Navegando por Autor "Sciarrino, Fabio"

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Ab initio experimental violation of Bell inequalities
(Physical Review Research, 2022-02-28) Poderini, Davide; Polino, Emanuele; Rodari, Giovanni; Suprano, Alessia; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
The violation of a Bell inequality is the paradigmatic example of device-independent quantum information: The nonclassicality of the data is certified without the knowledge of the functioning of devices. In practice, however, all Bell experiments rely on the precise understanding of the underlying physical mechanisms. Given that, it is natural to ask: Can one witness nonclassical behavior in a truly black-box scenario? Here, we propose and implement, computationally and experimentally, a solution to this ab initio task. It exploits a robust automated optimization approach based on the stochastic Nelder-Mead algorithm. Treating preparation and measurement devices as black boxes, and relying on the observed statistics only, our adaptive protocol approaches the optimal Bell inequality violation after a limited number of iterations for a variety photonic states, measurement responses, and Bell scenarios. In particular, we exploit it for randomness certification from unknown states and measurements. Our results demonstrate the power of automated algorithms, opening a venue for the experimental implementation of device-independent quantum technologies
Causal Networks and Freedom of Choice in Bell’s Theorem
(PRX Quantum, 2021-11-03) Araújo, Rafael Chaves Souto; Moreno Filho, Marcos George Magalhães; Polino, Emanuele; Poderini, Davide; Agresti, Iris; Suprano, Alessia; Barros, Mariana Rodrigues; Carvacho, Gonzalo; Wolfe, Elie; Canabarro, Askery; Spekkens, Robert W.; Sciarrino, Fabio
Bell’s theorem is typically understood as the proof that quantum theory is incompatible with local-hidden-variable models. More generally, we can see the violation of a Bell inequality as witnessing the impossibility of explaining quantum correlations with classical causal models. The violation of a Bell inequality, however, does not exclude classical models where some level of measurement dependence is allowed, that is, the choice made by observers can be correlated with the source generating the systems to be measured. Here, we show that the level of measurement dependence can be quantitatively upper bounded if we arrange the Bell test within a network. Furthermore, we also prove that these results can be adapted in order to derive nonlinear Bell inequalities for a large class of causal networks and to identify quantumly realizable correlations that violate them.
Criteria for nonclassicality in the prepare-and-measure scenario
(Physical Review Research, 2020-10-20) Poderini, Davide; Brito, Samuraí Gomes de Aguiar; Nery, Ranieri Vieira; Sciarrino, Fabio; Araújo, Rafael Chaves Souto
Quantum communication networks involving the preparation, sharing, and measurement of quantum states are ubiquitous in quantum information. Of particular relevance within this context is to understand under which conditions a given quantum resource can give rise to correlations incompatible with a classical explanation. Here we consider the so-called prepare-and-measure scenario, in which a quantum or classical message with bounded dimension is transmitted between two parties. In this scenario we derive criteria witnessing whether a set of quantum states can lead or not to nonclassical correlations. Based on that, we show that quantum resources that can only give rise to classical correlations in the simplest prepare-and-measure scenario can have their nonclassicality witnessed if we increase the number of preparations or measurements.
Device-independent test of a delayed choice experiment
(American Physical Society, 2019-08-12) Polino, Emanuele; Agresti, Iris; Poderini, Davide; Carvacho, Gonzalo; Milani, Giorgio; Lemos, Gabriela Barreto; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
The wave or particle duality has long been considered a fundamental signature of the nonclassical behavior of quantum phenomena, especially in a delayed choice experiment, where the experimental setup revealing either the particle or the wave nature of the system is decided after the system has entered the apparatus. However, as counterintuitive as it might seem, usual delayed choice experiments do have a simple causal explanation. Here, we take a different route and under a natural assumption about the dimensionality of the system under examination, we present an experimental proof of the nonclassicality of a delayed choice experiment based on the violation of a dimension witness inequality. Our conclusion is reached in a device-independent and detection loophole-free manner, that is, based solely on the observed data and without the need of special assumptions about the measurement apparatus
Device-independent witness for the nonobjectivity of quantum dynamics
(Physical Review A, 2023-09-05) Araújo, Rafael Chaves Souto; Rodari, Giovanni; Moreno Filho, Marcos George Magalhães; Polino, Emanuele; Nery, Ranieri Vieira; Suprano, Alessia; Duarte, Cristhiano; Sciarrino, Fabio; Poderini, Davide
Quantum Darwinism offers an explanation for the emergence of classical objective features (those we are used to at macroscopic scales) from quantum properties at the microscopic level. The interaction of a quantum system with its surroundings redundantly proliferates information to many parts of the environment, turning it accessible and objective to different observers. However, given that one cannot probe the quantum system directly, only its environment, how to determine whether an unknown quantum property can be deemed objective? Here we propose a probabilistic framework to analyze this question and show that objectivity implies a Bell-like inequality. Among several other results, we show quantum violations of this inequality, a device-independent proof of the nonobjectivity of quantum correlations. We also implement a photonic experiment where the temporal degree of freedom of photons is the quantum system of interest, while their polarization acts as the environment. Employing a fully black-box approach, we achieve the violation of a Bell-like inequality, thus certifying the nonobjectivity of the underlying quantum dynamics in a fully device-independent framework
Experimental Connection between the Instrumental and Bell Inequalities
(Multidisciplinary Digital Publishing, 2019-07-18) Agresti, Iris; Carvacho, Gonzalo; Poderini, Davide; Aolita, Leandro; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
An investigated process can be studied in terms of the causal relations among the involved variables, representing it as a causal model. Some causal models are particularly relevant, since they can be tested through mathematical constraints between the joint probability distributions of the observables. This is a valuable tool because, if some data violates the constraints of a causal model, the implication is that the observed statistics is not compatible with that causal structure. Strikingly, when non-classical correlations come to play, a discrepancy between classical and quantum causal predictions can arise, producing a quantum violation of the classical causal constraints. The simplest scenario admitting such quantum violation is given by the instrumental causal processes. Here, we experimentally violate an instrumental test on a photonic platform and show how the quantum correlations violating the CHSH inequality can be mapped into correlations violating an instrumental test, despite the different forms of non-locality they display. Indeed, starting from a Bell-like scenario, we recover the violation of the instrumental scenario through a map between the two behaviours, which includes a post-selection of data and then we test an alternative way to violate the CHSH inequality, adopting the instrumental process platform
Experimental genuine tripartite nonlocality in a quantum triangle network
(PRX Quantum, 2022-09-21) Araújo, Rafael Chaves Souto; Poderini, Davide; Polino, Emanuele; Agresti, Iris; Vera, Gonzalo Alfredo Carvacho; Canabarro, Askery; Wolfe, Elie; Suprano, Alessia; Sciarrino, Fabio
Quantum networks are the center of many of the recent advances in quantum science, not only leading to the discovery of new properties in the foundations of quantum theory but also allowing for novel communication and cryptography protocols. It is known that networks beyond that in the paradigmatic Bell’s theorem imply new and sometimes stronger forms of nonclassicality. Due to a number of practical difficulties, however, the experimental implementation of such networks remains far less explored. Going beyond what has been previously tested, here we verify the nonlocality of an experimental triangle network, consisting of three independent sources of bipartite entangled photon states interconnecting three distant parties. By performing separable measurements only and evaluating parallel chained Bell inequalities, we show that such networks can lead to a genuine form of tripartite nonlocality, where classical models are unable to mimic the quantum predictions even if some of the parties are allowed to communicate
Experimental nonclassicality in a causal network without assuming freedom of choice
(Nature Communications, 2023-02-17) Polino, Emanuele; Poderini, Davide; Rodari, Giovanni; Agresti, Iris; Suprano, Alessia; Carvacho, Gonzalo; Wolfe, Elie; Canabarro, Askery; Moreno Filho, Marcos George Magalhães; Milani, Giorgio; Spekkens, Robert W.; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
In a Bell experiment, it is natural to seek a causal account of correlations wherein only a common cause acts on the outcomes. For this causal structure, Bell inequality violations can be explained only if causal dependencies are modeled as intrinsically quantum. There also exists a vast landscape of causal structures beyond Bell that can witness nonclassicality, in some cases without even requiring free external inputs. Here, we undertake a photonic experiment realizing one such example: the triangle causal network, consisting of three measurement stations pairwise connected by common causes and no external inputs. To demonstrate the nonclassicality of the data, we adapt and improve three known techniques: (i) a machine-learning-based heuristic test, (ii) a data-seeded inflation technique generating polynomial Bell-type inequalities and (iii) entropic inequalities. The demonstrated experimental and data analysis tools are broadly applicable paving the way for future networks of growing complexity
Experimental test of quantum causal influences
(Sciance Advances, 2022-02-25) Agresti, Iris; Poderini, Davide; Polacchi, Beatrice; Miklin, Nikolai; Gachechiladze, Mariami; Suprano, Alessia; Polino, Emanuele; Milani, Giorgio; Carvacho, Gonzalo; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
Since Bell’s theorem, it is known that local realism fails to explain quantum phenomena. Bell inequality violations manifestly show the incompatibility of quantum theory with classical notions of cause and effect. As recently found, however, the instrumental scenario—a pivotal tool in causal inference—allows for nonclassicality signatures going beyond this paradigm. If we are not limited to observational data and can intervene in our setup, then we can witness quantum violations of classical bounds on the causal influence among the involved variables even when no Bell-like violation is possible. That is, through interventions, the quantum behavior of a system that would seem classical can be demonstrated. Using a photonic setup—faithfully implementing the instrumental causal structure and switching between observation and intervention run by run—we experimentally witness such a nonclassicality. We also test quantum bounds for the causal influence, showing that they provide a reliable tool for quantum causal modeling
Machine-learning-based device-independent certification of quantum networks
(Physical Review Research, 2023-04-10) D’Alessandro, Nicola; Polacch, Beatrice; Moreno Filho, Marcos George Magalhães; Polino, Emanuele; Araújo, Rafael Chaves Souto; Agresti, Iris; Sciarrino, Fabio
Witnessing nonclassical behavior is a crucial ingredient in quantum information processing. For that, one has to optimize the quantum features a given physical setup can give rise to, which is a hard computational task currently tackled with semidefinite programming, a method limited to linear objective functions and that becomes prohibitive as the complexity of the system grows. Here, we propose an alternative strategy, which exploits a feedforward artificial neural network to optimize the correlations compatible with arbitrary quantum networks. A remarkable step forward with respect to existing methods is that it deals with nonlinear optimization constraints and objective functions, being applicable to scenarios featuring independent sources and nonlinear entanglement witnesses. Furthermore, it offers a significant speedup in comparison with other approaches, thus allowing to explore previously inaccessible regimes. We also extend the use of the neural network to the experimental realm, a situation in which the statistics are unavoidably affected by imperfections, retrieving device-independent uncertainty estimates on Bell-like violations obtained with independent sources of entangled photon states. In this way, this work paves the way for the certification of quantum resources in networks of growing size and complexity
Maximal qubit violation of n-locality inequalities in a star-shaped quantum network
(IOP Publishing, 2017-11-14) Andreoli, Francesco; Carvacho, Gonzalo; Santodonato, Luca; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
Bellʼs theorem was a cornerstone for our understanding of quantum theory and the establishment of Bell non-locality played a crucial role in the development of quantum information. Recently, its extension to complex networks has been attracting growing attention, but a deep characterization of quantum behavior is still missing for this novel context. In this work we analyze quantum correlations arising in the bilocality scenario, that is a tripartite quantum network where the correlations between the parties are mediated by two independent sources of states. First, we prove that non-bilocal correlations witnessed through a Bell-state measurement in the central node of the network form a subset of those obtainable by means of a local projective measurement. This leads us to derive the maximal violation of the bilocality inequality that can be achieved by arbitrary two-qubit quantum states and arbitrary local projective measurements.Wethen analyze in details the relation between the violation of the bilocality inequality and theCHSHinequality. Finally, we show how our method can be extended to the n-locality scenario consisting of n two-qubit quantum states distributed among n + 1nodes of a star-shaped network

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Navegando por Autor "Sciarrino, Fabio"