Any Physical Theory of Nature Must Be Boundlessly Multipartite Nonlocal
Quantum correlations are obtained when multiple parties perform independent measurements on a shared quantum state. For instance, (a) depicts quantum correlations arising from Alice, Bob and Charlie each performing measurements on the quantum state Q.
Bell’s seminal theorem proves that certain correlations predicted by quantum theory resist explanation in terms of any local theory based on shared randomness (for instance when the quantum state is replaced with a dice in (a)). But what about alternative explanations for quantum correlations, in terms of an exotic bipartite resources E of an undiscovered causal theory generalising quantum theory and shared randomness, as in (b)?
In our work we find that no such exotic causal theory can account for all quantum correlations, thus generalizing Bell’s theorem. We propose a concrete experiment as in figure a, which achieves correlations impossible to simulate with figure b scenario. This certifies the fact that No Bipartite-Nonlocal Causal Theory Can Explain Nature’s Quantum Correlations. It also opens a new technological challenge to experimentalists toward the realisation of our proposed experiment.
We generalise this result to more general causal theory based on k-partite (and not only bipartite) systems and shared randomness.
On a more practical aspect, our work questions the standard notion of genuine multipartite nonlocality and propose a stricter redefinition of it, more compatible with the notion of no-signalling.