1,721,083 research outputs found
Computable Entanglement Cost under Positive Partial Transpose Operations
Full text linkQuantum information theory is plagued by the problem of regularizations, which require the evaluation of formidable asymptotic quantities. This makes it computationally intractable to gain a precise quantitative understanding of the ultimate efficiency of key operational tasks such as entanglement manipulation. Here, we consider the problem of computing the asymptotic entanglement cost of preparing noisy quantum states under quantum operations with positive partial transpose (PPT). By means of an analytical example, a previously claimed solution to this problem is shown to be incorrect. Building on a previous characterization of the PPT entanglement cost in terms of a regularized formula, we construct instead a hierarchy of semidefinite programs that bypasses the issue of regularization altogether, and converges to the true asymptotic value of the entanglement cost. Our main result establishes that this convergence happens exponentially fast, thus yielding an efficient algorithm that approximates the cost up to an additive error epsilon in time poly(D, log(1/epsilon)), where D is the underlying Hilbert space dimension. To our knowledge, this is the first time that an asymptotic entanglement measure is shown to be efficiently computable despite no closed-form formula being available
Uno strumento UAV sperimentale per una metodologia sempre più consolidata: il caso di Via di Nocera a Pompei (Una herramienta VANT experimental para una metodología cada vez más consolidada: el caso de Via di Nocera en Pompeya)
No full textIl rilevamento aereo nel campo dell’architettura e dell’archeologia sta riscuotendo sempre maggior interesse, soprattutto a seguito dello sviluppo tecnologico dei VANT (vehículo aéreo no tripulado), più famosi nella terminologia inglese UAV (Unmanned Aereal Vehicles). Negli ultimi anni, infatti, questi mezzi hanno migliorato notevolmente le loro caratteristiche, come il tempo di volo, la stabilità e il payload. Parallelamente, abbiamo assistito alla miniaturizzazione del sensore che, non a discapito della qualità, ha aperto nuove frontiere per il rilievo fotogrammetrico fino a pochi anni fa quasi inimmaginabili. A questo sviluppo ha contribuito anche il progresso dei software di fotogrammetria digitale, con l’introduzione sempre più spinta di nuovi algoritmi di calcolo – Structure from Motion –, per la restituzione di orto-immagini ad alta definizione, spesso requisito necessario per la documentazione e la conservazione del patrimonio archeologico. Proprio in questo filone di ricerca si colloca il caso studio, qui presentato a scopo principalmente divulgativo, del rilievo fotogrammetrico da mezzo UAV di Via di Nocera, nell’area archeologica degli scavi di Pompei, patrimonio Unesco dal lontano 1997. Dal processamento delle immagini, a valle di un’accurata campagna topografica con la battuta dei necessari Ground Control Point (GCP), è stata restituita un’orto-immagine ad alta risoluzione e georeferenziata. L’analisi metrica rispetto i GCP ha riscontrato errori puntuali inferiori ai 2 cm, validando metricamente le rappresentazioni per elaborati in scala 1:50 (o anche più grande), essenziali per una dettagliata documentazione del basolato e permettendone, così, una nuova lettura anche del relativo stato di degrado.El levamiento aéreo en el campo de la arquitectura y arqueología, está demostrando siempre mayor interés, principalmente luego del desarrollo tecnológico de los VANT (Vehículo aéreo no tripulado) o más conocido en su término en inglés UAV (Unmanned Aereal Vehicles). En los últimos años, en efecto, estos medios han mejorado notablemente sus características, ya sea, el tiempo y la estabilidad del vuelo, y el payload. Al mismo tiempo, hemos sido testigos del desarrollo tecnológico de los sensores pasivos, capaces en pocos años, de la miniaturización, que abre nuevas fronteras en el levantamiento fotogramétrico, situación que pocos años atrás era inimaginable. En este desarrollo, ha contribuido también el progreso de los softwares de fotogrametría digital, con la introducción de nuevos algoritmos de cálculo – Structure from Motion –, para la restitución de orto-fotos de alta definición, muchas veces requisito esencial para la documentación y la conservación del patrimonio arqueológico. Precisamente en este ámbito de investigación se localiza el caso de estudio, aquí presentado con un objetivo principalmente divulgativo, del levamiento fotogramétrico por medio de VANT de Via di Nocera, en el área arqueológica de Pompeya, patrimonio de la Unesco desde el lejano 1997. Desde el procesamiento de las imágenes, capturadas a partir de una precisa campaña de recolección y con la utilización de los necesarios Ground Control Point (GCP), ha sido restituida una orto-imagen de alta resolución y geo-referenciada. El análisis métrico respecto los GCP ha evidenciado errores puntuales inferiores a los 2 cm, validando métricamente la representación para piezas graficas en escala 1:50 (o incluso más grande), esencial para una detallada documentación del pavimento y permitiendo también, de esta manera, una nueva lectura del relativo estado de degrado
Maximum tolerable excess noise in continuous-variable quantum key distribution and improved lower bound on two-way capacities
Full text linkThe two-way capacities of quantum channels determine the ultimate entanglement and secret-key distribution rates achievable by two distant parties that are connected by a noisy transmission line, in the absence of quantum repeaters. Since repeaters will likely be expensive to build and maintain, a central open problem of quantum communication is to understand what performances are achievable without them. Here we find a new lower bound on the energy-constrained and unconstrained two-way quantum and secret-key capacities of all phase-insensitive bosonic Gaussian channels, namely thermal attenuator, thermal amplifier and additive Gaussian noise, which are realistic models for the noise affecting optical fibres or free-space links. Ours is the first non-zero lower bound on the two-way quantum capacity in the parameter range where the (reverse) coherent information becomes negative, and it shows explicitly that entanglement distribution is always possible when the channel is not entanglement breaking. This completely solves a crucial open problem of the field, namely establishing the maximum excess noise, which is tolerable in continuous-variable quantum key distribution. In addition, our construction is fully explicit; that is, we devise and optimize a concrete entanglement distribution and distillation protocol that works by combining recurrence and hashing protocols
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Quantum communication on the bosonic loss-dephasing channel
Full text linkQuantum optical systems are typically affected by two types of noise: photon loss and dephasing. Despite extensive research on each noise process individually, a comprehensive understanding of their combined effect is still lacking. A crucial problem lies in determining the values of loss and dephasing for which the resulting loss-dephasing channel is antidegradable, implying the absence of codes capable of correcting its effect or, alternatively, capable of enabling quantum communication. A conjecture [Quantum 6, 821 (2022)2521-327X10.22331/q-2022-09-29-821] suggested that the bosonic loss-dephasing channel is not antidegradable if the loss is below 50%. In this paper we refute this conjecture, specifically proving that for any value of the loss, if the dephasing is above a critical value, then the bosonic loss-dephasing channel is antidegradable. While our result identifies a large parameter region where quantum communication is not possible, we also prove that if two-way classical communication is available, then quantum communication - and thus quantum key distribution - is always achievable, even for high values of loss and dephasing
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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