Proof That a Complex Quantum Network Is Truly Quantum

I n 1964, John Stewart Bell predicted that correlations betweenmeasurements made by two parties on a pair of entangled particles could confirm the fundamental nonclassical nature of the quantumworld. In the past few years, researchers have performed various tests of Bell’s predictions that were rigorous enough to rule out classical explanations. Now researchers in China and Spain have done the same for a more complex system—a quantum network in which three parties makemeasurements on pairs of entangled particles generated by two sources [1]. The researchers say that their “stringent” confirmation of quantum phenomena is encouraging for the development of future secure quantum communication networks.