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Using standard telecommunication equipment, we present an experimental platform for investigating Parity-Time (PT) symmetry in a two-dimensional synthetic lattice with tunable gain, loss and phase structure. By exploiting the fibre nonlinearity, dissipative TT solitons and wave collapse are realized, for which the later one gives rise to an amplified, self-accelerating nonlinear wave packet. Following a concept developed in [1], we present a novel protocol for realizing TT-symmetry in a 2D synthetic lattices based on mixed short and long-range coupling. Our set-up (see Fig. 1a) consists of two pairs of slightly dissimilar coupled fibre loops, where each fibre patch is approximately 30 km long. As shown in Fig. 1a, a 20ns long optical pulse at 1550 nm wavelength is injected into the outer left loop and splits at the first 50/50 coupler at the entrance of the two inner loops. The lengths of the inner two loops differ by ΔLinner ≈ 600m (ΔTinner ≈ 3μs), while that of the outer loops by ΔLouter ≈ 6m (ΔTouter = 30ns). After passing through the second 50/50 coupler, the pulses split again and propagate through the outer loops back to the first 50/50 coupler. After a mean round trip time Τ ≈ 300 μs, an ordered sequence of four pulses arrives at the first coupler and starts its journey again. In each roundtrip m pulse delay increases / decreases by ΔTinner, if the pulse passes the long / short inner loop which is equivalent to a step to the right / left on an effective 2D synthetic lattice denoted by x. Afterwards, by propagating through the outer long / short loop, the pulse delay increases / decreases by ΔTouter, respectively, and thus the pulse makes a step up / down on the lattice denoted by y. In this way, any path through the 2D lattice can be interpreted as a combination of roundtrips through the four different loops. The arrival times of the pulses at the photodetector are measured, sampled electronically and mapped onto a 2D discrete lattice spanned by the χ and y coordinates (see Fig. 1b). This synthetic lattice is, for instance, equivalent to a 2D waveguide array and both 50/50 couplers resemble the effect of wave coupling to neighbouring waveguides in horizontal or vertical direction. TT-symmetry can be realized by applying a balanced gain/loss space-dependent protocol on the synthetic lattice (red and blue arrows in Fig. 1b). Unbroken TT-symmetry is only realized if in addition phase modulation scheme is applied (orange diagonal regions in Fig. 1b).

Over the last few years, parity-time ($\mathcal{P}\mathcal{T}$) symmetry has been the focus of considerable attention. Ever since, pseudo-Hermitian notions have permeated a number of fields ranging from optics to atomic and topological physics, as well as optomechanics, to mention a few. Unlike their Hermitian counterparts, nonconservative systems do not exhibit a priori real eigenvalues and hence unitary evolution. However, once $\mathcal{P}\mathcal{T}$ symmetry is introduced, such dissipative systems can surprisingly display a real eigenspectrum, thus ensuring energy conservation during evolution. In optics, $\mathcal{P}\mathcal{T}$ symmetry can be readily established by incorporating, in a balanced way, regions having an equal amount of optical gain and loss. However, thus far, all optical realizations of such $\mathcal{P}\mathcal{T}$ symmetry have been restricted to a single transverse dimension (1D), such as arrays of optical waveguides or active coupled cavity arrangements. In most cases, only the loss function was modulated---a restrictive aspect that is only appropriate for linear systems. Here, we present an experimental platform for investigating the interplay between $\mathcal{P}\mathcal{T}$ symmetry and nonlinearity in two-dimensional (2D) environments, where nonlinear localization and soliton formation can be observed. In contrast to typical dissipative solitons, we demonstrate a one-parameter family of soliton solutions that are capable of displaying attributes similar to those encountered in nonlinear conservative arrangements. For high optical powers, this new family of $\mathcal{P}\mathcal{T}$ solitons tends to collapse on a discrete network---thus giving rise to an amplified, self-accelerating structure.

MSSM-like string models from the compactification of the heterotic string on toroidal orbifolds (of the kind $T^6/P$) have distinct phenomenological properties, like the spectrum of vector-like exotics, the scale of supersymmetry breaking, and the existence of non-Abelian flavor symmetries. We show that these characteristics depend crucially on the choice of the underlying orbifold point group $P$. In detail, we use boosted decision trees to predict $P$ from phenomenological properties of MSSM-like orbifold models. As this works astonishingly well, we can utilize machine learning to predict the orbifold origin of the MSSM., 24 pages, 6 figures

We investigate linear and nonlinear light propagation in weakly perturbed AlGaAs slab waveguides whose fabrication induced local defects give rise to random multiple scattering. The statistical evaluation of the intensity distribution at the rear facet reveals an abundance of high-amplitude events in the linear regime, contradicting the predictions of the central limit theorem by far. In this article we propose weak successive backscattering to be a generator of high-amplitude events, while a focusing nonlinearity of the Kerr type counterintuitively diminishes them, thus effectively randomizing the intensity distribution. We support our experimental findings with numerical simulations based on the stray matrix approach introduced by Edrei et al. [Phys. Rev. Lett. 62, 2120 (1989)PRLTAO0031-900710.1103/PhysRevLett.62.2120]. This novel proposed mechanism for the formation of extreme events in the context of linear wave dynamics represents the first encounter of the phenomenon in the limit of weak scattering.

As time flow dictates all evolution, its effective reversal is a topic of active research in a broad range of disciplines, including acoustics, hydrodynamics and optics. This multifarious set of environments is reflected by a great diversity of approaches to observe various echoes of wave functions. Here, we experimentally demonstrate time reversal of a pulse sequence propagating through a photonic mesh lattice realized by two coupled loops of telecommunication fibres. Our system features a symmetric band structure, which allows for almost perfect reversal of its evolution by exchanging the population between two opposing bands. The protocol applied is based on a non-adiabatic and instantaneous exchange of eigenstates resulting in highly efficient time reversal of a pulse chain.

We experimentally investigate a topologically non-trivial edge state of the SSH model. In the presence of nonlinearity topological protection breaks down resulting in efficient energy transfer from a nonlinear wave to the edge state.

We experimentally demonstrate nonlinear wave collapse at mW-power levels in a genuine 1D system formed by coupled optical fibers. Based on time-multiplexing we realize an effective two-dimensional lattice and observe strong nonlinearly driven field compression.

Photonic mesh lattices provide a versatile experimental platform for studying light propagation in a discrete environment. By using standard telecommunication equipment it is possible to influence the evolution of light by actively modulating its amplitude and phase. Fiber components thus offer a ready to use solution for creating a \( \mathbb{PT} \)-symmetric optical network. Here, we analyze two classes of \( \mathbb{PT} \)-symmetric networks: those with local \( \mathbb{PT} \) symmetry fulfilling the conditions of \( \mathbb{PT} \) symmetry in each time step but not globally and those with global \( \mathbb{PT} \) symmetry providing completely real eigenvalues. We investigate light propagation in these dissipative media in the linear and nonlinear regime and discuss nonlinear localization as well as the formation of soliton. Furthermore, Bloch oscillations are induced by an external phase modulation and are found to restore pseudo-Hermitian propagation.

RSC Advances 7(28), 17164-17172 (2017). doi:10.1039/C7RA01140G, Published by RSC Publishing, London

A thermal process window to form a stable metallurgical compound of AlSn6Cu and Al99.5 was obtained by initial experiments. A special mould system for a horizontal continuous composite casting process was developed, supported by finite element simulations. Preliminary 2D models were used to identify the main process variables influencing the temperature in the region where the compound is formed between the two layers. The thickness ratio of the layers and the initial temperature of the AlSn6Cu substrate strip were found to be the most important parameters. The special bilayer mould system was manufactured and implemented into an existing continuous casting device upgraded by a second furnace to hold the additional pure aluminium. A stable casting process was achieved. The quality of the manufactured compound was assessed by metallographic specimens cut from the obtained bilayers. Based on temperature measurements, a full 3D finite element model was developed to gain a more realistic description of the temperature and fluid flow conditions in the composite casting, especially in the margin regions.

An action generates an equal and opposite reaction. If it were possible, however, for one of the two bodies to have negative mass, they would accelerate each other. A situation analogous to this is now realized in an optical system. Solitons moving in an optical mesh lattice exhibit either an effective positive or negative mass, thus enabling observation of self-acceleration.

Mit der EU-Wasserrahmenrichtlinie wurde unter anderem eine europaische Strategie gegen die Wasserverschmutzung eingefuhrt. Ein wichtiges Element dieser Strategie ist der kombinierte Ansatz, wonach bei Einleitungen in Gewasser emissionsseitige Begrenzungen und gleichzeitig Immissionsgrenzwerte anzuwenden sind, die nach Verdunnung des Abwassers im Vorfluter eingehalten werden mussen. Mit den EU-Verordnungen REACH und CLP wurden vor wenigen Jahren die Rahmenbedingungen fur die europaische Chemikalienpolitik neu aufgestellt. Ein Kernelement ist die Verpflichtung von Herstellern oder Importeuren von chemischen Stoffen uber eine Tonne pro Jahr, diese zu registrieren und damit umfangreiche Daten uber die sichere Verwendung von Chemikalien vorzulegen. Diese Daten und Informationen sowie die Instrumente von REACH (z. B. Zulassung, Beschrankung) stehen nun auch den anderen Umweltsektoren und insbesondere der Wasserwirtschaft zur Verfugung und sollten mittelfristig auch den Zielen der Strategie gegen die Wasserverschmutzung dienen.

A number of concurrent, relaxed priority queues have recently been proposed and implemented. Results are commonly reported for a throughput benchmark that uses a uniform distribution of keys from a large integer range, a balanced mixture of operations, and mostly for single systems. We have conducted more extensive benchmarking of three recent, relaxed priority queues on four different types of systems with different key ranges and distributions. While we can show superior throughput and scalability for our own k-LSM priority queue for the uniform key distribution, the picture changes drastically for other distributions, both with respect to achieved throughput and relative merit of the priority queues. The throughput benchmark alone is thus not sufficient to characterize the performance of concurrent priority queues. Our priority queue, benchmark code and full set of results are publicly available to foster comparison.

Photonic mesh lattices are easily realized in the temporal domain using standard telecom equipment. This allows for the experimental demonstration of quantum walks and of various effects of linear and nonlinear dynamics in discrete systems.

Uber die Halfte der Studenten vergessen jahrlich ihre Passworter und verursachen deshalb enorme Verwaltungsprobleme. Die Universitat Regensburg hat einen neuen Weg beschritten, um dem Herr zu werden. Sie setzt auf die sensorlose Biometrie Tippverhalten.

Controlling light transport in nonlinear active environments is a topic of considerable interest in the field of optics. In such complex arrangements, of particular importance is to devise strategies to subdue chaotic behaviour even in the presence of gain/loss and nonlinearity, which often assume adversarial roles. Quite recently, notions of parity-time (PT) symmetry have been suggested in photonic settings as a means to enforce stable energy flow in platforms that simultaneously employ both amplification and attenuation. Here we report the experimental observation of optical solitons in PT-symmetric lattices. Unlike other non-conservative nonlinear arrangements where self-trapped states appear as fixed points in the parameter space of the governing equations, discrete PT solitons form a continuous parametric family of solutions. The possibility of synthesizing PT-symmetric saturable absorbers, where a nonlinear wave finds a lossless path through an otherwise absorptive system is also demonstrated., Parity–time symmetry can impose a stable energy flow in photonic systems with simultaneous amplification and attenuation. Here, Wimmer et al. demonstrate optical solitons belonging to a continuous parametric family of solutions in a parity–time-symmetric lattice and observe saturable absorber action.

We report the first experimental observation of optical solitons in PT-symmetric temporal lattices. By utilizing a judicious balance between gain, loss and nonlinear effects, such self-trapped states remain stable over long distances.

Light propagation in periodic environments is often associated with a number of interesting and potentially useful processes. If a crystalline optical potential is also linearly ramped, light can undergo periodic Bloch oscillations, a direct outcome of localized Wannier-Stark states and their equidistant eigenvalue spectrum. Even though these effects have been extensively explored in conservative settings, this is by no means the case in non-Hermitian photonic lattices encompassing both amplification and attenuation. Quite recently, Bloch oscillations have been predicted in parity-time-symmetric structures involving gain and loss in a balanced fashion. While in a complex bulk medium, one intuitively expects that light will typically follow the path of highest amplification, in a periodic system this behavior can be substantially altered by the underlying band structure. Here, we report the first experimental observation of Bloch oscillations in parity-time-symmetric mesh lattices. We show that these revivals exhibit unusual properties like secondary emissions and resonant restoration of PT symmetry. In addition, we present a versatile method for reconstructing the real and imaginary components of the band structure by directly monitoring the light evolution during a cycle of these oscillations., Comment: Published online 07 December 2015 in Scientific Reports

The time evolution of the resistance of amorphous thin films of the phase change materials Ge2Sb2Te5, GeTe and AgIn-Sb2Te is measured during annealing at T = 80°C. The annealing process is interrupted by several fast temperature dips to determine the changing temperature dependence of the resistance. This procedure enables us to identify to what extent the resistance increase over time can be traced back to an increase in activation energy EA or to a rise of the prefactor R*. We observe that, depending on the material, the dominating contribution to the increase in resistance during annealing can be either a change in activation energy (Ge2Sb2Te5) or a change in prefactor (AgIn-Sb2Te). In the case of GeTe, both contribute about equally. We conclude that any phenomenological model for the resistance drift in amorphous phase change materials that is based on the increase of one parameter alone (e.g., the activation energy) cannot claim general validity.

Die plattformunabhangige und organisationsubergreifende Web Services-Interoperabilitat setzt ein leistungsfahiges und flexibles Autorisierungssystem voraus. In diesem Beitrag wird ein Zugriffskontrollsystem vorgestellt, welches sich durch das Zusammenspiel lokaler und verteilter Autorisierung auszeichnet. Im Zusammenhang mit lokaler Zugriffskontrolle, bei der Rechte innerhalb einer Organisation ausgewertet werden, stellt sich insbesondere die Herausforderung, Autorisierungsregeln zu konsolidieren: Da die Funktionalitat von Web Services haufig auf weitere Anwendungen und Betriebsmittel wie Datenbanksystemen aufsetzt, ergeben sich Abhangigkeiten bezuglich der Autorisierung, die uberpruft und eingehalten werden mussen. Uber eine verteilte Zugriffskontrolle wird der Aufbau von Kollaborationsnetzwerken ermoglicht. Der Augenmerk liegt im Folgenden auf schwach gekoppelten Zusammenschlussen, die die Autonomie der beteiligten Organisationen beibehalt. Skalierbarkeit und Effizenz werden durch den Einsatz von rollenbasierter Zugriffskontrolle einerseits und dem Caching ahnlicher, wiederkehrender Autorisierungen andererseits erreicht.

In this letter, bipolar fast-pulse switching in TiO2 -based nanocrossbar devices was investigated. A dedicated measurement setup was used to measure the transient currents during 5-ns resistive switching. Transient peak currents for the set and reset processes were as high as 200 and 230 μA, respectively. The currents observed during fast-pulse switching are explained and simulated by Joule heating, which is needed for fast oxygen-vacancy movement. The measured transient currents enable a further optimization of resistive switches based on TiO2.

Priority queues are data structures which store keys in an ordered fashion to allow efficient access to the minimal (maximal) key. Priority queues are essential for many applications, e.g., Dijkstra's single-source shortest path algorithm, branch-and-bound algorithms, and prioritized schedulers. Efficient multiprocessor computing requires implementations of basic data structures that can be used concurrently and scale to large numbers of threads and cores. Lock-free data structures promise superior scalability by avoiding blocking synchronization primitives, but the \emph{delete-min} operation is an inherent scalability bottleneck in concurrent priority queues. Recent work has focused on alleviating this obstacle either by batching operations, or by relaxing the requirements to the \emph{delete-min} operation. We present a new, lock-free priority queue that relaxes the \emph{delete-min} operation so that it is allowed to delete \emph{any} of the $\rho+1$ smallest keys, where $\rho$ is a runtime configurable parameter. Additionally, the behavior is identical to a non-relaxed priority queue for items added and removed by the same thread. The priority queue is built from a logarithmic number of sorted arrays in a way similar to log-structured merge-trees. We experimentally compare our priority queue to recent state-of-the-art lock-free priority queues, both with relaxed and non-relaxed semantics, showing high performance and good scalability of our approach., Comment: Short version as ACM PPoPP'15 poster

Hyperobjects are efficient mechanisms to coordinate accesses to shared variables and data-structures in task-parallel programming models, where each thread can operate on its own coordinated local view of the shared data. Synchronization between local views is restricted to occur at well-defined points in the execution, and can be left to the hyperobject implementation. This paper provides a general model for hyperobjects that does not require programming language or runtime support and may therefore be used with any task-parallel programming system. We show that hyperobjects can be efficiently implemented in a wait-free manner, meaning that all concurrent accesses to a hyperobject are guaranteed to complete in a bounded number of steps. The novel finisher hyperobject presented in this paper provides transitive termination detection for task-parallel programs. It can be used to efficiently implement task synchronization primitives like finish. However, finishers can also be used to manage reference-counted resources, e.g. shared pointers and copy-on-write pointers. Finally, we provide a wait-free variant of the associative reducer hyperobject known from the Cilk++ programming language.

Pheet, a task-scheduling framework that allows for easy customization of internal data-structures, is a research vehicle for experimenting with high-level application and low-level architectural support for task-parallel programming models. Pheet is highly configurable, and allows comparison between different implementations of data structures used in the scheduler, as well as comparisons between entirely different schedulers (typically using work-stealing). Pheet is being used to investigate high-level task-parallel support mechanisms that allow applications to influence scheduling decisions and behavior. One such mechanism, that we use in this work, is scheduling strategies. Previous Pheet benchmarking was done on conventional multicore architectures from AMD and Intel. In this paper we discuss the performance of Pheet on a prototype Intel Xeon Phi coprocessor with 61 cores. We compare these results to Pheet on three conventional multicore architectures. Using two benchmarks from the mostly non-numerical/combinatorial Pheet suite we find that the Xeon Phi coprocessor provides considerably better scalability than the other architectures, with more than a 70x speedup on the 61-core Knights Corner prototype system when using 4-way SMT, although not achieving the same absolute performance. For our research, the Xeon Phi coprocessor is an interesting architecture for implementing and evaluating fine-grained task-parallel parallel algorithm implementations.

Work-stealing systems are typically oblivious to the nature of the tasks they are scheduling. They do not know or take into account how long a task will take to execute or how many subtasks it will spawn. Moreover, task execution order is typically determined by an underlying task storage data structure, and cannot be changed. There are thus possibilities for optimizing task parallel executions by providing information on specific tasks and their preferred execution order to the scheduling system. We investigate generalizations of work-stealing and introduce a framework enabling applications to dynamically provide hints on the nature of specific tasks using scheduling strategies . Strategies can be used to independently control both local task execution and steal order. Strategies allow optimizations on specific tasks, in contrast to more conventional scheduling policies that are typically global in scope. Strategies are composable and allow different, specific scheduling choices for different parts of an application simultaneously. We have implemented a work-stealing system based on our strategy framework. A series of benchmarks demonstrates beneficial effects that can be achieved with scheduling strategies.