Showing papers by "University of Paderborn published in 2012"

A survey of information-centric networking

Abstract: The information-centric networking (ICN) concept is a significant common approach of several future Internet research activities. The approach leverages in-network caching, multiparty communication through replication, and interaction models decoupling senders and receivers. The goal is to provide a network infrastructure service that is better suited to today?s use (in particular. content distribution and mobility) and more resilient to disruptions and failures. The ICN approach is being explored by a number of research projects. We compare and discuss design choices and features of proposed ICN architectures, focusing on the following main components: named data objects, naming and security, API, routing and transport, and caching. We also discuss the advantages of the ICN approach in general.

Dual-polarity plasmonic metalens for visible light

Abstract: Surface topography and refractive index profile dictate the deterministic functionality of a lens. The polarity of most lenses reported so far, that is, either positive (convex) or negative (concave), depends on the curvatures of the interfaces. Here we experimentally demonstrate a counter-intuitive dual-polarity flat lens based on helicity-dependent phase discontinuities for circularly polarized light. Specifically, by controlling the helicity of the input light, the positive and negative polarity are interchangeable in one identical flat lens. Helicity-controllable real and virtual focal planes, as well as magnified and demagnified imaging, are observed on the same plasmonic lens at visible and near-infrared wavelengths. The plasmonic metalens with dual polarity may empower advanced research and applications in helicity-dependent focusing and imaging devices, angular-momentum-based quantum information processing and integrated nano-optoelectronics.

Dispersionless Phase Discontinuities for Controlling Light Propagation

Abstract: Ultrathin metasurfaces consisting of a monolayer of subwavelength plasmonic resonators are capable of generating local abrupt phase changes and can be used for controlling the wavefront of electromagnetic waves. The phase change occurs for transmitted or reflected wave components whose polarization is orthogonal to that of a linearly polarized (LP) incident wave. As the phase shift relies on the resonant features of the plasmonic structures, it is in general wavelength-dependent. Here, we investigate the interaction of circularly polarized (CP) light at an interface composed of a dipole antenna array to create spatially varying abrupt phase discontinuities. The phase discontinuity is dispersionless, that is, it solely depends on the orientation of dipole antennas, but not their spectral response and the wavelength of incident light. By arranging the antennas in an array with a constant phase gradient along the interface, the phenomenon of broadband anomalous refraction is observed ranging from visible to ...

The electronic structure and optical response of rutile, anatase and brookite TiO2.

Abstract: In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO2 brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn‐Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange‐correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe‐Salpeter equation for Coulomb correlated electron‐hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO2 are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description. (Some figures may appear in colour only in the online journal)

Global Large-Data Solutions in a Chemotaxis-(Navier–)Stokes System Modeling Cellular Swimming in Fluid Drops

Abstract: In the modeling of collective effects arising in bacterial suspensions in fluid drops, coupled chemotaxis-(Navier–)Stokes systems generalizing the prototype have been proposed to describe the spont

Lithium niobate on insulator (LNOI) for micro-photonic devices

Abstract: The state-of-the-art of high-refractive-index-contrast single-crystalline thin lithium niobate (LiNbO3) films as a new platform for high-density integrated optics is reviewed. Sub-micrometer thin LiNbO3 films are obtained by “ion-slicing”. They can be bonded by two different techniques to a low-index substrate to obtain “lithium niobate on insulator” (LNOI) even as wafer of 3” diameter. Different micro- and nano-structuring techniques have been used to successfully develop micro-photonic devices. To be specific, the fabrication and characterization of LNOI photonic wires with cross-section < 1 µm2, periodically poled LNOI photonic wires for second harmonic generation, electro-optically tunable microring resonators, free standing microrings for hybrid integration, and photonic crystal structures are described.

STIR: software for tomographic image reconstruction release 2

Abstract: We present a new version of STIR (Software for Tomographic Image Reconstruction), an open source object-oriented library implemented in C++ for 3D positron emission tomography reconstruction. This library has been designed such that it can be used for many algorithms and scanner geometries, while being portable to various computing platforms. This second release enhances its flexibility and modular design and includes additional features such as Compton scatter simulation, an additional iterative reconstruction algorithm and parametric image reconstruction (both indirect and direct). We discuss the new features in this release and present example results. STIR can be downloaded from http://stir.sourceforge.net.

A 2D Quantum Walk Simulation of Two-Particle Dynamics

Abstract: Multidimensional quantum walks can exhibit highly nontrivial topological structure, providing a powerful tool for simulating quantum information and transport systems. We present a flexible implementation of a two-dimensional (2D) optical quantum walk on a lattice, demonstrating a scalable quantum walk on a nontrivial graph structure. We realized a coherent quantum walk over 12 steps and 169 positions by using an optical fiber network. With our broad spectrum of quantum coins, we were able to simulate the creation of entanglement in bipartite systems with conditioned interactions. Introducing dynamic control allowed for the investigation of effects such as strong nonlinearities or two-particle scattering. Our results illustrate the potential of quantum walks as a route for simulating and understanding complex quantum systems.

StreamKM++: A clustering algorithm for data streams

Abstract: We develop a new k-means clustering algorithm for data streams of points from a Euclidean space. We call this algorithm StreamKM++. Our algorithm computes a small weighted sample of the data stream and solves the problem on the sample using the k-means++ algorithm of Arthur and Vassilvitskii (SODA '07). To compute the small sample, we propose two new techniques. First, we use an adaptive, nonuniform sampling approach similar to the k-means++ seeding procedure to obtain small coresets from the data stream. This construction is rather easy to implement and, unlike other coreset constructions, its running time has only a small dependency on the dimensionality of the data. Second, we propose a new data structure, which we call coreset tree. The use of these coreset trees significantly speeds up the time necessary for the adaptive, nonuniform sampling during our coreset construction.We compare our algorithm experimentally with two well-known streaming implementations: BIRCH [Zhang et al. 1997] and StreamLS [Guha et al. 2003]. In terms of quality (sum of squared errors), our algorithm is comparable with StreamLS and significantly better than BIRCH (up to a factor of 2). Besides, BIRCH requires significant effort to tune its parameters. In terms of running time, our algorithm is slower than BIRCH. Comparing the running time with StreamLS, it turns out that our algorithm scalesmuch better with increasing number of centers. We conclude that, if the first priority is the quality of the clustering, then our algorithm provides a good alternative to BIRCH and StreamLS, in particular, if the number of cluster centers is large. We also give a theoretical justification of our approach by proving that our sample set is a small coreset in low-dimensional spaces.

Cognition, action, and object manipulation.

Abstract: Although psychology is the science of mental life and behavior, little attention has been paid to the means by which mental life is translated into behavior. One domain in which links between cognition and action have been explored is the manipulation of objects. This article reviews psychological research on this topic, with special emphasis on the tendency to grasp objects differently depending on what one plans to do with the objects. Such differential grasping has been demonstrated in a wide range of object manipulation tasks, including grasping an object in a way that reveals anticipation of the object’s future orientation, height, and required placement precision. Differential grasping has also been demonstrated in a wide range of behaviors, including 1-hand grasps, 2-hand grasps, walking, and transferring objects from place to place as well as from person to person. The populations in which the tendency has been shown are also diverse, including nonhuman primates as well as human adults, children, and babies. The tendency is compromised in a variety of clinical populations and in children of a surprisingly advanced age. Verbal working memory is compromised as well if words are memorized while object manipulation tasks are performed; the recency portion of the serial position curve is reduced in this circumstance. In general, the research reviewed here points to rich connections between cognition and action as revealed through the study of object manipulation. Other implications concern affordances, Donders’ law, naturalistic observation, and the teaching of psychology.

Upper Critical Solution Temperature of Poly(N-acryloyl glycinamide) in Water: A Concealed Property

Abstract: Polymers showing an upper critical solution temperature (UCST) in water are rare. Recently, the nonionic homopolymer poly(N-acryloyl glycinamide) (poly(NAGA)) has been shown to exhibit a sharp uppe...

Do not snoop my habits: preserving privacy in the smart grid

Abstract: The recent deployment of smart grids has proven to bring numerous advantages in terms of energy consumption reduction in both homes and businesses. A more accurate measurement of up-to-date electricity necessities through smart meters utilization leads to an enhancement in the ability of monitoring, controlling and predicting energy use. Nevertheless, it has associated drawbacks related to the privacy of customers as well, since such management might reveal their personal habits and behavior, which electrical appliances they are using at each moment, whether they are at home or not, and so on. In this article we present a privacy enhanced architecture for smart metering aimed to tackle this threat by means of a new and novel protocol encrypting individual measurements while allowing the electricity supplier to access the aggregation of the corresponding decrypted values. The technique being used is named additively homomorphic encryption, and enables the direct connection and exchange of data between electricity suppliers and final users, while preserving the privacy of the latter.

Self-affine elastic contacts: percolation and leakage.

Abstract: We study fluid flow at the interfaces between elastic solids with randomly rough, self-affine surfaces. We show by numerical simulation that elastic deformation lowers the relative contact area at which contact patches percolate in comparison to traditional approaches to seals. Elastic deformation also suppresses leakage through contacts even far away from the percolation threshold. Reliable estimates for leakage can be obtained by combining Persson's contact mechanics theory with a slightly modified version of Bruggeman's effective-medium solution of the Reynolds equation.

Stabilization in a two-species chemotaxis system with a logistic source

Abstract: We study a system of three partial differential equations modelling the spatio-temporal behaviour of two competitive populations of biological species both of which are attracted chemotactically by the same signal substance. More precisely, we consider the initial-boundary value problem for under homogeneous Neumann boundary conditions in a bounded domain , n ≥ 1, with smooth boundary.When 0 ≤ a1 < 1 and 0 ≤ a2 < 1, this system possesses a uniquely determined spatially homogeneous positive equilibrium (u, v). We show that given any such a1 and a2 and any positive diffusivities d1 and d2 and cross-diffusivities χ1 and χ2, this steady state is globally asymptotically stable within a certain nonempty range of the logistic growth coefficients μ1 and μ2.

Molecular dynamics and experimental study of conformation change of poly(N-isopropylacrylamide) hydrogels in mixtures of water and methanol.

Abstract: The conformation transition of poly(N-isopropylacrylamide) hydrogel as a function of the methanol mole fraction in water/methanol mixtures is studied both experimentally and by atomistic molecular dynamics simulation with explicit solvents. The composition range in which the conformation transition of the hydrogel occurs is determined experimentally at 268.15, 298.15, and 313.15 K. In these experiments, cononsolvency, i.e., collapse at intermediate methanol concentrations while the hydrogel is swollen in both pure solvents, is observed at 268.15 and 298.15 K. The composition range in which cononsolvency is present does not significantly depend on the amount of cross-linker. The conformation transition of the hydrogel is caused by the conformation transition of the polymer chains of its backbone. Therefore, conformation changes of single backbone polymer chains are studied by massively parallel molecular dynamics simulations. The hydrogel backbone polymer is described with the force field OPLS-AA, water with the SPC/E model, and methanol with the model of the GROMOS-96 force field. During simulation, the mean radius of gyration of the polymer chains is monitored. The conformation of the polymer chains is studied at 268, 298, and 330 K as a function of the methanol mole fraction. Cononsolvency is observed at 268 and 298 K, which is in agreement with the present experiments. The structure of the solvent around the hydrogel backbone polymer is analyzed using H-bond statistics and visualization. It is found that cononsolvency is caused by the fact that the methanol molecules strongly attach to the hydrogel's backbone polymer, mainly with their hydroxyl group. This leads to the effect that the hydrophobic methyl groups of methanol are oriented toward the bulk solvent. The hydrogel+solvent shell hence appears hydrophobic and collapses in water-rich solvents. As more methanol is present in the solvent, the effect disappears again.

Limits on the deterministic creation of pure single-photon states using parametric down-conversion

Abstract: Parametric down-conversion (PDC) is one of the most widely used methods to create pure single-photon states for quantum information applications. However, little attention has been paid to higher-order photon components in the PDC process, yet these ultimately limit the prospects of generating single photons of high quality. In this paper we investigate the impact of higher-order photon components and multiple frequency modes on the heralding rates and single-photon fidelities. This enables us to determine the limits of PDC sources for single-photon generation. Our results show that a perfectly single-mode PDC source in conjunction with a photon-number-resolving detector is ultimately capable of creating single-photon Fock states with unit fidelity and a maximal state creation probability of 25$%$. Hence, an array of 17 switched sources is required to build a deterministic ($g$99$%$ emission probability) pure single-photon source.

Error tolerance of the boson-sampling model for linear optics quantum computing

Abstract: Linear optics quantum computing is a promising approach to implementing scalable quantum computation. However, this approach has very demanding physical resource requirements. Recently, Aaronson and Arkhipov(e-print arXiv:1011.3245) showed that a simplified model, which avoids the requirement for fast feed-forward and postselection, while likely not capable of solving BQP-complete problems efficiently, can solve an interesting sampling problem believed to be classically hard. Loss and mode mismatch are the dominant sources of error in such systems. We provide evidence that even lossy systems or systems with mode mismatch are likely to be classically hard to solve. This is of practical interest to experimentalists wishing to demonstrate such systems since it suggests that, even with errors in their implementation, they are likely implementing an algorithm that is classically hard to solve. Our results also equivalently apply to the multiwalker quantum walk model.

Learning behavior models for hybrid timed systems

Abstract: A tailored model of a system is the prerequisite for various analysis tasks, such as anomaly detection, fault identification, or quality assurance. This paper deals with the algorithmic learning of a system's behavior model given a sample of observations. In particular, we consider real-world production plants where the learned model must capture timing behavior, dependencies between system variables, as well as mode switches--in short: hybrid system's characteristics. Usually, such model formation tasks are solved by human engineers, entailing the well-known bunch of problems including knowledge acquisition, development cost, or lack of experience. Our contributions to the outlined field are as follows. (1) We present a taxonomy of learning problems related to model formation tasks. As a result, an important open learning problem for the domain of production system is identified: The learning of hybrid timed automata. (2) For this class of models, the learning algorithm HyBUTLA is presented. This algorithm is the first of its kind to solve the underlying model formation problem at scalable precision. (3) We present two case studies that illustrate the usability of this approach in realistic settings. (4) We give a proof for the learning and runtime properties of HyBUTLA.

Frequency tuning of piezoelectric energy harvesters by magnetic force

Abstract: A piezoelectric energy harvester is an electromechanical device that converts ambient mechanical vibration into electric power. Most existing vibration energy harvesting devices operate effectively at a single frequency only, dictated by the design of the device. This frequency must match the frequency of the host structure vibration. However, real world structural vibrations rarely have a specific constant frequency. Therefore, piezoelectric harvesters that generate usable power across a range of exciting frequencies are required to make this technology commercially viable. Currently known harvester tuning techniques have many limitations, in particular they miss the ability to work during harvester operation and most often cannot perform a precise tuning. This paper describes the design and testing of a vibration energy harvester with tunable resonance frequency, wherein the tuning is accomplished by changing the attraction force between two permanent magnets by adjusting the distance between the magnets. This tuning technique allows the natural frequency to be manipulated before and during operation of the harvester. Furthermore the paper presents a physical description of the frequency tuning effect. The experimental results achieved with a piezoelectric bimorph fit the calculated results very well. The calculation and experimental results show that using this tuning technique the natural frequency of the harvester can be varied efficiently within a wide range: in the test setup, the natural frequency of the piezoelectric bimorph could be increased by more than 70%.

Procrastination in a distance university setting

Abstract: Procrastination, putting off until tomorrow what should have been done today, is a self-regulation failure that is widespread among students. Although plenty of research has emerged regarding academic procrastination, hardly any research endeavor regarding procrastination in distance university settings exists. This lack of research is even more astonishing when considering that the demands on self-regulation are higher in distance education settings than in traditional university settings. The present (questionnaire) study was intended to shed light on procrastination in an actual distance university setting by exploring its relationship to grades, learning strategies (e.g., cognitive, meta-cognitive strategies), and life satisfaction in students from a distance university in comparison to students from a traditional university.

Technology for enhancing statistical reasoning at the school level

Abstract: The purpose of this chapter is to provide an updated overview of digital technologies relevant to statistics education, and to summarize what is currently known about how these new technologies can support the development of students’ statistical reasoning at the school level. A brief literature review of trends in statistics education is followed by a section on the history of technologies in statistics and statistics education. Next, an overview of various types of technological tools highlights their benefits, purposes and limitations for developing students’ statistical reasoning. We further discuss different learning environments that capitalize on these tools with examples from research and practice. Dynamic data analysis software applications for secondary students such as Fathom and TinkerPlots are discussed in detail. Examples are provided to illustrate innovative uses of technology. In the future, these uses may also be supported by a wider range of new tools still to be developed. To summarize some of the findings, the role of digital technologies in statistical reasoning is metaphorically compared with travelling between data and conclusions, where these tools represent fast modes of transport. Finally, we suggest future directions for technology in research and practice of developing students’ statistical reasoning in technology-enhanced learning environments.

Hybrid functionals and GW approximation in the FLAPW method

Abstract: We present recent advances in numerical implementations of hybrid functionals and the GW approximation within the full-potential linearized augmented-plane-wave (FLAPW) method. The former is an approximation for the exchange–correlation contribution to the total energy functional in density-functional theory, and the latter is an approximation for the electronic self-energy in the framework of many-body perturbation theory. All implementations employ the mixed product basis, which has evolved into a versatile basis for the products of wave functions, describing the incoming and outgoing states of an electron that is scattered by interacting with another electron. It can thus be used for representing the nonlocal potential in hybrid functionals as well as the screened interaction and related quantities in GW calculations. In particular, the six-dimensional space integrals of the Hamiltonian exchange matrix elements (and exchange self-energy) decompose into sums over vector–matrix–vector products, which can be evaluated easily. The correlation part of the GW self-energy, which contains a time or frequency dependence, is calculated on the imaginary frequency axis with a subsequent analytic continuation to the real axis or, alternatively, by a direct frequency convolution of the Green function G and the dynamically screened Coulomb interaction W along a contour integration path that avoids the poles of the Green function. Hybrid-functional and GW calculations are notoriously computationally expensive. We present a number of tricks that reduce the computational cost considerably, including the use of spatial and time-reversal symmetries, modifications of the mixed product basis with the aim to optimize it for the correlation self-energy and another modification that makes the Coulomb matrix sparse, analytic expansions of the interaction potentials around the point of divergence at k = 0, and a nested density and density-matrix convergence scheme for hybrid-functional calculations. We show CPU timings for prototype semiconductors and illustrative results for GdN and ZnO.