Quantifying causal influences in the presence of a quantum common cause

TL;DR

This paper introduces a framework to quantify causal influences in quantum systems, revealing that all pure entangled states violate classical causal bounds even without Bell inequality violations, challenging traditional causality notions.

Contribution

It presents a novel method to estimate causal influences in quantum scenarios independent of interventions and Bell inequalities, highlighting quantum entanglement's impact on causality.

Findings

All pure bipartite entangled states violate classical causal bounds.

The framework applies to classical, quantum, and post-quantum common causes.

Violations occur even in scenarios where Bell inequalities are not violated.

Abstract

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.