Quantum correlations with no causal order

TL;DR

This paper introduces a quantum framework that allows for correlations without a definite causal order, challenging traditional notions of causality and suggesting space-time may emerge from quantum phenomena.

Contribution

It develops a new quantum framework for multipartite correlations that does not assume a fixed causal order, revealing correlations that violate causal inequalities.

Findings

Discovery of correlations violating causal inequalities

All known causal order-respecting correlations are encompassed by the framework

Classical limit restores definite causal order

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? We address these questions from the standpoint of quantum mechanics in a new framework for multipartite correlations which 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.