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URI Permanente para esta coleçãohttps://repositorio.ufrn.br/handle/123456789/30128

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Nonlocality in sequential correlation scenarios
(New Journal of Physics, 2014-03-28) Gallego, Rodrigo; Würflinge, Lars Erik; Araújo, Rafael Chaves Souto; Acín, Antonio; Navascués, Miguel
As first shown by Popescu (1995 Phys. Rev. Lett. 74 2619), some quantum states only reveal their nonlocality when subjected to a sequence of measurements while giving rise to local correlations in standard Bell tests. Motivated by this manifestation of 'hidden nonlocality' we set out to develop a general framework for the study of nonlocality when sequences of measurements are performed. Similar to Gallego et al (2013 Phys. Rev. Lett. 109 070401) our approach is operational, i.e. the task is to identify the set of allowed operations in sequential correlation scenarios and define nonlocality as the resource that cannot be created by these operations. This leads to a characterization of sequential nonlocality that contains as particular cases standard nonlocality and hidden nonlocality
Causal structures from entropic information: geometry and novel scenarios
(New Journal of Physics, 2014-04-03) Araújo, Rafael Chaves Souto; Luft, Lukas; Gross, David
Bell's theorem in physics, as well as causal discovery in machine learning, both face the problem of deciding whether observed data is compatible with a presumed causal relationship between the variables (for example, a local hidden variable model). Traditionally, Bell inequalities have been used to describe the restrictions imposed by causal structures on marginal distributions. However, some structures give rise to non-convex constraints on the accessible data, and it has recently been noted that linear inequalities on the observable entropies capture these situations more naturally. In this paper, we show the versatility of the entropic approach by greatly expanding the set of scenarios for which entropic constraints are known. For the first time, we treat Bell scenarios involving multiple parties and multiple observables per party. Going beyond the usual Bell setup, we exhibit inequalities for scenarios with extra conditional independence assumptions, as well as a limited amount of shared randomness between the parties. Many of our results are based on a geometric observation: Bell polytopes for two-outcome measurements can be naturally imbedded into the convex cone of attainable marginal entropies. Thus, any entropic inequality can be translated into one valid for probabilities. In some situations the converse also holds, which provides us with a rich source of candidate entropic inequalities
Machine learning nonlocal correlations
(American Physical Society, 2019-05-22) Silva, Askery Alexandre Canabarro Barbosa da; Brito, Samuraí Gomes de Aguiar; Araújo, Rafael Chaves Souto
The ability to witness nonlocal correlations lies at the core of foundational aspects of quantum mechanics and its application in the processing of information. Commonly, this is achieved via the violation of Bell inequalities. Unfortunately, however, their systematic derivation quickly becomes unfeasible as the scenario of interest grows in complexity. To cope with that, here, we propose a machine learning approach for the detection and quantification of nonlocality. It consists of an ensemble of multilayer perceptrons blended with genetic algorithms achieving a high performance in a number of relevant Bell scenarios. As we show, not only can the machine learn to quantify nonlocality, but discover new kinds of nonlocal correlations inaccessible with other current methods as well. We also apply our framework to distinguish between classical, quantum, and even postquantum correlations. Our results offer a novel method and a proof-of-principle for the relevance of machine learning for understanding nonlocality
Unveiling phase transitions with machine learning
(American Physical Society, 2019-07-22) Canabarro, Askery Alexandre Canabarro Barbosa da; Fanchini, Felipe Fernandes; Malvezzi, André Luiz; Pereira, Rodrigo Gonçalves; Araújo, Rafael Chaves Souto
The classification of phase transitions is a central and challenging task in condensed matter physics. Typically, it relies on the identification of order parameters and the analysis of singularities in the free energy and its derivatives. Here, we propose an alternative framework to identify quantum phase transitions, employing both unsupervised and supervised machine learning techniques. Using the axial next-nearest-neighbor Ising (ANNNI) model as a benchmark, we show how unsupervised learning can detect three phases (ferromagnetic, paramagnetic, and a cluster of the antiphase with the floating phase) as well as two distinct regions within the paramagnetic phase. Employing supervised learning we show that transfer learning becomes possible: a machine trained only with nearest-neighbor interactions can learn to identify a new type of phase occurring when next-nearest-neighbor interactions are introduced. All our results rely on few- and low-dimensional input data (up to twelve lattice sites), thus providing a computational friendly and general framework for the study of phase transitions in many-body systems
Statistical properties of the quantum internet
(American Physical Society, 2020-05-27) Brito, Samuraí Gomes de Aguiar; Silva, Askery Alexandre Canabarro Barbosa da; Araújo, Rafael Chaves Souto; Cavalcanti, Daniel
Steady technological advances are paving the way for the implementation of the quantum internet, a network of locations interconnected by quantum channels. Here we propose a model to simulate a quantum internet based on optical fibers and employ network-theory techniques to characterize the statistical properties of the photonic networks it generates. Our model predicts a continuous phase transition between a disconnected and a highly connected phase and that the typical photonic networks do not present the small world property. We compute the critical exponents characterizing the phase transition, provide quantitative estimates for the minimum density of nodes needed to have a fully connected network and for the average distance between nodes. Our results thus provide quantitative benchmarks for the development of a quantum internet
Data-driven study of the COVID-19 pandemic via age-structured modelling and prediction of the health system failure in Brazil amid diverse intervention strategies
(Public Library of Science, 2020-07-30) Silva, Askery Alexandre Canabarro Barbosa da; Tenório, Elayne; Martins, Renato; Martins, Laís; Brito, Samuraí Gomes de Aguiar; Araújo, Rafael Chaves Souto
In this work we propose a data-driven age-structured census-based SIRD-like epidemiological model capable of forecasting the spread of COVID-19 in Brazil. We model the current scenario of closed schools and universities, social distancing of people above sixty years old and voluntary home quarantine to show that it is still not enough to protect the health system by explicitly computing the demand for hospital intensive care units. We also show that an urgent intense quarantine might be the only solution to avoid the collapse of the health system and, consequently, to minimize the quantity of deaths. On the other hand, we demonstrate that the relaxation of the already imposed control measures in the next days would be catastrophic
Entanglement-based quantum communication complexity beyond Bell nonlocality
(npj Quantum Information, 2022-02-03) Ho, Joseph; Moreno Filho, Marcos George Magalhães; Brito, Samuraí Gomes de Aguiar; Graffitti, Francesco; Morrison, Christopher L.; Nery, Ranieri Vieira; Pickston, Alexander; Proietti, Massimiliano; Rabelo, Rafael; Fedrizzi, Alessandro; Araújo, Rafael Chaves Souto
Efficient distributed computing offers a scalable strategy for solving resource-demanding tasks, such as parallel computation and circuit optimisation. Crucially, the communication overhead introduced by the allotment process should be minimised—a key motivation behind the communication complexity problem (CCP). Quantum resources are well-suited to this task, offering clear strategies that can outperform classical counterparts. Furthermore, the connection between quantum CCPs and non-locality provides an information-theoretic insight into fundamental quantum mechanics. Here we connect quantum CCPs with a generalised non-locality framework—beyond Bell’s paradigmatic theorem—by incorporating the underlying causal structure, which governs the distributed task, into a so-called non-local hidden-variable model. We prove that a new class of communication complexity tasks can be associated with Bell-like inequalities, whose violation is both necessary and sufficient for a quantum gain. We experimentally implement a multipartite CCP akin to the guess-your-neighbour-input scenario, and demonstrate a quantum advantage when multipartite Greenberger-Horne-Zeilinger (GHZ) states are shared among three users
Enhancing entanglement and total correlation dynamics via local unitaries
(Physical Review A, 2022-02-22) Varela, Joab Morais; Nery, Ranieri Vieira; Moreno Filho, Marcos George Magalhães; Viana, Alice Caroline de Oliveira; Landi, Gabriel; Araújo, Rafael Chaves Souto
The interaction with the environment is one of the main obstacles to be circumvented in practical implementations of quantum information tasks. The use of local unitaries, while not changing the initial entanglement present in a given state, can enormously change its dynamics through a noisy channel, and consequently its ability to be used as a resource. In this way, local unitaries provide an easy and accessible way to enhance quantum correlations in a variety of different experimental platforms. Given an initial entangled state and a certain noisy channel, what are the local unitaries providing the most robust dynamics? In this paper we solve this question considering two-qubit states, together with paradigmatic and relevant noisy channels, showing its consequences for teleportation protocols and identifying cases where the most robust states are not necessarily the ones imprinting the least information about themselves into the environment. We also derive a general multipartite law relating the interplay between the total correlations in the system and environment with their mutual information built up over the noisy dynamics. Finally, we employ the IBM Quantum Experience to provide a proof-of-principle experimental implementation of our results
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
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
Uma visão da Ciência das Redes sobre o Instituto Nacional de Ciência e Tecnologia em Informação Quântica (INCT-IQ)
(Revista Brasileira de Ensino de Física, 2022-08-09) Oliveira, Rute; Sanz, Liliana; Araújo, Rafael Chaves Souto
A ciência das redes é um campo multidisciplinar que oferece um arcabouço amplo para se estudar propriedades estatísticas de uma variedade de fenômenos. No cerne do seu sucesso, está o fato de que os sistemas, por mais complexos que sejam seus constituintes ou interações, podem ser representados por um simples grafo, um conjunto de nós conectados por arestas. Nesta abordagem, processos de natureza muito diferentes, como a internet, colaborações científicas, ou redes de proteínas, se tornam semelhantes do ponto de vista da rede, o que nos permite não somente entender de maneira unificada as redes naturais mas também otimizar e projetar redes artificiais mais eficientes. Dentro deste contexto, este artigo tem dois objetivos. Primeiramente, apresentar os principais conceitos da ciência das redes, tais como grafos, propriedade de mundo pequeno, distribuição de conectividade entre outros, assim como alguns dos principais modelos de redes já propostos. O segundo objetivo é aplicar este ferramental para analisar uma rede real, mais precisamente a rede de pesquisadores do Instituto Nacional de Ciência e Tecnologia de Informação Quântica. Nossos resultados mostram que do ponto de vista estatístico a rede estudada é bem descrita por uma lei de potência truncada, com um alto grau de interconectividade entre os participantes. Um aglomerado, formado por 8 comunidades menores, contém 85% dos cientistas da rede. O número médio de colaborações da rede é próximo de 5 e a média de artigos publicados está acima de 13 durante o período de duração do projeto. A rede possui um alto grau de agregação, com valor de hCi = 0.4, mostrando que os colaboradores de um dado cientista também tendem a colaborar entre si.
Uso do aplicativo Tinkercad para enriquecer a prática pedagógica em disciplinas de eletricidade no Ensino Médio
(Revista do Professor de Física, 2022-12-07) Miguez, Maria Luiza; Sousa, Jefferson Maia de; Sobreira, Fernando Wellysson de Alencar; Almeida, Carlos Alberto Santos
Cursos de ensino médio integrados apresentam em seus componentes curriculares disciplinas aplicadas de Física, que objetivam introduzir conceitos fundamentais devendo trabalhar tanto a teoria como a prática correspondente. A conjugação dessas abordagens distintas muitas vezes implica em uma delas ser parcialmente negligenciada. A parte prática é a que sofre com a falta de tempo, somada à falta de estrutura laboratorial recorrente nas instituições de ensino no Brasil. Ferramentas virtuais que simulam laboratórios têm potencial para conjugadas às práticas presenciais enriquecer e fortalecer os conhecimentos teóricos que já são protagonistas. Apresentaremos uma atividade desenvolvida para utilizar o TinkerCAD no estudo de circuitos, especificamente para o estudo de associação de resistores
Information causality in multipartite scenarios
(Physical Review A, 2023-04-07) Pollyceno, Lucas; Araújo, Rafael Chaves Souto; Rabelo, Rafael
The Bell nonlocality is one of the most intriguing and counterintuitive phenomena displayed by quantum systems. Interestingly, such stronger-than-classical quantum correlations are somehow constrained, and one important question to the foundations of quantum theory is whether there is a physical, operational principle responsible for those constraints. One candidate is the information causality principle, which, in some particular cases, is proven to hold for quantum systems and to be violated by stronger-than-quantum correlations. In multipartite scenarios, though, it is known that the original formulation of the information causality principle fails to detect even extremal stronger-than-quantum correlations, thus suggesting that a genuinely multipartite formulation of the principle is necessary. In this work, we advance towards this goal, reporting a different formulation of the information causality principle in multipartite scenarios. By proposing a change of perspective, we obtain multipartite informational inequalities that work as necessary criteria for the principle to hold. We prove that such inequalities hold for all quantum resources and forbid some stronger-than-quantum ones. Finally, we show that our approach can be strengthened if multiple copies of the resource are available, or, counterintuitively, if noisy communication channels are employed
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
Witnessing nonclassicality in a causal structure with three observable variables
(PRX Quantum, 2023-04-20) Lauand, Pedro; Poderini, Davide; Nery, Ranieri Vieira; Moreno Filho, Marcos George Magalhães; Pollyceno, Lucas; Rabelo, Rafael; Araújo, Rafael Chaves Souto
Seen from the modern lens of causal inference, Bell’s theorem is nothing other than the proof that a specific classical causal model cannot explain quantum correlations. It is thus natural to move beyond Bell’s paradigmatic scenario and consider different causal structures. For the specific case of three observable variables, it is known that there are three nontrivial causal networks. Two of those are known to give rise to quantum nonclassicality: the instrumental and the triangle scenarios. Here we analyze the third and remaining one, which we name the Evans scenario, akin to the causal structure underlying the entanglement-swapping experiment. We prove a number of results about this elusive scenario and introduce new and efficient computational tools for its analysis that can also be adapted to deal with more general causal structures. We do not solve its main open problem—whether quantum nonclassical correlations can arise from it—but give a significant step in this direction by proving that postquantum correlations, analogous to the paradigmatic Popescu-Rohrlich box, do violate the constraints imposed by a classical description of the Evans causal structure
Quantifying quantum causal influences
(Physical Review A, 2023-08-28) Hutter, Lucas; Araújo, Rafael Chaves Souto; Nery, Ranieri Vieira; Moreno Filho, Marcos George Magalhães; Brod, Daniel Jost
Causal influences are at the core of any empirical science, the reason why its quantification is of paramount relevance for the mathematical theory of causality and applications. Quantum correlations, however, challenge our notion of cause and effect, implying that tools and concepts developed over the years having in mind a classical world have to be reevaluated in the presence of quantum effects. Here, we propose the quantum version of the most common causality quantifier, the average causal effect, measuring how much a target quantum system is changed by interventions on its presumed cause. Not only does it offer an innate manner to quantify causation in two-qubit gates but also in alternative quantum computation models such as the measurement-based version, suggesting that causality can be used as a proxy for optimizing quantum algorithms. Considering quantum teleportation, we show that any pure entangled state offers an advantage in terms of causal effects as compared to separable states. This broadness of different uses showcases that, just as in the classical case, the quantification of causal influence has foundational and applied consequences and can lead to a yet totally unexplored tool for quantum information science
O uso pedagógico do filme “Radioactive” no estudo da física moderna
(Jornal Mato-Grossense de Física, 2023-09-29) Oliveira, Willer Frank de Sousa; Sobreira, Fernando Wellysson de Alencar; Miguez, Maria Luiza
Neste trabalho, pretendemos investigar os diversos conceitos apresentados no filme “Radioactive” de 2019, lançado em plataformas de streaming. No filme é apresentado o contexto histórico vivido pela famosa cientista polonesa Marie Curie, a primeira mulher a receber prêmios Nobel de Física e de Química e se tornar professora na Universidade de Paris. Dentre os diversos conceitos presentes no filme são apresentados o decaimento radioativo da matéria, que contribui para o entendimento da estrutura atômica, a relação do uso de radiação em processos terapêuticos, o uso de raio-X em campos de batalha etc. A investigação desses conceitos foi realizada através de uma abordagem qualitativa utilizando-se de revisão bibliográfica e teve como resultado a produção de material que poderá ser utilizado em sala de aula por professores e estudantes para tratar de Física e Química moderna, permitindo ainda a integração destes conceitos com o nosso cotidiano, possibilitando sua utilização como ponto de partida para discussões interdisciplinares
Estimating the volume of correlation sets in causal networks
(Physical Review A, 2024-01-24) Camillo, Giulio; Lauand, Pedro; Poderini, Davide; Rabelo, Rafael; Araújo, Rafael Chaves Souto
Causal networks beyond that in the paradigmatic Bell's theorem can lead to new kinds and applications of nonclassical behavior. Their study, however, has been hindered by the fact that they define a nonconvex set of correlations and only very incomplete or approximated descriptions have been obtained so far, even for the simplest scenarios. Here we take a different stance on the problem and consider the relative volume of classical or nonclassical correlations a given network gives rise to, considering distances to sets of interest and how they distribute too. Among other results, we show instances where the inflation technique, arguably the most disseminated tool in the community, is unable to detect a significant portion of the nonclassical behaviors, up to three copies of each source, and that a concentration phenomenon of distances happens in one of them. Interestingly, we also show that the use of interventions, a central tool in causal inference, can enhance substantially our ability to witness nonclassicality
Detecting quantum phase transitions in a frustrated spin chain via transfer learning of a quantum classifier algorithm
(Physical Review A, 2024-05-20) Ferreira-Martins, André Juan; Silva, Leandro; Palhares Júnior, Alberto Bezerra de; Pereira, Rodrigo; Soares-Pinto, Diogo O.; Araújo, Rafael Chaves Souto; Canabarro, Askery
The classification of phases and the detection of phase transitions are central and challenging tasks in diverse fields. Within physics, these rely on the identification of order parameters and the analysis of singularities in the free energy and its derivatives. Here, we propose an alternative framework to identify quantum phase transitions. Using the axial next-nearest-neighbor Ising (ANNNI) model as a benchmark, we show how machine learning can detect three phases (ferromagnetic, paramagnetic, and a cluster of the antiphase with the floating phase). Employing supervised learning, we demonstrate the feasibility of transfer learning. Specifically, a machine trained only with nearest-neighbor interactions can learn to identify a new type of phase occurring when next-nearest-neighbor interactions are introduced. We also compare the performance of common classical machine learning methods with a version of the quantum nearest neighbors (QNN) algorithm
Observational-interventional Bell inequalities
(Physical Review A, 2024-10-21) Poderini, Davide; Nery, Ranieri Vieira; Moreno Filho, Marcos George Magalhães; Zamora, Santiago; Lauand, Pedro; Araújo, Rafael Chaves Souto
Generalizations of Bell's theorem, particularly within quantum networks, are now being analyzed through the lens of causal inference. However, the exploration of interventions, a central concept in causality theory, remains significantly unexplored. In this work, we take an initial step in this direction, by analyzing the instrumental scenario and proposing hybrid Bell inequalities integrating observational and interventional data. Focusing on binary outcomes with any number of inputs, we obtain the complete characterization of the observational-interventional polytope, equivalent to a Hardy-like Bell inequality, albeit describing a distinct quantum experiment. To illustrate its applications, we show a significant enhancement regarding threshold detection efficiencies for quantum violations also showing the use of this hybrid approach in quantum steering scenarios

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