Quantum nonlocality

The concepts of entanglement and nonlocality are now recognized as defining features of quantum theory. Distant observers sharing a quantum system prepared in an entangled state, can establish strong correlations, which could provably not been achieved in any theory satisfying a natural constraint of locality. The theoretical and experimental explorations of quantum entanglement and nonlocality achieved tremendous progress in recent years. 

Moreover, the discovery of a general theory of nonlocality, featuring nonlocal correlations more powerful than those of quantum mechanics (but nevertheless consistent with relativity) has raised deep questions about the foundations of quantum theory. Why is nonlocality limited in quantum physics? Is there a physical principle from which quantum theory would emerge? There has been tremendous progress in this research area worldwide during the last years. This line of research offers potential to deepen our understanding of the foundations of quantum mechanics.

The Triangle Network. Image taken from Renou et al. PHYSICAL REVIEW LETTERS 123, 140401 (2019).