1,721,034 research outputs found
Symmetrizing dynamics: from classical to quantum applications
Full text linkAmong the issues regarding networked systems, the “consensus problem” and the related algorithms have received a significant share of attention during the last ten years. In this problem the network agents asymptotically have to attain agreement on the value of some objective variable under local communication constraints. A number of algorithms have been developed to address this problem, among which the celebrated gossip algorithm. The latter relays on switching dynamics and, under rather weak assumptions, exhibits robust convergence under variations in the interaction constraints, i.e. the network topology.
In this dissertation we reinterpret the goal of the consensus problem as a symmetrisation problem, and we address it by a switching-type dynamics based on convex combinations of actions of a finite group. In order to study the convergence of our class of algorithms we lift the dynamics to an abstract, group-theoretic level that allow us to derive general conditions for convergence. Such conditions, in fact, are independent of the particular group action, and focus only on the group itself and the way the iterations are selected. Convergence is guaranteed provided that some mild assumptions on the selection rule for the iterations are fulfilled. Furthermore, this class of algorithms retains the robustness features and unsupervised character of the consensus algorithm.
Our reformulation allow to devise algorithms for application as diverse as randomized discrete Fourier transform and random state generation. We pose a special emphasis on the extension of the consensus problem to the quantum domain. In this setting we highlight how, due to the richer mathematical structure over which the internal state is encoded, the definition of the consensus goal admits various extensions, each of them exhibiting different features. We also propose a suitable dissipative dynamics enacting the symmetrising gossip interactions and then use our general result on convergence to prove it ensures asymptotic convergence.
Beside the technical results, one of the main contributions of our work is a new, generalized view point on consensus, which allows us to extend the robustness of consensus-inspired algorithms to new problems in apparently unrelated fields. This reinforces the role of consensus algorithms as fundamental tools for distributed computing, both in the classical and the quantum setting
Quantum Tomography under Prior Information
No full textWe provide a detailed analysis of the question: how many measurement settings or outcomes are needed in order to identify an unknown quantum state which is constrained by prior information? We show that if the prior information restricts the possible states to a set of lower dimensionality, then topological obstructions can increase the required number of outcomes by a factor of two over the number of real parameters needed to characterize the set of all states. Conversely, we show that almost every measurement becomes informationally complete with respect to the constrained set if the number of outcomes exceeds twice the Minkowski dimension of the set. We apply the obtained results to determine the minimal number of outcomes of measurements which are informationally complete with respect to states with rank constraints. In particular, we show that the minimal number of measurement outcomes (POVM elements) necessary to identify all pure states in a d-dimensional Hilbert space is 4d-3-c(d) α(d) for some c(d) ∈ [1, 2] and α(d) being the number of ones appearing in the binary expansion of (d-1)
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
A new perspective on gossip iterations: From Symmetrization to quantum consensus
No full textThis paper extends the gossip algorithm, widely studied in the literature on distributed computing and control algorithms, to networks of quantum systems. In doing so, we reinterpret the classical algorithm and the average consensus task as a symmetrization problem with respect to the action of the permutation group. This allows us to extend in a natural way the gossip consensus algorithm to the quantum setting and prove its convergence properties to symmetric states while preserving the expectation of permutation-invariant global observables
- …
