Navegando por Autor "Moreno Filho, Marcos George Magalhães"

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Causal inference with imperfect instrumental variables
(Journal Of Causal Inference, 2022-05-06) Miklin, Nikolai; Gachechiladze, Mariami; Moreno Filho, Marcos George Magalhães; Araujo, Rafael Chaves Souto
Instrumental variables allow for quantification of cause and effect relationships even in the absence of interventions. To achieve this, a number of causal assumptions must be met, the most important of which is the independence assumption, which states that the instrument and any confounding factor must be independent. However, if this independence condition is not met, can we still work with imperfect instrumental variables? Imperfect instruments can manifest themselves by violations of the instrumental inequalities that constrain the set of correlations in the scenario. In this article, we establish a quantitative relationship between such violations of instrumental inequalities and the minimal amount of measurement dependence required to explain them for the case of discrete observed variables. As a result, we provide adapted inequalities that are valid in the presence of a relaxed measurement dependence assumption in the instrumental scenario. This allows for the adaptation of existing and new lower bounds on the average causal effect for instrumental scenarios with binary outcomes. Finally, we discuss our findings in the context of quantum mechanics
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.
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
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
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
Events in quantum mechanics are maximally non-absolute.
(Quantum: the open journal for quantum science, 2022-08-24) Moreno Filho, Marcos George Magalhães; Nery, Ranieri Vieira; Duarte, Cristhiano; Araujo, Rafael Chaves Souto
The notorious quantum measurement problem brings out the difficulty to reconcile two quantum postulates: the unitary evolution of closed quantum systems and the wave-function collapse after a measurement. This problematics is particularly highlighted in the Wigner's friend thought experiment, where the mismatch between unitary evolution and measurement collapse leads to conflicting quantum descriptions for different observers. A recent no-go theorem has established that the (quantum) statistics arising from an extended Wigner's friend scenario is incompatible when one try to hold together three innocuous assumptions, namely no-superdeterminism, parameter independence and absoluteness of observed events. Building on this extended scenario, we introduce two novel measures of non-absoluteness of events. The first is based on the EPR2 decomposition, and the second involves the relaxation of the absoluteness hypothesis assumed in the aforementioned no-go theorem. To prove that quantum correlations can be maximally non-absolute according to both quantifiers, we show that chained Bell inequalities (and relaxations thereof) are also valid constraints for Wigner's experiment
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
General Method for Classicality Certification in the Prepare and Measure Scenario
(PRX Quantum, 2021-07-20) Araújo, Rafael Chaves Souto; Moreno Filho, Marcos George Magalhães; Nery, Ranieri Vieira; Brito, Samuraí Gomes de Aguiar; Gois, Carlos de; Rabelo, Rafael
Preparing and measuring physical systems are the operational building blocks of any physical experiment, and to describe them is the first purpose of any physical theory. Remarkably, even when only uncharacterized preparation and measurement devices are present, it is sometimes possible to distinguish between the behaviors of quantum and classical systems from only observational data. Certifying the physical origin of measurement statistics in the prepare and measure scenario is of primal importance for developing quantum networks, distributing quantum keys, and certifying randomness, to mention a few applications, but, surprisingly, no general methods to do so are known. We progress on this problem by crafting a general, sufficient condition to certify that a given set of preparations can only generate classical statistics, for any number of generalized measurements. As an application, we employ the method to demonstrate nonclassicality activation in the prepare and measure scenario, also considering its application in random access codes. Following that, we adapt our method to certify, again through a sufficient condition, whether a given set of measurements can never give rise to nonclassical behaviors, irrespective of what preparations they may act upon. This, in turn, allows us to find a large set of incompatible measurements that cannot be used to demonstrate nonclassicality, thus showing incompatibility is not sufficient for nonclassicality in the prepare and measure scenario.
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
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
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
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

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Navegando por Autor "Moreno Filho, Marcos George Magalhães"