Causal classification of spatiotemporal quantum correlations

TL;DR

This paper establishes criteria to identify atemporal quantum correlations from measurement outcomes, revealing an intrinsic arrow of time and a new measure of spatial quantum correlation distinct from entanglement.

Contribution

It introduces necessary and sufficient conditions for classifying quantum correlations as atemporal, highlighting their asymmetry under time reversal and their relation to causal structures.

Findings

Quantum correlations can be classified as atemporal or not.

Atemporal correlations exhibit asymmetry under time reversal.

Certain quantum correlations have an intrinsic arrow of time.

Abstract

From correlations in measurement outcomes alone, can two otherwise isolated parties establish whether such correlations are atemporal? That is, can they rule out that they have been given the same system at two different times? Classical statistics says no, yet quantum theory disagrees. Here, we introduce the necessary and sufficient conditions by which such quantum correlations can be identified as atemporal. We demonstrate the asymmetry of atemporality under time reversal, and reveal it to be a measure of spatial quantum correlation distinct from entanglement. Our results indicate that certain quantum correlations possess an intrinsic arrow of time, and enable classification of general quantum correlations across space-time based on their (in)compatibility with various underlying causal structures.