Experimental violation of n-locality in a star quantum network[1]

Abstract

The launch of a satellite capable of distributing entanglement through long distances and the first loophole-free violation of Bell inequalities are milestones indicating a clear path for the establishment of quantum networks. However, nonlocality in networks with independent entanglement sources has only been experimentally verified in simple tripartite networks, via the violation of bilocality inequalities. Here, by using a scalable photonic platform, we implement star-shaped quantum networks consisting of up to five distant nodes and four independent entanglement sources. We exploit this platform to violate the chained n-locality inequality and thus witness, in a device-independent way, the emergence of nonlocal correlations among the nodes of the implemented networks. These results open new perspectives for quantum information processing applications in the relevant regime where the observed correlations are compatible with standard local hidden variable models but are nonclassical if the independence of the sources is taken into account.

Experimental genuine tripartite nonlocality in a quantum triangle network

“…In spite of significant theoretical progress, understanding the correlations that such quantum networks can give rise to (see for instance the recent review [55]), the experimental side of this effort [56][57][58][59][60][61][62][63][64] is far less unexplored, due to a number of practical difficulties. In the particular case of a photonic platform, the most prominent of these difficulties stems from the probabilistic nature of the photons' generation and the synchronization process that becomes exponentially more demanding as the number of entanglement sources increase.…”

“…Furthermore, in many cases there * fabio.sciarrino@uniroma1.it is a need for entangled measurements, a requirement that can only be partially achieved with linear optics, or which requires device-dependent assumptions for the experimental implementation [56,57]. These issues are worsened when the independence of the sources is required to be physically justified, for instance, by using different and non-synchronized lasers to pump the generation crystals that act as sources [63]. For this reason, only two examples of photonic networks involving independent sources have been implemented in a device-independent manner so far, one concerning the bilocality scenario [56-58, 62, 64] and the other a star network with four sources [63].…”

“…These issues are worsened when the independence of the sources is required to be physically justified, for instance, by using different and non-synchronized lasers to pump the generation crystals that act as sources [63]. For this reason, only two examples of photonic networks involving independent sources have been implemented in a device-independent manner so far, one concerning the bilocality scenario [56-58, 62, 64] and the other a star network with four sources [63].…”

“…We experimentally realize the quantum triangle network using a platform based on the scheme of Ref. [63], showing that we can achieve genuine tripartite nonlocality without the (demanding!) employment of tripartite sources [69,70].…”