Experimental Demonstration that No Tripartite-Nonlocal Causal Theory Explains Nature's Correlations

TL;DR

This paper experimentally demonstrates that quantum correlations from tripartite entangled states cannot be explained by any causal theory limited to bipartite resources, confirming the fundamental tripartite nature of quantum entanglement.

Contribution

The authors experimentally verify tripartite quantum correlations that cannot be reproduced by bipartite causal theories, introducing a new device-independent witness and extending results to four-partite states.

Findings

Experimental violation of bipartite causal theories using GHZ_3 states

High-fidelity GHZ_3 states achieved (fidelity 0.9741)

Correlations incompatible with bipartite-only causal explanations

Abstract

Quantum theory predicts the existence of genuinely tripartite-entangled states, which cannot be obtained from local operations over any bipartite entangled states and unlimited shared randomness. Some of us recently proved that this feature is a fundamental signature of quantum theory. The state $\left|{GHZ}_3\right\rangle=(\left|000\right\rangle+\left|111\right\rangle)/\sqrt{2}$ gives rise to tripartite quantum correlations which cannot be explained by any causal theory limited to bipartite nonclassical common causes of any kind (generalising entanglement) assisted with unlimited shared randomness. Hence, any conceivable physical theory which would reproduce quantum predictions will necessarily include genuinely tripartite resources. In this work, we verify that such tripartite correlations are experimentally achievable. We derive a new device-independent witness capable of falsifying causal theories wherein nonclassical resources are merely bipartite. Using a high-performance photonic $\left|{GHZ}_3\right\rangle$ states with fidelities of $ 0.9741\pm0.002$, we provide a clear experimental violation of that witness by more than $26.3$ standard deviation, under the locality and fair sampling assumption. We generalise our work to the $\left|{GHZ}_4\right\rangle$ state, obtaining correlations which cannot be explained by any causal theory limited to tripartite nonclassical common causes assisted with unlimited shared randomness.