Showing papers by "University of Paderborn published in 2017"

Advanced capabilities for materials modelling with Quantum ESPRESSO.

Abstract: Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software

Advanced capabilities for materials modelling with Quantum ESPRESSO

Abstract: Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

Nonlinear photonic metasurfaces

Abstract: Compared with conventional optical elements, 2D photonic metasurfaces, consisting of arrays of antennas with subwavelength thickness (the ‘meta-atoms’), enable the manipulation of light–matter interactions on more compact platforms. The use of metasurfaces with spatially varying arrangements of meta-atoms that have subwavelength lateral resolution allows control of the polarization, phase and amplitude of light. Many exotic phenomena have been successfully demonstrated in linear optics; however, to meet the growing demand for the integration of more functionalities into a single optoelectronic circuit, the tailorable nonlinear optical properties of metasurfaces will also need to be exploited. In this Review, we discuss the design of nonlinear photonic metasurfaces — in particular, the criteria for choosing the materials and symmetries of the meta-atoms — for the realization of nonlinear optical chirality, nonlinear geometric Berry phase and nonlinear wavefront engineering. Finally, we survey the application of nonlinear photonic metasurfaces in optical switching and modulation, and we conclude with an outlook on their use for terahertz nonlinear optics and quantum information processing. Photonic metasurfaces can be used to control the polarization, phase and amplitude of light. Nonlinear metasurfaces enable giant nonlinear optical chirality, realization of the geometric Berry phase, wavefront engineering, and optical switching and modulation, and hold potential for on-chip applications.

Beam switching and bifocal zoom lensing using active plasmonic metasurfaces

Abstract: Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators. While the larger counterparts typically rely on mechanical actuation, this can be undesirable in some cases on a microscopic scale due to inherent space restrictions. Here, we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6. In particular, we demonstrate beam switching and bifocal lensing, thus, paving the way for a plethora of active optical elements employing plasmonic metasurfaces, which follow the same design principles.

Gaussian Boson Sampling.

Abstract: Boson sampling has emerged as a tool to explore the advantages of quantum over classical computers as it does not require universal control over the quantum system, which favors current photonic experimental platforms. Here, we introduce Gaussian Boson sampling, a classically hard-to-solve problem that uses squeezed states as a nonclassical resource. We relate the probability to measure specific photon patterns from a general Gaussian state in the Fock basis to a matrix function called the Hafnian, which answers the last remaining question of sampling from Gaussian states. Based on this result, we design Gaussian Boson sampling, a #P hard problem, using squeezed states. This demonstrates that Boson sampling from Gaussian states is possible, with significant advantages in the photon generation probability, compared to existing protocols.

SINR-Based DoS Attack on Remote State Estimation: A Game-Theoretic Approach

Abstract: We consider remote state estimation of cyberphysical systems under signal-to-interference-plus-noise ratio-based denial-of-service attacks. A sensor sends its local estimate to a remote estimator through a wireless network that may suffer interference from an attacker. Both the sensor and the attacker have energy constraints. We first study an associated two-player game when multiple power levels are available. Then, we build a Markov game framework to model the interactive decision-making process based on the current state and information collected from previous time steps. To solve the associated optimality (Bellman) equations, a modified Nash Q-learning algorithm is applied to obtain the optimal solutions. Numerical examples and simulations are provided to demonstrate our results.

The customer value proposition: evolution, development, and application in marketing

Abstract: The customer value proposition (CVP) has a critical role in communicating how a company aims to provide value to customers. Managers and scholars increasingly use CVP terminology, yet the concept remains poorly understood and implemented; relatively little research on this topic has been published, considering the vast breadth of investigations of the value concept. In response, this article offers a comprehensive review of fragmented CVP literature, highlighting the lack of a strong theoretical foundation; distinguishes CVPs from related concepts; proposes a conceptual model of the CVP that includes antecedents, consequences, and moderators, together with several research propositions; illustrates the application of the CVP concept to four contrasting companies; and advances a compelling agenda for research.

Hot isostatic pressing of IN718 components manufactured by selective laser melting

Abstract: Selective laser melting and other additive manufacturing (AM) techniques have recently attracted substantial interest of both researchers and the processing industry. The freedom of design leads to completely new possibilities for constructions and, thus, to entirely new products. In the selective laser melting (SLM) process, the components are produced layer-wise using a laser beam. SLM is a powder bed based AM process and is characterized by the complete melting of the utilized powder material. Employing SLM, complex three-dimensional parts and light weight structures can be produced directly from 3D CAD data. However, although SLM is a very promising technology, there are still challenges to solve. In the present study, a close look is taken at the porosity. Under cyclic loading, pores can act as stress raisers and lead to premature crack initiations, which reduce the fatigue strength of the material. Hot isostatic pressing (HIP) offers the possibility to reduce the porosity. HIP combines high pressure and high temperature to produce materials with superior properties. The influence of the HIP process parameters on the density and microstructure of IN718 SLM components is investigated by means of micro X-ray computed tomography and scanning electron microscopy. The results of the experiments show that the majority of pores can be densified by means of HIP. On the other hand, some pores cannot be densified. The reason for this is seen in entrapped argon gas from the SLM process.

Control-Flow Integrity: Precision, Security, and Performance

Abstract: Memory corruption errors in C/C++ programs remain the most common source of security vulnerabilities in today’s systems. Control-flow hijacking attacks exploit memory corruption vulnerabilities to divert program execution away from the intended control flow. Researchers have spent more than a decade studying and refining defenses based on Control-Flow Integrity (CFI); this technique is now integrated into several production compilers. However, so far, no study has systematically compared the various proposed CFI mechanisms nor is there any protocol on how to compare such mechanisms. We compare a broad range of CFI mechanisms using a unified nomenclature based on (i) a qualitative discussion of the conceptual security guarantees, (ii) a quantitative security evaluation, and (iii) an empirical evaluation of their performance in the same test environment. For each mechanism, we evaluate (i) protected types of control-flow transfers and (ii) precision of the protection for forward and backward edges. For open-source, compiler-based implementations, we also evaluate (iii) generated equivalence classes and target sets and (iv) runtime performance.

Ultrathin Nonlinear Metasurface for Optical Image Encoding

Abstract: Security of optical information is of great importance in modern society. Many cryptography techniques based on classical and quantum optics have been widely explored in the linear optical regime. Nonlinear optical encryption in which encoding and decoding involve nonlinear frequency conversions represents a new strategy for securing optical information. Here, we demonstrate that an ultrathin nonlinear photonic metasurface, consisting of meta-atoms with 3-fold rotational symmetry, can be used to hide optical images under illumination with a fundamental wave. However, the hidden image can be read out from second harmonic generation (SHG) waves. This is achieved by controlling the destructive and constructive interferences of SHG waves from two neighboring meta-atoms. In addition, we apply this concept to obtain gray scale SHG imaging. Nonlinear metasurfaces based on space variant optical interference open new avenues for multilevel image encryption, anticounterfeiting, and background free image reconstruction.

Beamnet: End-to-end training of a beamformer-supported multi-channel ASR system

Abstract: This paper presents an end-to-end training approach for a beamformer-supported multi-channel ASR system. A neural network which estimates masks for a statistically optimum beamformer is jointly trained with a network for acoustic modeling. To update its parameters, we propagate the gradients from the acoustic model all the way through feature extraction and the complex valued beamforming operation. Besides avoiding a mismatch between the front-end and the back-end, this approach also eliminates the need for stereo data, i.e., the parallel availability of clean and noisy versions of the signals. Instead, it can be trained with real noisy multi-channel data only. Also, relying on the signal statistics for beamforming, the approach makes no assumptions on the configuration of the microphone array. We further observe a performance gain through joint training in terms of word error rate in an evaluation of the system on the CHiME 4 dataset.

Predictors of teachers' use of ICT in school --- the relevance of school characteristics, teachers' attitudes and teacher collaboration

Abstract: This paper is based on the research question of what predictors (school characteristics, teachers' attitudes, teacher collaboration and background characteristics) determine secondary school teachers' frequency of computer use in class. The use of new technologies by secondary school teachers for educational purposes is an important factor regarding school and teaching processes. The use of digital media in schools is, among other things, associated with the goal of supporting learning processes and improving the quality of education. This contribution identifies relevant factors by means of multiple regression analyses of the teachers' frequency of computer use for instruction in five countries (the Netherlands, Denmark, Australia, Poland and Germany) to get an idea of how to support the frequency of the use of computers in class. The analyses and findings are based on the International Association for the Evaluation of Educational Achievement (IEA) study of International Computer and Information Literacy Study 2013 (ICILS), which investigates the computer and information literacy (CIL) of secondary school students and the contexts in which students develop CIL in 21 countries. Antecedents concerning school characteristics, teachers' attitudes and teacher collaboration on the process level and background characteristics of secondary school teachers (N = 8.920) are examined in order to gain further insight into the nature and effect of predictors for secondary school teachers' in-class use of information and communications technology. The analyses show that there are more country-specific results than similarities between the countries selected. In all countries, antecedents concerning teachers' attitudes are more relevant for teachers' in-class use of computers than school characteristics or teacher collaboration on the process level.

A generalized solution concept for the Keller–Segel system with logarithmic sensitivity: global solvability for large nonradial data

Abstract: The chemotaxis system $$\begin{aligned} \left\{ \begin{array}{l} u_t = \Delta u - \chi abla \cdot \left( \frac{u}{v} abla v\right) , \\ v_t=\Delta v - v+u, \end{array} \right. \end{aligned}$$ is considered in a bounded domain $$\Omega \subset \mathbb {R}^n$$ with smooth boundary, where $$\chi >0$$ . An apparently novel type of generalized solution framework is introduced within which an extension of previously known ranges for the key parameter $$\chi $$ with regard to global solvability is achieved. In particular, it is shown that under the hypothesis that $$\begin{aligned} \chi <\left\{ \begin{array}{ll} \infty \qquad &{} \text{ if } n=2, \\ \sqrt{8} \qquad &{} \text{ if } n=3, \\ \frac{n}{n-2} \qquad &{} \text{ if } n\ge 4, \end{array} \right. \end{aligned}$$ for all initial data satisfying suitable assumptions on regularity and positivity, an associated no-flux initial-boundary value problem admits a globally defined generalized solution. This solution inter alia has the property that $$\begin{aligned} u\in L^1_{loc}(\overline{\Omega }\times [0,\infty )). \end{aligned}$$

7th Open Challenge on Question Answering over Linked Data (QALD-7)

Abstract: The past years have seen a growing amount of research on question answering (QA) over Semantic Web data, shaping an interaction paradigm that allows end users to profit from the expressive power of Semantic Web standards while, at the same time, hiding their complexity behind an intuitive and easy-to-use interface. On the other hand, the growing amount of data has led to a heterogeneous data landscape where QA systems struggle to keep up with the volume, variety and veracity of the underlying knowledge.

Sensor Scheduling in Variance Based Event Triggered Estimation With Packet Drops

Abstract: This paper considers a remote state estimation problem with multiple sensors observing a dynamical process, where sensors transmit local state estimates over an independent and identically distributed (i.i.d.) packet dropping channel to a remote estimator. At every discrete time instant, the remote estimator decides whether each sensor should transmit or not, with each sensor transmission incurring a fixed energy cost. The channel is shared such that collisions will occur if more than one sensor transmits at a time. Performance is quantified via an optimization problem that minimizes a convex combination of the expected estimation error covariance at the remote estimator and expected energy usage across the sensors. For transmission schedules dependent only on the estimation error covariance at the remote estimator, this work establishes structural results on the optimal scheduling which show that: 1) for unstable systems, if the error covariance is large then a sensor will always be scheduled to transmit and 2) there is a threshold-type behavior in switching from one sensor transmitting to another. Specializing to the single sensor case, these structural results demonstrate that a threshold policy (i.e., transmit if the error covariance exceeds a certain threshold and don't transmit otherwise) is optimal. We also consider the situation where sensors transmit measurements instead of state estimates, and establish structural results including the optimality of threshold policies for the single sensor, scalar case. These results provide a theoretical justification for the use of such threshold policies in variance based event triggered estimation. Numerical studies confirm the qualitative behavior predicted by our structural results.

Optically excited structural transition in atomic wires on surfaces at the quantum limit

Abstract: Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

Effects of signal-dependent motilities in a Keller–Segel-type reaction–diffusion system

Abstract: This work considers the Keller–Segel-type parabolic system ut = Δ(uϕ(v)), x ∈ Ω,t > 0, vt = Δv − v + u,x ∈ Ω,t > 0, (⋆) in a smoothly bounded convex domain Ω ⊂ ℝn, n ≥ 2, under no-flux boundary conditions, which has recently been proposed as a model for processes of stripe pattern formation via so-called “self-trapping” mechanisms In the two-dimensional case, in stark contrast to the classical Keller–Segel model in which large-data solutions may blow up in finite time, for all suitably regular initial data the associated initial value problem is seen to possess a globally-defined bounded classical solution, provided that the motility function ϕ ∈ C3([0,∞)) ∩ W1,∞((0,∞)) is uniformly positive In the corresponding higher-dimensional setting, it is shown that certain weak solutions exist globally, where in the particular three-dimensional case this solution actually is bounded and classical if the initial data are suitably small in the norm of L2(Ω) × W1,4(Ω) Finally, if still n = 3 but merely the physically interpretable quantity ∥ϕ′∥ L∞((0,∞))∫Ωu0 is appropriately small, then the above-weak solutions are proved to become eventually smooth and bounded

Nonlinear Metasurface for Simultaneous Control of Spin and Orbital Angular Momentum in Second Harmonic Generation.

Abstract: The spin and orbital angular momentum (SAM and OAM) of light is providing a new gateway toward high capacity and robust optical communications. While the generation of light with angular momentum is well studied in linear optics, its further integration into nonlinear optical devices will open new avenues for increasing the capacity of optical communications through additional information channels at new frequencies. However, it has been challenging to manipulate the both SAM and OAM of nonlinear signals in harmonic generation processes with conventional nonlinear materials. Here, we report the generation of spin-controlled OAM of light in harmonic generations by using ultrathin photonic metasurfaces. The spin manipulation of OAM mode of harmonic waves is experimentally verified by using second harmonic generation (SHG) from gold meta-atom with 3-fold rotational symmetry. By introducing nonlinear phase singularity into the metasurface devices, we successfully generate and measure the topological charges o...

Gaussian Boson sampling

Abstract: We present the protocol for Gaussian Boson Sampling with single-mode squeezed states. We eliminate heralding and show that our proposal with the Hafnian matrix function can retain the higher photon number contributions at the input.

Single-pixel computational ghost imaging with helicity-dependent metasurface hologram.

Abstract: Different optical imaging techniques are based on different characteristics of light. By controlling the abrupt phase discontinuities with different polarized incident light, a metasurface can host a phase-only and helicity-dependent hologram. In contrast, ghost imaging (GI) is an indirect imaging modality to retrieve the object information from the correlation of the light intensity fluctuations. We report single-pixel computational GI with a high-efficiency reflective metasurface in both simulations and experiments. Playing a fascinating role in switching the GI target with different polarized light, the metasurface hologram generates helicity-dependent reconstructed ghost images and successfully introduces an additional security lock in a proposed optical encryption scheme based on the GI. The robustness of our encryption scheme is further verified with the vulnerability test. Building the first bridge between the metasurface hologram and the GI, our work paves the way to integrate their applications in the fields of optical communications, imaging technology, and security.

Crashworthiness and numerical simulation of hybrid aluminium-CFRP tubes under axial impact

Abstract: In the present study, the crashworthiness and the numerical simulation of circular hybrid aluminium-CFRP tubes are investigated. It can be shown that hybrids provide significant lightweight potential. The specific energy absorption is 37% higher compared to a pure aluminium structure. The post-crash analysis is done by computed tomography methods. The failure of the hybrid component shows a mixture of energy absorption mechanisms of its pure materials. Hybrid-specific energy absorption mechanisms compensate the limited primary energy absorption mechanisms of the CFRP and metal components. The simulation of axial-loaded composite structures is challenging the currently available simulation methods. In the present work, a multi shell model is used for the simulation of the hybrid structure. This approach enables an efficient design of CFRP-aluminium hybrid components. First approaches towards the simulation of hybrid specific failure modes are given.

Volumetric Generation of Optical Vortices with Metasurfaces

Abstract: Recent advances in metasurfaces, i.e., two-dimensional arrays of engineered nanoscale inclusions that are assembled onto a surface, have revolutionized the way to control electromagnetic waves with ultrathin, compact components. The generation of optical vortex beams, which carry orbital angular momentum, has emerged as a vital approach to applications ranging from high-capacity optical communication to parallel laser fabrication. However, the typically bulky elements used for the generation of optical vortices impose a fundamental limit toward on-chip integration with subwavelength footprints. Here, we investigate and experimentally demonstrate a three-dimensional volumetric optical vortices generation based on light–matter interaction with a high-efficiency dielectric metasurface. By employing the concepts of Dammann vortex gratings and spiral Dammann zone plates, a 3D optical vortex array with micrometer spatial separation is achieved from visible to near-infrared wavelengths. Importantly, we show that...

Critical mass for infinite-time aggregation in a chemotaxis model with indirect signal production

Abstract: We study the Neumann initial-boundary problem for the chemotaxis system    ut = ∆u−∇ · (u∇v), x ∈ Ω, t > 0, 0 = ∆v − μ(t) + w, x ∈ Ω, t > 0, τwt + δw = u, x ∈ Ω, t > 0, (⋆) in the unit disk Ω := B1(0) ⊂ R, where δ ≥ 0 and τ > 0 are given parameters and μ(t) := − ∫ Ω w(x, t)dx, t > 0. It is shown that this problem exhibits a novel type of critical mass phenomenon with regard to the formation of singularities, which drastically differs from the well-known threshold property of the classical Keller-Segel system, as obtained upon formally taking τ → 0, in that it refers to blow-up in infinite time rather than in finite time: Specifically, it is first proved that for any sufficiently regular nonnegative initial data u0 and w0, (⋆) possesses a unique global classical solution. In particular, this shows that in sharp contrast to classical Keller-Segel-type systems reflecting immediate signal secretion by the cells themselves, the indirect mechanism of signal production in (⋆) entirely rules out any occurrence of blow-up in finite time. However, within the framework of radially symmetric solutions it is next proved that • whenever δ > 0 and ∫ Ω u0 8πδ, one can find initial data such that ∫ Ω u0 = m, and such that for the corresponding solution we have ‖u(·, t)‖L∞(Ω) → ∞ as t → ∞.

Structural lithium ion battery electrolytes via reaction induced phase-separation

Abstract: For the realization of structural batteries, electrolytes where both higher ionic conductivity and stiffness are combined need to be developed. The present study describes the formation of a struct ...

CogniCrypt: supporting developers in using cryptography

Abstract: Previous research suggests that developers often struggle using low-level cryptographic APIs and, as a result, produce insecure code. When asked, developers desire, among other things, more tool support to help them use such APIs. In this paper, we present CogniCrypt, a tool that supports developers with the use of cryptographic APIs. CogniCrypt assists the developer in two ways. First, for a number of common cryptographic tasks, CogniCrypt generates code that implements the respective task in a secure manner. Currently, CogniCrypt supports tasks such as data encryption, communication over secure channels, and long-term archiving. Second, CogniCrypt continuously runs static analyses in the background to ensure a secure integration of the generated code into the developer’s workspace. This video demo showcases the main features of CogniCrypt: youtube.com/watch?v=JUq5mRHfAWY.

An integrated location-inventory-routing humanitarian supply chain network with pre- and post-disaster management considerations

Abstract: Efficiency is a key success factor in complex supply chain networks. It is imperative to ensure proper flow of goods and services in humanitarian supply chains in response to a disaster. To this end, we propose a multi-echelon humanitarian logistic network that considers the location of central warehouses, managing the inventory of perishable products in the pre-disaster phase, and routing the relief vehicles in the post-disaster phase. An epsilon-constraint method, a non-dominated sorting genetic algorithm (NSGA-II), and a modified NSGA-II called reference point based non-dominated sorting genetic algorithm-II (RPBNSGA-II) are proposed to solve this mixed integer linear programming (MILP) problem. The analysis of variance (ANOVA) is used to analyze the results showing that NSGA-II performs better than the other algorithms with small size problems while RPBNSGA-II outperforms the other algorithms with large size problems.

Monitoring multiple myeloma by next-generation sequencing of V(D)J rearrangements from circulating myeloma cells and cell-free myeloma DNA.

Abstract: Recent studies suggest that circulating tumor cells and cell-free DNA may represent powerful non-invasive tools for monitoring disease in patients with solid and hematologic malignancies. Here, we conducted a pilot study in 27 myeloma patients to explore the clonotypic V(D)J rearrangement for monitoring circulating myeloma cells and cell-free myeloma DNA. Next-generation sequencing was used to define the myeloma V(D)J rearrangement and for subsequent peripheral blood tracking after treatment initiation. Positivity for circulating myeloma cells/cell-free myeloma was associated with conventional remission status (P<0.001) and 91% of non-responders/progressors versus 41% of responders had evidence of persistent circulating myeloma cells/cell-free myeloma DNA (P<0.001). About half of the partial responders showed complete clearance of circulating myeloma cells/cell-free myeloma DNA despite persistent M-protein, suggesting that these markers are less inert than the M-protein, rely more on cell turnover and, therefore, decline more rapidly after initiation of effective treatment. Positivity for circulating myeloma cells and for cell-free myeloma DNA were associated with each other (P=0.042), but discordant in 30% of cases. This indicates that cell-free myeloma DNA may not be generated entirely by circulating myeloma cells and may reflect overall tumor burden. Prospective studies need to define the predictive potential of high-sensitivity determination of circulating myeloma cells and DNA in the monitoring of multiple myeloma.