Model Order Selection in DoA Scenarios via Cross-Entropy based Machine Learning Techniques

In this paper, we present a machine learning approach for estimating the number of incident wavefronts in a direction of arrival scenario. In contrast to previous works, a multilayer neural network with a cross-entropy objective is trained. Furthermore, we investigate an online training procedure that allows an adaption of the neural network to imperfections of an antenna array without explicitly calibrating the array manifold. We show via simulations that the proposed method outperforms classical model order selection schemes based on information criteria in terms of accuracy, especially for a small number of snapshots and at low signal-to-noise-ratios. Also, the online training procedure enables the neural network to adapt with only a few online training samples, if initialized by offline training on artificial data

Die Expansivität Europas und ihre Folgen = The expansion of Europe and its consequences. ZEI Discussion Paper C 247/2018

Europe was created by expansion and still expands continuously until today. This expansivity created dubious consequences in Eastern Europe. On the other hand, the political, economic, and cultural problems of globality also originated from European expansion, but not exclusively from colonialism. Therefore, instead of the generalized hate of the heirs of the victims and the generalized contrition of the heirs of the perpetrators a common differentiated approach to history from both sides is the order of the day

Macroeconometric evaluation of active labour market policies in Austria

The analysis of active labour market policies (ALMP) at the macroeconomic level measures net effects on labour market outcomes. These net effects consist of direct treatment effects on participants as well as indirect effects on non-participants and on the economy as a whole, e.g. deadweight, substitution and displacement effects. This paper contributes to the empirical studies of macroeconometric evaluation of ALMP by considering the regional effects on both the matching process and the job-seeker rate. This joint view permits us to draw conclusions on how ALMP achieves the goals set by policy makers. To this end, we use an exclusive data set on Austrian job-seekers in the years 2001 to 2007 and employ contemporaneous GMM and quasi-ML estimators to take into account both the simultaneity of ALMP and spatial interrelations between employment office districts. The empirical results indicate that a large number of participants in job schemes in the non-profit sector, wage subsidies, and apprenticeships cause particularly favourable effects on the regional matching function and the job-seeker rate.

Fluid Vesicles in Shear Flow

The shape dynamics of fluid vesicles is governed by the coupling of the flow within the two-dimensional membrane to the hydrodynamics of the surrounding bulk fluid. We present a numerical scheme which is capable of solving this flow problem for arbitrarily shaped vesicles using the Oseen tensor formalism. For the particular problem of simple shear flow, stationary shapes are found for a large range of parameters. The dependence of the orientation of the vesicle and the membrane velocity on shear rate and vesicle volume can be understood from a simplified model.Comment: 10 pages, LaTeX2e, psfig, 5 eps figures included, to appear in Phys. Rev. Lett., 199

Entanglement generation in a system of two atomic quantum dots coupled to a pool of interacting bosons

We discuss entanglement generation in a closed system of one or two atomic quantum dots (qubits) coupled via Raman transitions to a pool of cold interacting bosons. The system exhibits rich entanglement dynamics, which we analyze in detail in an exact quantum mechanical treatment of the problem. The bipartite setup of only one atomic quantum dot coupled to a pool of bosons turns out to be equivalent to two qubits which easily get entangled being initially in a product state. We show that both the number of bosons in the pool and the boson-boson interaction crucially affect the entanglement characteristics of the system. The tripartite system of two atomic quantum dots and a pool of bosons reduces to a qubit-qutrit-qubit realization. We consider entanglement possibilities of the pure system as well as of reduced ones by tracing out one of the constituents, and show how the entanglement can be controlled by varying system parameters. We demonstrate that the qutrit, as expected, plays a leading role in entangling of the two qubits and the maximum entanglement depends in a nontrivial way on the pool characteristics.Comment: 16 pages, 6 figure

Decoherence Models for Discrete-Time Quantum Walks and their Application to Neutral Atom Experiments

We discuss decoherence in discrete-time quantum walks in terms of a phenomenological model that distinguishes spin and spatial decoherence. We identify the dominating mechanisms that affect quantum walk experiments realized with neutral atoms walking in an optical lattice. From the measured spatial distributions, we determine with good precision the amount of decoherence per step, which provides a quantitative indication of the quality of our quantum walks. In particular, we find that spin decoherence is the main mechanism responsible for the loss of coherence in our experiment. We also find that the sole observation of ballistic instead of diffusive expansion in position space is not a good indicator for the range of coherent delocalization. We provide further physical insight by distinguishing the effects of short and long time spin dephasing mechanisms. We introduce the concept of coherence length in the discrete-time quantum walk, which quantifies the range of spatial coherences. Unexpectedly, we find that quasi-stationary dephasing does not modify the local properties of the quantum walk, but instead affects spatial coherences. For a visual representation of decoherence phenomena in phase space, we have developed a formalism based on a discrete analogue of the Wigner function. We show that the effects of spin and spatial decoherence differ dramatically in momentum space.Comment: 32 pages, 10 figures, 1 table, replaced fig. 10 in the new versio

Anisotropic backscatter in ice-penetrating radar data: potential mechanisms and implications

Airborne and ground-based radar have been used extensively in the past to measure ice thickness and to investigate the internal structure of ice sheets in terms of layering. The main reflection mechanisms for internal reflections are changes in density, conductivity, and crystal orientation fabric, which alter thepermittivity of the ice. Linking the different mechanisms to the individual reflection horizons enables thededuction of glaciological parameters like accumulation rates or age-depth estimates. If no sample material from snow pits or ice-cores are available, multi-frequency and multi-polarization measurements must be applied to distinguish between the different reflection mechanisms. The backscattered power of horizons caused by changes in conductivity varies with the center frequency whereas in the case of horizons originating from changing crystal orientation the backscattered power is dependent on the polarization plane of the carrier signal.In this study we examine a sample data set near the German summer station Kohnen (drill site for theEPICA-EDML ice core) on the Antarctic plateau. The data were acquired with an airplane sliding on ground, producing varying incident polarization with a circular profile and several cross profiles with different headings. We find that the backscattered power changes with varying antenna orientation (i.e. polarization). In the upper third of the ice column the backscatter has two maxima with a 180° symmetry. The maxima align with the direction of minimal surface strain. At approximately 900 m depth the anisotropy is shifted by 90° in heading azimuth, with the maxima now being parallel to the maximum in surface strain. This dataset is unique, as airborne systems (primarily designed for the sounding of ice thickness) are usually not used for ground-based applications. The observed anisotropy appears clearly and is intriguing as the reason for it is entirely unknown. As primary suspects we consider the role of changing crystal orientation and ellipsoidal shaped air bubbles. The effect is visible from 200 1400 m. It appears distributed along the entire interval, and not restricted to individual layers. It seems that the polarization dependence becomes visible by a changing background level of the acquired signal, which is otherwise largely dominated by layer-like, polarization independent reflections. Hence we apply a (semi-analytical) volume scattering model in order to understand the different reflection mechanisms better. From ice-core measurements it is known that the crystals in the upper hundred meters are only weakly aligned (if at all), and it is unclear how the crystal orientation changes overshort depth intervals (~10 m). The rotation of the anisotropy coincides with the clathrate transition in the ice core and thus we first focus on the effect of anisotropic air bubbles. In an in-coherent approach we treat the ice matrix as a random medium and use the vector radiative transfer theory to incorporate boundary conditions. In a second step we model the effect of crystal orientation to estimate both, the degree of alignment and the statistical variance in the permittivity tensor needed to generate the observed pattern in backscatter. Doing so, we eventually aim at pinning down the mechanisms for the anisotropy in the upper interval, lower interval and the interrelation of the two by a shift of 90°.Anisotropic air bubbles as well as aligned crystal orientation allow to deduce stress and strain rates and a potential change thereof along depth. So far it is largely unclear, how surface strain rates relate with strain rates within the ice. If one of the two suspected mechanisms can be excluded or confirmed, this study may serve as a case study for future polarimetric surveys with low-frequency radars, in order to supply ice-sheet modelling with adequate boundary conditions - including changes in the internal structure of ice sheets along depth