People can only achieve perfect agreement on a random choice if they share a common cause, and this rule is hard-coded into the quantum world.
This new coordination principle states that randomized agreement across a network requires a shared causal link. This mathematical proof applies to both classical and quantum systems, providing a way to certify that two events are related. Information theorists previously lacked a rigorous way to define this requirement for perfect coordination. This discovery allows for the creation of more secure and reliable quantum communication networks. It effectively provides a 'trust' metric that can be verified through the fundamental laws of physics. It proves that coordination is not just a choice but a physical necessity driven by shared history.
Coordination Requires a Common Cause in Quantum Theory
arXiv · 2605.03120
We propose a novel causal principle that is a genuinely multipartite extension of Reichenbach's common cause principle, namely, the coordination principle: parties in a network can achieve perfect randomized coordination--in particular, agree on a uniformly random output--only if they all share a common cause. We show that this principle does not follow from the standard no-signaling and independence principles by providing an explicit theory satisfying all these principles while violating the c