But are Bell inequalities the only signature of the incompatibility between quantum correlations and causality theory? Motivated by this question, we introduce a general framework able to estimate causal influences between two variables, without the need of interventions and irrespectively of the classical, quantum, or even postquantum nature of a common cause. In particular, by considering the simplest instrumental scenario-for which violation of Bell inequalities is not possible-we show that every pure bipartite entangled state violates the classical bounds on causal influence, thus, answering in negative to the posed question and opening a new venue to explore the role of causality within quantum theory.
量子力学挑战了我们对自然中因果关系的直觉。一些基本概念，包括赖兴巴赫的共同原因原则或局域实在论的概念，必须重新考虑。传统上，这通过违反贝尔不等式来证明。但是贝尔不等式是量子关联与因果理论不相容的唯一标志吗？受这个问题的启发，我们引入了一个通用框架，能够估计两个变量之间的因果影响，而无需干预，且与共同原因的经典、量子甚至后量子性质无关。特别是，通过考虑最简单的工具场景——对于这种场景，不可能违反贝尔不等式——我们表明，每一个纯二分纠缠态都违反了因果影响的经典界限，因此，对所提出的问题给出了否定回答，并为探索因果关系在量子理论中的作用开辟了一个新途径。
