Quantum-coherent mixtures of causal relations

TL;DR

This paper demonstrates a quantum-coherent mixture of cause-effect and common cause relations in a quantum system, revealing richer causal structures and providing criteria to witness quantum coherence in causal relations.

Contribution

The authors experimentally realize and characterize a nonclassical causal mixture in a quantum optics setup, advancing understanding of quantum causality beyond classical frameworks.

Findings

Demonstration of a quantum-coherent mixture of causal relations

Derivation of criteria for witnessing quantum causal coherence

Experimental realization using quantum optics

Abstract

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 upon 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.