Quantifying causal influences in the presence of a quantum common cause
This work addresses foundational issues in quantum mechanics and causality theory, offering a novel approach to explore quantum correlations beyond Bell inequalities.
The authors tackled the problem of whether Bell inequalities are the only signature of quantum correlations' incompatibility with causality theory by introducing a framework to estimate causal influences without interventions, regardless of the nature of a common cause. They showed that every pure bipartite entangled state violates classical bounds on causal influence in an instrumental scenario, answering the question negatively and opening new avenues in quantum causality.
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 post-quantum 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.