  Non-locality stands nowadays not only as one of the cornerstones of quantum
theory, but also plays a crucial role in quantum information processing.
Several experimental investigations of nonlocality have been carried out over
the years. In spite of their fundamental relevance, however, all previous
experiments do not consider a crucial ingredient that is ubiquitous in quantum
networks: the fact that correlations between distant parties are mediated by
several, typically independent, sources of quantum states. Here, using a
photonic setup we investigate a quantum network consisting of three spatially
separated nodes whose correlations are mediated by two independent sources.
This scenario allows for the emergence of a new kind of non-local correlations
that we experimentally witness by violating a novel Bell inequality. Our
results provide the first experimental proof-of-principle of generalizations of
Bell's theorem for networks, a topic that has attracted growing attention and
promises a novel route for quantum communication protocols.
