Non-Local Boxes for Networks

Nonlocal boxes are conceptual tools that capture the essence of the phenomenon of quantum non-locality, central to modern quantum theory and quantum technologies. We introduce network nonlocal boxes tailored for quantum networks under the natural assumption that these networks connect independent sources and do not allow signaling. Hence, these boxes satisfy the No-Signaling and Independence (NSI) principle. For the case of boxes without inputs, connecting pairs of sources and producing binary outputs, we prove that there is an essentially unique network nonlocal box with local random outputs and maximal 2-box correlations: $E_2=\sqrt{2}-1$.