Centrum Wiskunde and Informatica

Amsterdam, Netherlands

Centrum Wiskunde and Informatica

Amsterdam, Netherlands
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Meixner B.,Centrum Wiskunde and Informatica
ACM Computing Surveys | Year: 2017

Hypervideos and interactive multimedia presentations allow the creation of fully interactive and enriched video. It is possible to organize video scenes in a nonlinear way. Additional information can be added to the video ranging from short descriptions to images and more videos. Hypervideos are video-based but also provide navigation between video scenes and additional multimedia elements. Interactive multimedia presentations consist of different media with a temporal and spatial synchronization that can be navigated via hyperlinks. Their creation and description requires description formats,multimediamodels, and standards- as well as players. Specialized authoring tools with advanced editing functions allow authors to manage all media files, link and arrange them to an overall presentation, and keep an overview during the whole process. They considerably simplify the creation process compared to writing and editing description documents in simple text editors. Data formats need features that describe interactivity and nonlinear navigation while maintaining temporal and spatial synchronization. Players should be easy to use with extended feature sets keeping elements synchronized. In this article, we analyzed more than 400 papers for relevant work in this field. From the findings we discovered a set of trends and unsolved problems, and propose directions for future research. © 2017 ACM.

Van Aert S.,University of Antwerp | Batenburg K.J.,Centrum Wiskunde and Informatica | Batenburg K.J.,University of Antwerp | Rossell M.D.,ETH Zurich | And 2 more authors.
Nature | Year: 2011

Determining the three-dimensional (3D) arrangement of atoms in crystalline nanoparticles is important for nanometre-scale device engineering and also for applications involving nanoparticles, such as optoelectronics or catalysis. A nanoparticle's physical and chemical properties are controlled by its exact 3D morphology, structure and composition. Electron tomography enables the recovery of the shape of a nanoparticle from a series of projection images. Although atomic-resolution electron microscopy has been feasible for nearly four decades, neither electron tomography nor any other experimental technique has yet demonstrated atomic resolution in three dimensions. Here we report the 3D reconstruction of a complex crystalline nanoparticle at atomic resolution. To achieve this, we combined aberration-corrected scanning transmission electron microscopy, statistical parameter estimation theory and discrete tomography. Unlike conventional electron tomography, only two images of the target - a silver nanoparticle embedded in an aluminium matrix - are sufficient for the reconstruction when combined with available knowledge about the particle's crystallographic structure. Additional projections confirm the reliability of the result. The results we present help close the gap between the atomic resolution achievable in two-dimensional electron micrographs and the coarser resolution that has hitherto been obtained by conventional electron tomography. ©2011 Macmillan Publishers Limited. All rights reserved.

Buhrman H.,Centrum Wiskunde and Informatica | Cleve R.,University of Waterloo | Cleve R.,Perimeter Institute for Theoretical Physics | Massar S.,Free University of Colombia | De Wolf R.,Centrum Wiskunde and Informatica
Reviews of Modern Physics | Year: 2010

Quantum information processing is the emerging field that defines and realizes computing devices that make use of quantum mechanical principles such as the superposition principle, entanglement, and interference. Until recently the common notion of computing was based on classical mechanics and did not take into account all the possibilities that physically realizable computing devices offer in principle. The field gained momentum after Shor developed an efficient algorithm for factoring numbers, demonstrating the potential computing powers that quantum computing devices can unleash. In this review the information counterpart of computing is studied. It was realized early on by Holevo that quantum bits, the quantum mechanical counterpart of classical bits, cannot be used for efficient transformation of information in the sense that arbitrary k -bit messages cannot be compressed into messages of k-1 qubits. The abstract form of the distributed computing setting is called communication complexity. It studies the amount of information, in terms of bits or in our case qubits, that two spatially separated computing devices need to exchange in order to perform some computational task. Surprisingly, quantum mechanics can be used to obtain dramatic advantages for such tasks. The area of quantum communication complexity is reviewed and it is shown how it connects the foundational physics questions regarding nonlocality with those of communication complexity studied in theoretical computer science. The first examples exhibiting the advantage of the use of qubits in distributed information-processing tasks were based on nonlocality tests. However, by now the field has produced strong and interesting quantum protocols and algorithms of its own that demonstrate that entanglement, although it cannot be used to replace communication, can be used to reduce the communication exponentially. In turn, these new advances yield a new outlook on the foundations of physics and could even yield new proposals for experiments that test the foundations of physics. © 2010 The American Physical Society.

Mancinska L.,University of Waterloo | Scarpa G.,Centrum Wiskunde and Informatica | Severini S.,University College London
IEEE Transactions on Information Theory | Year: 2013

We introduce two generalizations of Kochen-Specker (KS) sets: projective KS sets and generalized KS sets. We then use projective KS sets to characterize all graphs for which the chromatic number is strictly larger than the quantum chromatic number. Here, the quantum chromatic number is defined via a nonlocal game based on graph coloring. We further show that from any graph with separation between these two quantities, one can construct a classical channel for which entanglement assistance increases the one-shot zero-error capacity. As an example, we exhibit a new family of classical channels with an exponential increase. © 1963-2012 IEEE.

Vuurens J.B.P.,The Hague University of Applied Sciences | De Vries A.P.,Centrum Wiskunde and Informatica
IEEE Internet Computing | Year: 2012

The performance of information retrieval (IR) systems is commonly evaluated using a test set with known relevance. Crowdsourcing is one method for learning the relevant documents to each query in the test set. However, the quality of relevance learned through crowdsourcing can be questionable, because it uses workers of unknown quality with possible spammers among them. To detect spammers, the authors' algorithm compares judgments between workers; they evaluate their approach by comparing the consistency of crowdsourced ground truth to that obtained from expert annotators and conclude that crowdsourcing can match the quality obtained from the latter. © 2012 IEEE.

Basten H.J.S.,Centrum Wiskunde and Informatica
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2010

Context-free grammars are widely used but still hindered by ambiguity. This stresses the need for detailed detection methods that point out the sources of ambiguity in a grammar. In this paper we show how the approximative Noncanonical Unambiguity Test by Schmitz can be extended to conservatively identify production rules that do not contribute to the ambiguity of a grammar. We prove the correctness of our approach and consider its practical applicability. © 2010 Springer-Verlag.

Bosman P.A.N.,Centrum Wiskunde and Informatica
IEEE Transactions on Evolutionary Computation | Year: 2012

Algorithms that make use of the gradient, i.e., the direction of maximum improvement, to search for the optimum of a single-objective function have been around for decades. They are commonly accepted to be important and have been applied to tackle single-objective optimization problems in many fields. For multiobjective optimization problems, much less is known about the gradient and its algorithmic use. In this paper, we aim to contribute to the understanding of gradients for numerical, i.e., real-valued, multiobjective optimization. Specifically, we provide an analytical parametric description of the set of all nondominated, i.e., most promising, directions in which a solution can be moved such that the objective values either improve or remain the same. This result completes previous work where this set is described only for one particular case, namely when some of the nondominated directions have positive, i.e., nonimproving, components and the final set can be computed by taking the subset of directions that are all nonpositive. In addition we use our result to assess the utility of using gradient information for multiobjective optimization where the goal is to obtain a Pareto set of solutions that approximates the optimal Pareto set. To this end, we design and consider various multiobjective gradient-based optimization algorithms. One of these algorithms uses the description of the multiobjective gradient provided here. Also, we hybridize an existing multiobjective evolutionary algorithm (MOEA) with the various multiobjective gradient-based optimization algorithms. During optimization, the performance of the gradient-based optimization algorithms is monitored and the available computational resources are redistributed to allow the (currently) most effective algorithm to spend the most resources. We perform an experimental analysis using a few well-known benchmark problems to compare the performance of different optimization methods. The results underline that the use of a population of solutions that is characteristic of MOEAs is a powerful concept if the goal is to obtain a good Pareto set, i.e., instead of only a single solution. This makes it hard to increase convergence speed in the initial phase using gradient information to improve any single solution. However, in the longer run, the use of gradient information does ultimately allow for better fine-tuning of the results and thus better overall convergence. © 2011 IEEE.

Speelman F.,Centrum Wiskunde and Informatica
Leibniz International Proceedings in Informatics, LIPIcs | Year: 2016

Instantaneous non-local quantum computation requires multiple parties to jointly perform a quantum operation, using pre-shared entanglement and a single round of simultaneous communication. We study this task for its close connection to position-based quantum cryptography, but it also has natural applications in the context of foundations of quantum physics and in distributed computing. The best known general construction for instantaneous non-local quantum computation requires a pre-shared state which is exponentially large in the number of qubits involved in the operation, while efficient constructions are known for very specific cases only. We partially close this gap by presenting new schemes for efficient instantaneous non-local computation of several classes of quantum circuits, using the Clifford+T gate set. Our main result is a protocol which uses entanglement exponential in the T-depth of a quantum circuit, able to perform non-local computation of quantum circuits with a (poly-)logarithmic number of layers of T gates with quasi-polynomial entanglement. Our proofs combine ideas from blind and delegated quantum computation with the garden-hose model, a combinatorial model of communication complexity which was recently introduced as a tool for studying certain schemes for quantum position verification. As an application of our results, we also present an efficient attack on a recently-proposed scheme for position verification by Chakraborty and Leverrier. © Florian Speelman; licensed under Creative Commons License CC-BY.

Batenburg K.J.,Centrum Wiskunde and Informatica | Plantagie L.,Centrum Wiskunde and Informatica
IEEE Transactions on Image Processing | Year: 2012

Most reconstruction algorithms for transmission tomography can be subdivided in two classes: variants of filtered backprojection (FBP) and iterative algebraic methods. FBP is very fast and yields accurate results when a large number of projections are available, with high signal-to-noise ratio and a full angular range. Algebraic methods require much more computation time, yet they are more flexible in dealing with limited data problems and noise. In this paper, we propose an algorithm that combines the best of these two approaches: for a given linear algebraic method, a filter is computed that can be used within the FBP algorithm. The FBP reconstructions that result from using this filter strongly resemble the algebraic reconstructions and have many of their favorable properties, while the required reconstruction time is similar to standard-FBP. Based on a series of experiments, for both simulation data and experimental data, we demonstrate the merits of the proposed algorithm. © 1992-2012 IEEE.

Scarpa G.,Centrum Wiskunde and Informatica | Severini S.,University College London
IEEE Transactions on Information Theory | Year: 2012

The quantum chromatic number of a graph G is sandwiched between its chromatic number and its clique number, which are well-known NP-hard quantities. We restrict our attention to the rank-1 quantum chromatic number χ q (1)(G), which upper bounds the quantum chromatic number, but is defined under stronger constraints. We study its relation with the chromatic number χ(G) and the minimum dimension of orthogonal representations ξ(G). It is known that ξ(G) ≤ χ q (1)(G) ≤ χ(G). We answer three open questions about these relations: we give a necessary and sufficient condition to have ξ(G) = χ q (1)(G), we exhibit a class of graphs such that ξ (G) < χ q (1)(G), and we give a necessary and sufficient condition to have χ q (1)(G) < χ (G). Our main tools are Kochen-Specker sets, collections of vectors with a traditionally important role in the study of contextuality of physical theories and, more recently, in the quantification of quantum zero-error capacities. Finally, as a corollary of our results and a result by Avis on the quantum chromatic number, we give a family of Kochen-Specker sets of growing dimension. © 2006 IEEE.

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