Experimental activation of quantum nonlocality via a quantum network
Luis Villegas-Aguilar, Emanuele Polino, Sergei Slussarenko, Nora Tischler, Farzad Ghafari, Eric G. Cavalcanti, Geoff J. Pryde, Marco Túlio Quintino, Kiarn T. Laverick, Lynden K. Shalm
Quantum nonlocality arising from entangled particles cannot be explained by any classical notion of local causality. In the standard Bell scenario with a source and two measurement stations, entangled states can lose their ability to exhibit nonlocality due to noisy conditions. Given the fundamental and technological significance of nonlocality and the fact that these noisy entangled states can naturally arise in practical situations, it is of utmost importance to unveil their nonlocality. Here, we demonstrate that nonlocal correlations can be generated from quantum states that cannot show nonlocality in bipartite Bell scenarios. We show how to robustly characterize these states and experimentally activate their nonlocality when embedded in a photonic network, by violating classical constraints in this quantum network. These results have implications in foundations of quantum nonlocality and direct applications to quantum technologies by significantly increasing the robustness of nonlocal correlations to noise.
Quantum nonlocality arising from entangled particles cannot be explained by any classical notion of local causality. In the standard Bell scenario with a source and two measurement stations, entangled states can lose their ability to exhibit nonlocality due to noisy conditions. Given the fundamental and technological significance of nonlocality and the fact that these noisy entangled states can naturally arise in practical situations, it is of utmost importance to unveil their nonlocality. Here, we demonstrate that nonlocal correlations can be generated from quantum states that cannot show nonlocality in bipartite Bell scenarios. We show how to robustly characterize these states and experimentally activate their nonlocality when embedded in a photonic network, by violating classical constraints in this quantum network. These results have implications in foundations of quantum nonlocality and direct applications to quantum technologies by significantly increasing the robustness of nonlocal correlations to noise.
Bell nonlocality refers to correlations between two distant, entangled particles that challenge classical notions of local causality. Beyond its foundational significance, nonlocality is crucial for device-independent technologies like quantum key distribution and randomness generation. Nonlocality quickly deteriorates in the presence of noise, and restoring nonlocal correlations requires additional resources. These often come in the form of many instances of the input state and joint measurements, incurring a significant resource overhead. Here, we experimentally demonstrate that single copies of Bell-local states, incapable of violating any standard Bell inequality, can give rise to nonlocality after being embedded into a quantum network of multiple parties. We subject the initial entangled state to a quantum channel that broadcasts part of the state to two independent receivers and certify the nonlocality in the resulting network by violating a tailored Bell-like inequality. We obtain these results without making any assumptions about the prepared states, the quantum channel, or the validity of quantum theory. Our findings have fundamental implications for nonlocality and enable the practical use of nonlocal correlations in real-world applications, even in scenarios dominated by noise.
Bell’s nonlocality—correlations between two distant, entangled particles that cannot be explained by physical theories based on local causality—remains one of the most celebrated results of quantum foundations. However, these nonlocal correlations eventually disappear under the presence of noise. In this experimental work, we show that nonlocality, inaccessible for noisy states in the two-party scenario, can be activated in a three-node photonic network structure with a single copy of the state. Our results open up new possibilities for network-based quantum information processing applications, central for the development of a future quantum internet.
Nonlocal correlations arise from measurements on tripartite entangled states can be classified into two types, one genuinely 3−way nonlocal and other local with respect to some bipartition. Still, whether a genuine tripartite entangled quantum state can exhibit genuine 3−way nonlocality, remains a challenging problem so far as measurement context is concerned. Here we introduce a novel approach in this regard. We consider three genuine tripartite mixed entangled quantum states none of which is genuinely 3−way nonlocal in original Bell scenario, but they can exhibit genuine 3−way nonlocality when distributed in some suitable multiple entanglement swapping network. So, genuine 3−way nonlocality is a nonadditive resource, which can be activated. Our result implies that genuine 3−way nonlocality of quantum states strongly depends on the measurement context contrary to the belief that genuine 3−way nonlocality can be exhibited only in original Bell scenario.
The exploration of the concept of nonlocality beyond standard Bell scenarios in quantum network architectures unveils fundamentally new forms of correlations that hold a strong potential for future applications of quantum communication networks [1]. It is necessary to adapt theoretical advances to realistic configurations to materialize this potential. Here we consider a quantum triangle network, illustrated in Fig. 1, for which it was shown in theory that, remarkably, quantum nonlocality without inputs can be demonstrated for sources with an arbitrarily small level of independence [2]. We realize experimentally such correlated sources,ρAB,ρAC, andρBC, by carefully engineering the output state of a single AlGaAs multiplexed entangled-photon source [3], exploiting energy-matched channels cut in its broad spectrum. This simulated triangle network is then used to violate a Bell-like inequality that we derive to capture the effect of noise in the correlations present in our system [4]. We also rigorously validate our findings by analyzing the mutual information between the generated states. Our results allow us to deepen our understanding of network nonlocality while also pushing its practical relevance for quantum communication networks.
Nonlocality arising in networks composed of several independent sources gives rise to phenomena radically different from that in standard Bell scenarios. Over the years, the phenomenon of network nonlocality in the entanglement-swapping scenario has been well investigated and demonstrated. However, it is known that violations of the so-called bilocality inequality used in previous experimental demonstrations cannot be used to certify the nonclassicality of their sources. This has put forward a stronger concept for nonlocality in networks, called full network nonlocality. Here, we experimentally observe full network nonlocal correlations in a network where the source-independence, locality, and measurement-independence loopholes are closed. This is ensured by employing two independent sources, rapid setting generation, and spacelike separations of relevant events. Our experiment violates known inequalities characterizing nonfull network nonlocal correlations by over 5 standard deviations, certifying the absence of classical sources in the realization.
The certification of randomness is essential for both fundamental science and information technologies. Unlike traditional random number generators, randomness obtained from nonlocal correlations is fundamentally guaranteed to be unpredictable. However, it is also highly susceptible to noise. Here, we show that extending the conventional bipartite Bell scenario to hybrid quantum networks -- which incorporate both quantum channels and entanglement sources -- enhances the robustness of certifiable randomness. Our protocol even enables randomness to be certified from Bell-local states, broadening the range of quantum states useful for this task. Through both theoretical analysis and experimental validation in a photonic network, we demonstrate enhanced performance and improved noise resilience.