Kolloquium Sommersemester 2016

The CeNoS Kolloquium starts always at 16.30 s.t in Room 222 of the Institute for Applied Physics. From 16.15 on coffee is available.

Datum	Vortrag
04.04.2016	Sondertermin! 14:00 Uhr c.t. Integrated frequency combs meet quantum optics Dr. Michael Kues, Institut national de la recherche scientifique, Centre Energie Matériaux Télécommunications, Varennes (Québec), Canada Einladender: Prof. Dr. C. Fallnich The generation of optical quantum states on an integrated platform will enable low-cost and accessible advances for quantum technologies such as secure communications and quantum computation [1-4]. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index doped-glass platform – Hydex [5]) can enable the generation of pure heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, with photon frequencies corresponding to standard telecommunication channels spaced by 200 GHz. Exploiting a self-locked, intra-cavity excitation configuration, a highly stable integrated source of multiplexed heralded single photons is demonstrated, operating continuously for several weeks with less than 5% fluctuation. The measured photon bandwidth of 110 MHz is compatible with quantum memories, and high photon purity was confirmed though single photon auto-correlation measurements [6]. In turn, by simultaneously exciting two orthogonal polarization mode resonances, we introduce a new type of spontaneous four-wave mixing (FWM) to the toolbox of integrated photonics. In particular, we demonstrate the first realization of type-II spontaneous FWM (in analogy of type-II spontaneous parametric down-conversion in second-order media), which allows the direct generation of orthogonally-polarized photon pairs on a chip [7]. By using double-pulse excitation, we demonstrate the generation of time-bin entangled photon pairs [8] over the entire frequency comb spectrum. We measure qubit entanglement with visibilities above 90% and perform a tomographic density matrix reconstruction with a fidelity of 96%, enabling the implementation of quantum information processing protocols. Finally, the excitation field and the generated photons are intrinsically bandwidth-matched due to the resonant characteristics of the ring cavity, enabling the multiplication of Bell states and the generation of a four-photon time-bin entangled state. We confirm the generation of this four-photon entangled state through four-photon quantum interference with a measured visibility of 89% without background correction [9]. Integrated quantum frequency combs are thus a scalable and versatile platform for quantum information processing. References [1] D. Bonneau, J. W. Silverstone, M. G. Thompson, in Silicon Photonics III, L. Pavesi, D. J. Lockwood, Eds. (Springer, ed. 3rd, 2016), pp. 41–82. [2] H.J. Kimble,“The quantum internet,” Nature 453, 1023 (2008) [3] M. Kolobov, “The spatial behavior of nonclassical light,” Rev. Mod. Phys. 71, 1539 (1999) [4] P. Walther et al., “Experimental one-way quantum computing,” Nature. 434, 169 (2005) [5] D. J. Moss et al., “New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics,” Nature Photon. 7, 597 (2013). [6] C. Reimer et al., “Integrated frequency comb source of heralded single photons,” Opt. Express. 22, 1023 (2014) [7] C. Reimer et al., “Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip,” Nat. Commun. 6, 8236 (2015). [8] J. Brendel et al., “Pulsed energy-time entangled twin-photon source for quantum communication,” Phys. Rev. Lett. 86, 1392–1393 (2001). [9] C. Reimer et al., “Generation of multiphoton entangled quantum states by means of integrated frequency combs,” Science 351(6278), 1176-1180 (2016).
26.04.2016	Bulk and beyond: Scale bridging perspective on phase separation near surfaces and interfaces Prof. Dr. R. Spatschek, Forschungszentrum Jülich, Abteilung Thermochemie von Energiewerkstoffen Einladender: Prof. Dr. A. Heuer Phase separation is one of the key features of materials science with severe implications for various systems. Conventional thermodynamic concepts apply to bulk systems and deliver descriptions, which are of highest importance for many applied multicomponent systems. However, the aspect of elastic distortions, which arise due to lattice mismatches, are still not captured e.g. in thermodynamic databases. The situation becomes even more complex in the presence of surfaces and interfaces. We look at at this issue on various scales from a theoretical and computational perspective. As a result, we obtain a description which allows to include long-ranged elastic effects in a terminology accessible to atomistic and ab initio simulations. In particular find a drastic reduction of the solubility limit near free surfaces due to elastic coherency effects. This mechanism favours nucleation at free surfaces even in the absence of external stresses. We discuss the implications of this effect for hydrogen containing systems, where it may favor material failure due to hydrogen embrittlement, and compare the predictions to experimental observations.
03.05.2016	Risiko ! -- Modellierung von Risiken im Finanzbereich als Jobchance Dr. C. Romeike, Dr. Felix Huerkamp, Dr. C. Honisch, d-fine GmbH Einladender: Dr. O. Kamps Zur Webseite der Veranstaltung
10.05.2016	Quantenzustandsrekonstruktion und Nachweis von nichtklassischen Eigenschaften von Lichtfeldern Prof. Dr. Boris Hage, Universität Rostock Einladender: Prof. Dr. C. Fallnich Die Quanteneigenschaften (im Gegensatz zu den klassischen) von Lichtfeldern stellen die Grundlage für die modernen Forschungsfelder der optischen Quantenkommunikation, -kryptographie und -informationsverarbeitung dar. Die Rekonstruktion von optischen Quantenzuständen, etwa anhand der Wignerfunktion durch Quantenzustandstomographie, ist eine etablierte Methode. Wir berichten über unsere Anstrengungen den vermutlich wegen des Kerr-Effekts verschränkten Quantenzustand eines Solitonenmoleküls nach der Propagation durch eine optische Glasfaser zu rekonstruieren. Grundsätzlich ist die Rekonstruktion einer innerhalb der Messgenauigkeit eindeutigen Darstellung diese Quantenzustands möglich, jedoch sind anhand dessen im allgemeinen nur wenige Charakteristika ablesbar, etwa ob der Zustand auch eine rein klassische Darstellung hat und somit im einleitenden Sinne unbrauchbar ist. Daher beschäftigen uns mit der experimentellen Umsetzung von Nichtklassizitätskriterien, die in jüngster Zeit auf theoretischer Seite entwickelt wurden. Als Referenzobjekte mit wohl bekannten Eigenschaften verwenden wir hier gequetschte Zustände, die wir mithilfe der Dreiwellenmischung in entarteten optisch-parametrischen Verstärkern herstellen.
24.05.2016	Mitgliederversammlung
31.05.2016	Phase-field models for the growth and evolution of complex structures Prof. Dr. Mathis Plapp, Laboratoire PMC - Ecole Polytechnique Einladender: Prof.Dr. U. Thiele Complex ramified structures are generated naturally by growth and self-organization in a large number of processes. The ramification usually arises from morphological instabilities that are due to a coupling of interface motion to transport processes in the bulk. A complete understanding of structure formation therefore requires to follow the complex interface dynamics and the resulting nonlinear pattern selection processes. In recent years, the phase-field method has emerged as a method of choice for the numerical modelling of such free-boundary problems. It is based on phenomenological equations of out-of-equilibrium thermodynamics that are combined with free-energy functionals of Ginzburg-Landau type; interfaces are represented implicitly by profiles of suitable order parameters. Using relatively simple codes, structure evolution can be simulated qualitatively, and for some cases even quantitatively. I will first give an introduction to the principles of this method, taking as an example the growth of crystal dendrites during solidification, and then discuss other applications to structure formation in hard and soft matter systems.
14.06.2016	Spatiotemporal patterns with pde2path Prof. Dr. H. Uecker, Institut für Mathematik, Carl von Ossietzky Universität Oldenburg Einladender: Prof. Dr. U. Thiele The Matlab continuation and bifurcation package pde2path [1] has originally been developed for elliptic PDE over 2D spatial domains, but has recently been extended to also treat Hopf bifurcations. In this talk we review the basic ideas of pde2path, and give some examples of Hopf and other bifurcations for PDEs over 1,2 and 3 space dimensions. [1] www.staff.uni-oldenburg.de/hannes.uecker/pde2path/index.html
28.06.2016	Learning from fluctuations: The mechanics of active and passive cellular assemblies Prof. Dr. Timo Betz , Institut für Zellbiologie, WWU Münster Einladender: Dr. O. Kamps From a physics perspective living cells are impressive systems. They operate extremely reliably under nonlinear and non-equilibrium conditions, all embedded in a highly fluctuating background that is agitated by thermal Brownian motion. To understand the physical principles used by cells to perform their function, we use optical tweezers as well as high sensitive motion detectors to study both the mechanical properties of cells and the passive (thermal) and active (ATP dependent) fluctuations of cellular systems. This leads to new insights into the non-equilibrium physics used by living cells to maintain their organization even in a highly noisy environment. Combining the experimental data with non-equilibrium Langevin models we are able to extract molecular parameters such as forces and timescales from mesoscopic observables.
05.07.2016	Reward-based Learning and Decision Making Prof. Dr. Klaus Obermayer, TU Berlin - Institute of Software Engineering and Theoretical Computer Science - Neural Information Processing Einladender: PD Dr. C. Wolters Reinforcement learning provides a framework for making agents learn policies by feedback (reward) about whether their actions or action sequences were successful or not. Reinforcement learning also provides a framework for understanding, how humans learn and decide given reward information only. Standard reinforcement learning assumes that good decisions / actions / policies are the ones which maximize expected reward as a proxy of success. Humans and animals, on the other hand, often do not behave this way, and there is ample evidence for multiple reward-based learning systems as well as for multiple factors influencing learning and decision making. In my talk I will first address the interaction between stimulus-response and response-outcome learning in two tasks, where subjects are presented with both visual stimuli and rewards. One task involves implicit (shifts of covert visual attention) the other task explicit (button presses) decisions and actions. Using a model-based analysis I will show, how "bottom-up" stimulus and "top-down" reward information interact, and I will discuss the signatures of these interactions in neural signals measured with EEG and fMRI. Second I will address the interaction between risk and reward. I will present a new mathematical framework for including risk into reinforcement learning on Markov decision processes, and I will derive a risk-sensitive variant of the (model free) Q-learning scheme. The new framework is then applied to quantify the risk-sensitivity of human subjects playing a stock-market investment game. We find that the risk-sensitive variant provides a significantly better fit to the behavioral data and leads to an interpretation of the subject’s responses that is consistent with prospect theory. The analysis of simultaneously measured fMRI signals shows a significant correlation of the risk-sensitive valuation with neural activity in the striatum, cingulate cortex, and insula that is not present if standard Q-values are used.
12.07.2016	CeNoS Doktorandenkolloquium Theoretical analysis of structure formation on pre-patterned surfaces O. Buller, AG Heuer The Rayleigh-Plateau-like instability of ridges formed by molecules on pre-patterned substrates is studied by means of kinetic Monte Carlo (KMC) simulations and a thin film continuum model. We show systematically the qualitative agreement of the occurring instability in both models. In particular, we demonstrate that in the KMC model the transversal instability of ridges occurs on well defined scales which are significantly larger than the intrinsic scales of thermodynamical fluctuations. In the thin film model, the transversal instability for a single ridge and two weakly interacting ridges is investigated through a transversal linear stability analysis. We show the dispersion relations for transversal modulations and investigate their dependency on the system parameters. In regimes accessible to direct simulations, similar results are obtained for the KMC model. Sliding Drops - Dynamics of Large Ensembles M. Wilczek, AG Thiele We analyze the dynamics of a thin liquid film on a substrate using the thin film equation for partially wetting liquids. When including a lateral driving force due to, e.g., an inclined substrate and gravity, structures described by the equation, like drops and ridges, begin to move. In addition, also stability properties of the structures can change. In particular, large drops may undergo a pearling instability, where they emit small satellite droplets until their volume is small enough to be stable. We conduct direct numerical simulations on a large spatial domain in order to examine the interaction of many sliding drops. As the sliding velocity of the drops depends on their volume, larger drops overtake smaller drops and merge with them, possibly leading to an overall volume which is large enough for the pearling instability to occur. Studying a large ensemble, we find that this merging and pearling behaviour can lead to a stationary distribution of drop sizes, whose shape depends on the inclination angle of the substrate and the overall volume of liquid in the system. We explain the long-term evolution of the drop size distribution using stability properties of single drops obtained when studying families of droplet solutions with continuation techniques. Einladender: O. Kamps