Quantum correlations with no causal order

TLDR

Causal order is a fundamental concept in physics, yet its origin and necessity remain unclear, prompting investigation into whether it is an inherent property of nature. The study aims to explore whether quantum mechanics can accommodate correlations without a predefined causal order. The authors employ a quantum framework for multipartite correlations that assumes only local quantum validity, foregoing any global causal structure. The framework reproduces all known causal‑order correlations and reveals new correlations that violate a causal inequality, indicating that causal order emerges only in the classical limit.

Abstract

The idea that events obey a definite causal order is deeply rooted in our understanding of the world and at the basis of the very notion of time. But where does causal order come from, and is it a necessary property of nature? Here, we address these questions from the standpoint of quantum mechanics in a new framework for multipartite correlations that does not assume a pre-defined global causal structure but only the validity of quantum mechanics locally. All known situations that respect causal order, including space-like and time-like separated experiments, are captured by this framework in a unified way. Surprisingly, we find correlations that cannot be understood in terms of definite causal order. These correlations violate a 'causal inequality' that is satisfied by all space-like and time-like correlations. We further show that in a classical limit causal order always arises, which suggests that space-time may emerge from a more fundamental structure in a quantum-to-classical transition. Causal order is a concept that is engrained in the standard understanding of time, both in classical and quantum mechanics. Oreshkovet al.generalize the standard formalism of quantum theory to a framework with no pre-existing causal order, and find a new class of correlations that have no analogue in the classical world.