There May Be a Zoo of Non-classical Correlations Different than Bell’s

Viewpoint: Quantum Correlations Take a New Shape

Excerpts and salient points ~

+  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. 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.

+  All these studies involved the same “Bell scenario”: one common source prepares entanglement between two or more particles, and observers then perform randomly chosen, independent measurements on each particle. Over the last decade, however, researchers have started to investigate more complex networks of sources and measurements, hunting for new forms of nonclassicality that could possibly lead to quantum information applications. In the “bilocality” scenario [5], for instance, two independent sources share entangled pairs with three observers. This scenario can be used to study entanglement swapping—a way to entangle particles that do not directly interact. Another interesting scenario involves a triangular geometry: three independent sources on the triangle’s sides distribute entangled states to three measurement devices at the triangle’s corners.

+  Identifying these novel forms of correlation is only a first step. The nature of chosen states and measurements—inherently linked to the triangle geometry—is a strong hint that the correlations cannot be just an expression of standard Bell nonlocality. But further research is needed to conclusively prove that this is the case. Beyond the Bell scenario, and now the triangle scenario, there are many other scenarios in which new types of quantum correlations may be waiting to be discovered. Just as Bell nonlocality has proven to be a resource for cryptographic tasks—such as quantum key distribution and the generation of certifiably random bits—some of these new forms of quantum correlation could be put to work in new tasks tailored to the new scenarios. It’s hard to predict what these tasks might be, but the new work suggests that the triangle scenario might well be the best place to start looking for them.

Source:  APS physics.  Matthew F. Pusey,  Viewpoint: Quantum Correlations Take a New Shape…

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