Quantum Correlations Take a New Shape

Bell experiments provide perhaps the clearest demonstration of how radically quantum physics departs from classical physics. In such experiments, pairs of quantum particles are prepared in special entangled states and separately measured. The measurement outcomes are found to be correlated more strongly than allowed by any local, classical theory. The last few decades have seen intense theoretical and experimental efforts to investigate this phenomenon—with the first watertight demonstrations only recently achieved—and to apply it to quantum information. But all that work stuck to the same template used by John Stewart Bell over 50 years ago: two or more particles are jointly prepared by the same source and then independently measured. Now, Marc-Olivier Renou of the University of Geneva and colleagues have broken that mold with the discovery of a new form of quantum correlation arising in a more complex, triangular network of sources and observers [1]. This result offers a tantalizing hint that there may be a whole zoo of nonclassical correlations that are fundamentally different from those identified by Bell.