Circular polarization reversal of half-vortex cores in polariton condensates

Vortices are topological objects carrying quantized orbital angular momentum and have been widely studied in many physical systems for their applicability in information storage and processing. In systems with spin degree of freedom the elementary excitations are so called half-vortices, carrying a quantum rotation only in one of the two spin components. We study the spontaneous formation and stability of localized such half-vortices in semiconductor microcavity polariton condensates, non-resonantly excited by a linearly polarized ring-shaped pump. The TE-TM splitting of optical modes in the microcavity system leads to an effective spin-orbit coupling, resulting in solutions with discrete rotational symmetry. The cross-interaction between different spin components provides an efficient method to realize all-optical half-vortex core switching inverting its circular polarization state. This switching can be directly measured in the polarization resolved intensity in the vortex core region and it can also be applied to higher order half-vortex states.Comment: 8 pages, 8 figure

Exploring roads to functionality of polaritons in semiconductor microcavities

Optoelektronische Bauelemente gewannen in den letzten Jahren an Bedeutung zwecks ihrer möglichen Anwendungen im Bereich Informationsverarbeitung und Datenkommunikation. Halbleitermikrokavitäten haben sich als exzellente Plattformen für die Untersuchung fundamentaler Aspekte der Licht-Materie Wechselwirkung sowie die Erforschung funktionaler Aspekte herausgestellt. In diesen Strukturen entstehen Licht-Materie Quasiteilchen aus der starken Kopplung zwischen dem Lichtfeld in der Kavität und den Exzitonen im Halbleitermaterial. Die hybride Struktur dieser sogenannten Polaritonen zeichnet sich durch eine kleine effektive Masse, eine lange Kohärenzzeit und starke repulsive Wechselwirkungen aus, was sie experimentell zugänglich und gut geeignet für die Untersuchung nichtlinearer optischer Phänomene macht. Diese Arbeit konzentriert sich auf drei funktionale Aspekte von Polaritonen, die wir mit detaillierten numerischen und analytischen Berechnungen untersuchen. Der erste Teil fokussiert sich auf zwei verschiedene optische Schalter in einem resonant sowie nicht-resonant angeregten System, die auf der Polarisationsabhängigkeit der Polaritonen basieren. Der zweite Teil beschäftigt sich mit einer allgemeineren Charakterisierung von Polariton Kondensaten im Hinblick auf mögliche Anwendungen im Bereich der Quanteninformationstheorie. Innerhalb einer Theorie-Experiment Zusammenarbeit wird die Quantenkohärenz in verschiedenen Anregungskonfigurationen bestimmt.Optoelectronic devices have become increasingly important in recent years due to possible applications in information processing and data communication. The key point is the fundamental understanding of the underlying physics of the light-matter interaction. Semiconductor microcavities are excellent platforms to study fundamental aspects of the light-matter interaction as well as to explore functional aspects for possible applications. In these structures, light-matter quasiparticles arise due to a strong coupling between the cavity light field and the excitons in the semiconductor material. The hybrid nature of these so-called polaritons is characterized by a small effective mass, a long coherence time, and strong repulsive interactions, rendering them experimentally accessible and well-suited to investigate nonlinear optical phenomena. This thesis focuses on three different functional aspects of polaritons, which are explored with the help of extensive numerical and analytical calculations. In the first part, we investigate two distinct all- optical switching schemes based on the polaritons peculiar polarization dependence under resonant and nonresonant excitation. In the second part, we focus on a more general characterization of polariton condensates in terms of potential applications in quantum- information tasks. Within a theory-experiment cooperation, we determine the quantum coherence of a polariton condensates under spatially different excitation setups.by Matthias Pukrop ; [Examination Board: Prof. Dr. Stefan Schumacher, Prof. Dr. Torsten Meier, Prof. Dr. Cedric Meier, Dr. Uwe Gerstmann]Tag der Verteidigung: 24.02.2022Universität Paderborn, Dissertation, 202

On Cloud Assisted, Restricted, and Resource Constrained Scheduling

Wir untersuchen drei Scheduling-Problemfamilien auf ihre komplexitätstheoretische Härte und zeigen, welche algorithmischen Ansätze für diese Probleme geeignet sind. Im ersten Modell, das wir neu einführe und Server Cloud Scheduling nennen, werden große Abläufe als Sammlung von Jobs mit Abhängigkeiten gegeben. Die Jobs können auf einem Server ausgeführt werden, der beschränkt, aber kostenlos ist, oder in die Cloud ausgelagert werden, die unendliche Parallelität erlaubt, aber Kosten verursacht. Das Ziel ist die Minimierung der Laufzeit unter Einhaltung eines Budgets oder Minimierung der Kosten unter Einhaltung einer Frist. Unsere Ergebnisse beinhalten unter anderem FPTAS, die auf dynamischer Programmierung basieren, für recht allgemeine Fälle des Modells, und ein starkes NP-härte Ergebnis, das sogar dann gilt, wenn alle Joblängen 1 sind. Zweitens untersuchen wir das Restricted Assignment Interval Problem, bei dem Maschinen in einer bestimmten Reihenfolge gegeben sind und jeder Job nur auf einer konsekutiven Teilmenge der Maschinen ausgeführt werden kann. Ziel ist die Minimierung der Laufzeit. Es ist uns gelungen den ersten Approximationsalgorithmus mit einem konstanten Approximationsfaktor kleiner als 2 für das Problem zu liefern. Der Algorithmus erweitert frühere Ansätze der linearen Programmierung. Zuletzt betrachten wir Many Shared Resources Scheduling. Zusätzlich zu Maschinen und Jobs erhalten wir hier Ressourcen. Jeder Job benötigt eine Ressource, und Jobs, die dieselbe Ressource benötigen, können nicht parallel ausgeführt werden. Ziel ist die Laufzeit zu minimieren. Wir stellen zwei kombinatorische Approximationsalgorithmen vor, eine einfache und elegante 5/3-Approximation und eine technisch aufwendigere 3/2-Approximation. Unser erster Algorithmus schlägt bereits den Stand der Technik.We study three different families of scheduling problems, discuss their complexity theoretical hardness and show how the problems lend themselves to various algorithmic approaches. First, we introduce a new problem called server cloud scheduling, in which large tasks are given as a collection of smaller jobs with some precedence relation between themselves. Jobs can be processed on a purely sequential but free server or offloaded to the cloud, which allows infinite parallelization but incurs costs. The objective is either to minimize the makespan while adhering to a budget or to minimize the cost while adhering to a deadline. Some of our main results are dynamic programming based FPTAS for fairly general special cases of the model; and a strong NP-hardness result that holds even when all processing times are equal to 1. Second, we consider the restricted assignment interval problem, where the machines are given in some order, and each job is only eligible on a consecutive subset of the machines. The objective is to minimize the makespan. We give the first approximation algorithm with a constant approximation ratio less than 2 for restricted assignment interval. The algorithm refines and expands previous linear programming approaches. Last, we consider many shared resources scheduling. In addition to the machines and jobs, we are also given a set of resources. Each job may now require exclusive access to one of the resources during its processing time, or in other words, two jobs needing the same resource may not be scheduled in parallel. The goal is again to minimize the makespan. We present two combinatorial approximation algorithms, a simple and elegant 5/3-approximation and a technically more involved 3/2-approximation. Both of these beat the previous best-known algorithm, which was a 2-approximation.submitted by Simon Pukrop ; [Reviewers Prof. Dr. Friedhelm Meyer auf der Heide; Paderborn University, Prof. Dr. Klaus Jansen; Kiel University]Tag der Verteidigung: 26.06.2023Universität Paderborn, Dissertation, 202

Cognitive alterations in patients with non-affective psychotic disorder and their unaffected siblings and parents

Meijer J, Simons CJP, Quee PJ, Verweij K, GROUP Investigators. Cognitive alterations in patients with non-affective psychotic disorder and their unaffected siblings and parents

Externally Controlled Lotka-Volterra Dynamics in a Linearly Polarized Polariton Fluid

Spontaneous formation of transverse patterns is ubiquitous in nonlinear dynamical systems of all kinds. An aspect of particular interest is the active control of such patterns. In nonlinear optical systems this can be used for all-optical switching with transistor-like performance, for example realized with polaritons in a planar quantum-well semiconductor microcavity. Here we focus on a specific configuration which takes advantage of the intricate polarization dependencies in the interacting optically driven polariton system. Besides detailed numerical simulations of the coupled light-field exciton dynamics, in the present paper we focus on the derivation of a simplified population competition model giving detailed insight into the underlying mechanisms from a nonlinear dynamical systems perspective. We show that such a model takes the form of a generalized Lotka-Volterra system for two competing populations explicitly including a source term that enables external control. We present a comprehensive analysis both of the existence and stability of stationary states in the parameter space spanned by spatial anisotropy and external control strength. We also construct phase boundaries in non-trivial regions and characterize emerging bifurcations. The population competition model reproduces all key features of the switching observed in full numerical simulations of the rather complex semiconductor system and at the same time is simple enough for a fully analytical understanding of the system dynamics.Comment: 11 pages, 8 figure

Rückmeldungen aus Schulleistungstests an Lehrkräfte durch interaktive Informationsvisualisierungen

Teaching staff are expected to be able to use results from comprehensive student achieve-ment tests to improve their instruction. For these data-driven decision-making processes, skills that could be denoted as data literacy are required. Research has shown, however, that in some cases these competencies are not sufficiently pronounced and that feedback is often difficult to understand. Furthermore, the research demonstrated that it is necessary to in-crease the comprehensibility of feedback by using data visualizations. Regional state insti-tutes for education tasked with providing feedback supported teaching staff by offering fur-ther training and handouts on how to use external achievement test results. The matter of increasing the comprehensibility of feedback remains unaddressed. This gap in the research will be taken up in this thesis, which uses the example of comparative achievement tests to approach the question of whether the comprehensibility of achievement-test feedback could be increased by using a feedback system with interactive information visualizations. As part of a study with a cross-sectional design, twenty primary school teachers compared a familiar paper-based feedback for achievement-test results with a new, interactive feedback system. First, the teachers participating in the study assessed the perceived time required to answer the questions asked about the test results. Secondly, the teachers assessed how useful the feedback formats were for answering the questions. The results show that, on average, the questions asked about the data could be answered significantly more quickly using the interactive information visualizations. Furthermore, interactive information visualizations are assessed as being significantly more useful on average. Matrix visualizations were prov-en to have particularly positive effects. For the co-variables only one significant effect was perceived: Teachers with a high level of data literacy assessed the interactive feedback sys-tem as being faster. Moreover, the teachers gave statements regarding requirements for an interactive feedback system and their attitudes and experiences in working with achievement tests. The findings of this study indicate that feedback systems with a user- and demand-oriented graphic layout are becoming more important

(In-)Approximability Results for Interval, Resource Restricted, and Low Rank Scheduling

We consider variants of the restricted assignment problem where a set of jobs has to be assigned to a set of machines, for each job a size and a set of eligible machines is given, and the jobs may only be assigned to eligible machines with the goal of makespan minimization. For the variant with interval restrictions, where the machines can be arranged on a path such that each job is eligible on a subpath, we present the first better than 2-approximation and an improved inapproximability result. In particular, we give a (2-1/24)-approximation and show that no better than 9/8-approximation is possible, unless P=NP. Furthermore, we consider restricted assignment with R resource restrictions and rank D unrelated scheduling. In the former problem, a machine may process a job if it can meet its resource requirements regarding R (renewable) resources. In the latter, the size of a job is dependent on the machine it is assigned to and the corresponding processing time matrix has rank at most D. The problem with interval restrictions includes the 1 resource variant, is encompassed by the 2 resource variant, and regarding approximation the R resource variant is essentially a special case of the rank R+1 problem. We show that no better than 3/2, 8/7, and 3/2-approximation is possible (unless P=NP) for the 3 resource, 2 resource, and rank 3 variant, respectively. Both the approximation result for the interval case and the inapproximability result for the rank 3 variant are solutions to open challenges stated in previous works. Lastly, we also consider the reverse objective, that is, maximizing the minimal load any machine receives, and achieve similar results

Der Kampf des Gehirns gegen eindringende Krebszellen: Die organspezifische Abwehr

Auch heute noch versterben viele Menschen an den Folgen der Metastasierung durch solide Tumore (Lungenkrebs, Dickdarmkrebs, Brustkrebs etc.). Dennoch sind der genaue zeitliche Ablauf und die molekularen Mechanismen nur partiell verstanden. Vereinfacht kann dieser komplexe Prozess nach dem Aufenthaltsort der Krebszellen in drei Blöcke eingeteilt werden: i) die Phasen am Entstehungsorgan; ii) die Phasen im Blut- oder Lymphsystem; iii) und die Phasen im fremden Organ. In den letzten Jahren hat man durch genetische Untersuchungen festgestellt, dass sich erfolgreich metastasierte Krebszellen früher als erwartet vom Entstehungsort lösen und sich andernorts sogar weiter entwickeln (parallele Evolution). Zudem ist bekannt geworden, dass sich bei vielen Patienten Krebszellen im Blut oder Knochenmark nachweisen lassen, die aber erstaunlicherweise später keine Metastasen entwickeln. Somit müssen viele Krebszellen noch im Blut oder im fremden Organ erfolgreich aufgehalten werden. Aber gerade die letzten Phasen der Metastasierung im fremden Organ sind nur rudimentär erforscht. Interessanterweise ergaben einige Arbeiten, das beinahe alle Krebszellen kurz nach dem Austritt aus dem Blutgefäß in dem fremden Organ sehr schnell absterben. Somit scheint das Überleben in einer unbekannten Umgebung für die Krebszellen nicht nur das größte Hindernis, sondern auch der entscheidende Faktor über Leben und Tod zu sein. Aus diesen Gründen widmet sich unsere Arbeitsgruppe intensiv dieser letzten Phase der Metastasierung und der Frage, wie sich das fremde Organ gegen eindringende Krebszellen wehrt

SS-Mediziner zwischen Lagerdienst und Fronteinsatz : die personelle Besetzung der medizinischen Abteilung im Konzentrationslager Sachsenhausen 1936-1945

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