Beschreibung: Beschreibung: pizza


Vienna Theory Lunch Seminar

by Theodor Adaktylos, Andreas Gabriel, Niklas Johansson, Christoph Spengler and Nils-Ole Walliser

Tuesdays 12:15-13:30

held alternatively at:

TU Wien ( Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room E136 )

Uni Wien ( Boltzmanngasse 5, 5th floor, Schrödinger HS / large seminar room )


We thank our kind sponsors:





Wie auf vielen Universitäten praktiziert wollen wir ein Lunch-Seminar etablieren, das aktuelle Themen der Theoretischen Physik, die von DiplomandInnen, DoktorandInnen und PostDocs behandelt werden, aufgreift.

Das Niveau soll so gewählt werden, dass jeder Student und jede Studentin am Ende des Studiums dem Vortrag folgen kann. Die Vortragenden werden ermutigt keinen "perfekten" Vortrag zu halten, und sollen hauptsächlich Ihre Motivation, warum sie dieses Thema gewählt haben, wiedergeben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza, Sushi) gratis zur Verfügung gestellt.

We want to establish a lunch seminar as practiced at other universities. The focus is on recent theoretical research done by Master students, PhDs and PostDocs.

The seminar is designed for graduate students but should also be comprehensible to advanced undergraduate students. Pre-diploma students are particularly encouraged to attend so that they receive an overview of research activities conducted at both universities. Speakers are also encouraged to focus on their motivation for choosing their particular topic and to present open questions.

In order to avoid any "loss of time" we provide a free lunch (pizza, sushi).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailinglist eintragen oder Mail an dburke at oder an christoph.spengler at

Just attend! To receive informations via email go to the Mailinglist or drop an email to dburke at or christoph.spengler at


5 Oct 2010

Uni Wien

Maximilian Attems (TU Wien)

Instabilities in Quark Gluon Plasma

Abstract: The quark-gluon plasma (QGP) is a phase at extremely high temperatures formed by deconfined quarks and gluons as predicted by quantum chromodynamics (QCD). Non-Abelian plasma instabilities play a crucial role in the nonequilibrium dynamics of a weakly coupled QGP. The evolution of Chromo-Weibel instabilties can be carried out thanks to real-time lattice simulations. The developed hard-expanding-loop (HEL) formalism allows the (numerical) calculation of the time evolution of gluonic mean fields in full 3 dimensional Bjorken expansion.

12 Oct 2010

TU Wien

Note: unusual time! 12.00 sharp!

Christoph Saulder (Uni Wien)

Dynamics of the Local Group in different theories of gravity

Abstract: The Local Group, our home galaxy group, consists of two large galaxies, the Milky Way and M31 (Andromeda Galaxy), and about 40 known galaxies. The starting point of the investigation is the observed distribution of the galaxies in the Local Group, which differs from our expectations. The problem is that most galaxies in our galaxy group are arranged in a quite thin plane which does not correspond to the disc-plane of one of the two dominant spiral galaxies. A possible explanation for the observed distribution is an interaction of the Milky Way and M31 about 10 to 12 Gigayears ago. At this time gas from outer parts of the galaxies has been scattered in the orbital plane of these two galaxies. It is investigated for which members of the Local Group this model is realistic and to reproduce the results of Sawa and Fujimoto in 2005. Since the orbits of dwarf galaxies around the host galaxies are sensitive to the shape of the host’s potential and to dynamical friction. So the dynamics of the Local Group are a perfect laboratory to test the Dark Matter paradigm and also an alternative theory, which is called MOdified Newtonian Dynamics (MOND). In this theory the law of gravitation is modified for small accelerations which occur in the outer parts of galactic discs and also in a galaxy group. In order to do this, stellar dynamical numerical simulations using newly developed software were run. The programmes were capable of performing n-body calculations with Newtonian gravity (and Dark Matter halos) or deep-MOND gravity, Hubble expansion and dynamical friction (only in the case of Newtonian gravity). The initial conditions of the models are optimised using a genetic algorithm until the distribution after the integration is sufficiently close to the observed distribution. The results are very multifarious, in some aspects also surprising and show that the model is possible in the case of Newtonian gravity but it has significant problems in MOND.

19 Oct 2010

Uni Wien

Cancelled due to the Vollversammlung at the TU Wien!

Cancelled due to the Vollversammlung at the TU Wien!

Gandalf's talk will instead take place on Dec 14!

9 Nov 2010

TU Wien

Stefan Schauer (AIT)

Security Aspects of Quantum Key Distribution


Abstract:Within the last years extensive research in the field of quantum key distribution (QKD) has been done. It has been shown that a communication secured by quantum cryptography is not only theoretically

possible but is also practically realizable with today’s technical means. The relevance of quantum cryptography becomes even more apparent if the existence of a quantum computer is taken into account. Most of today’s communication, e.g. over the Internet, will become insecure if an adversary would be able to use a quantum computer. The security of QKD protocols is in principle guaranteed by the laws of

physics instead of computational assumptions. Nevertheless there are strategies for an adversary to obtain some information from QKD protocols. The aim of the legitimate communication parties is therefore

to identify methods to minimize that amount of information an adversary can get. In this talk I want to present the most basic attack strategies an adversary is able to apply on a QKD protocol and how much information is leaking out. Further, I want to focus on a special form of QKD protocols which are based on entanglement. In certain cases the application of entanglement gives an adversary the opportunity to obtain a large amount of information about the secret key.

16 Nov 2010

Uni Wien

Sabine Ertl (TU Wien)

Topologically Massive Gravity

Abstract: Topologically Massive Gravity (TMG) is quite an old theory in the field of 3 dimensional gravity. Despite age its attraction was recovered over the last few years as a possible toy model for quantum gravity. Its interesting features provide us with various kinds of vacuum and black hole solutions. One particular way of finding solutions to this theory is by considering its stationary axi-symmetric case, which is then nothing else than a particle-mechanics problem. This dimensionally reduced theory is then called Topologically Massive Mechanics (TMM). In this talk I will motivate three dimensional gravity and discuss general features of TMG. The main focus will be in finding and discussing solutions of TMG via the TMM approach.

23 Nov 2010

TU Wien

Massimiliano Rinaldi (Université de Genève)

Acoustic Black Holes

Abstract: In 1981 a seminal paper by W. Unruh showed that it is possible to generate thermal emission in a superfluid, which has the same characteristics of the Hawking radiation emitted by astrophysical black holes. This analogy opened up the possibility to observe the Hawking radiation in a laboratory, in a variety of condensed matter systems. In recent years, this possibility has become a feasible reality, boosting the interest in the so-called analogue models of gravity. In this talk, I will first review the fundamentals of Hawking radiation in both astrophysical and condensed matter systems, focussing in particular on Bose-Einstein condensates. Then I will discuss some recent developments and future perspectives of this fascinating analogy.

30 Nov 2010

Uni Wien

Daniel Reitzner (RCQI Bratislava)

Quantum Walks

Abstract: Quantum walks, quantum analogues of classical random walks, are yet another model of quantum computation and as such lead to new and efficient algorithms. I will present the properties of discrete-time quantum walks that are useful in constructing new algorithms, such as interference and fast mixing time and I will show how they can be exploited. One example will be quantum-walk search, where analogy with Grover search can be drawn. I will finalize the talk with another example - Ambainis' algorithm for element distinctness.

Note: unusual day!

9 Dec 2010

TU Wien

Xu-Guang Huang (Goethe Universität Frankfurt)

Bulk viscosities of magnetized quark matter and neutron star phenomenology

Abstract: Neutron star is a kind of remnant resulting from gravitational collapse of a massive star during supernova explosion. Due to the magnetic flux conservation during the collapsing process, the resultant neutron star can have intense magnetic field. In strong magnetic fields the transport coefficients of star matter may become anisotropic. We determine the general form of the complete set of transport coefficients in the presence of a strong magnetic field, and calculate explicitly the bulk viscosities transverse and parallel to the B-field respectively for strange quark matter, which arise due to the non-leptonic weak processes u + s <--> u + d. As an application, we discuss the effects of these bulk viscosities on the r-mode instability of a rotating neutron star. We find that the instability region can be significantly enlarged if the star is composed by quarks and has very large magnetic field, making a magnetized strange star more susceptible to r-mode instability than its unmagnetized counterparts.

Extra talk!

13 Dec 2010

TU Wien

Bindusar Sahoo (ICTP)

Electrified plasma in AdS/CFT correspondence

Abstract: Strongly coupled plasma are quite interesting to study within the framework of AdS/CFT, because of its possible relevance to quark-gluon plasma one observes in heavy-ion collisions . Although the plasma is studied by perturbing it and observing its linear responses, sometimes the situation demands going beyond the linearized approximation and taking back reactions to the gravity equations of motion into account. In this talk I will focus on one of my collaborative project in which we study a system which indeed needs full back reactions on the metric. In this work we construct new gravity backgrounds holographic dual to neutral plasma with U(1) global symmetry in the presence of constant electric field, considering its full back-reactions to the metric. As the electric field and the induced current cause a net energy in-flow to the system, the plasma is continually heated up and the corresponding gravity solution has an expanding horizon. After proposing a consistent late-time expansion scheme, we present analytic solutions in the scheme up to next-leading order, and our solutions are new time-dependent solutions of 5D asymptotic AdS Einstein-Maxwell(-Chern-Simons) theory. To extract dual CFT stress tensor and U(1) current from the solutions, we perform a rigorous holographic renormalization of Einstein-Maxwell-Chern-Simons theory including full back-reactions, which can in itself be an interesting addition to literatures. As by-products, we obtain interesting modifications of energy-momentum/current Ward identities due to the U(1) symmetry and its triangle anomaly.

14 Dec 2010

Uni Wien

Gandalf Lechner (Uni Wien)

Deformation of quantum field theories

Abstract: Quantum field theories on deformed, noncommutative spacetimes are currently investigated as possible candidates for models incorporating some effects of a still elusive theory of quantum gravity, and also as a tool to better understand the yet unsolved problem of constructing interacting quantum field theories in a non-perturbative manner. In this talk, a gentle introduction to noncommutative spaces will be given, focussing on the mathematical analogy to Heisenberg's commutation relations from quantum mechanics. After recalling how quantum mechanics can be viewed as a deformation of classical mechanics, a possible strategy for deforming quantum field theories on usual Minkowski space to quantum field theories on a fuzzy, noncommutative Minkowski space, is explained. Basic features of these models, such as their locality and covariance properties, are presented. In the end, a short account of some ongoing research projects about the relation between noncommutative quantum field theories in Euclidean and Lorentzian signature ("Wick rotation"), models of locally noncommutative field theories, and thermal aspects of deformed quantum field theories is given.

11 Jan 2011

Uni Wien

Tomas Rybar (RCQI Bratislava)

Quantum finite-depth memory channels

Abstract: We analyze the depth of the memory of quantum memory channels generated by a fixed unitary transformation describing the interaction between the principal system and internal degrees of freedom of the process device. We investigate the simplest case of a qubit memory channel with a two-level memory system. In particular, we explicitly characterize all interactions for which the memory depth is finite. We show that the memory effects are either infinite, or they disappear after at most two uses of the channel. Memory channels of finite depth can be to some extent controlled and manipulated by so-called reset sequences. We show that actions separated by the sequences of inputs of the length of the memory depth are independent and constitute memoryless channels.

18 Jan 2011

TU Wien

Nan Su (Goethe Universität Frankfurt)

QCD Thermodynamics at Intermediate Coupling

Abstract: The weak-coupling expansion of the QCD free energy is known to order g_s^6*log{g_s}, however, the resulting series is poorly convergent at phenomenologically relevant temperatures. I will discuss how the gauge invariant hard-thermal-loop perturbation theory (HTLpt) reorganization of the calculation improves the convergence of the successive approximations to the QCD free energy. I will present new results of an HTLpt calculation of QCD thermodynamics to three loops. The results of this calculation are consistent with lattice data down to 2-3T_c. This is a non-trivial result since, in this temperature regime, the QCD coupling constant is neither infinitesimally weak nor infinitely strong with g_s~2, or equivalently alpha_s~0.3. Therefore, we have a crucial test of the quasiparticle picture in the intermediate coupling regime. Our results suggest that HTLpt provides a systematic framework that can be used to calculate static and dynamic quantities for temperatures relevant at LHC.

25 Jan 2011

Uni Wien

Meera Parish (Cavendish Laboratory, Cambridge)

Trimers, molecules and polarons in polarized atomic Fermi gases

Abstract: In this talk, I will consider an atomic Fermi gas in the limit of extreme spin imbalance, where one has a single spin-down impurity atom interacting attractively with a spin-up atomic Fermi gas. By constructing variational wave functions for polarons, molecules and trimers, I will explore the quantum phase transitions between each of these bound states as a function of mass ratio and interaction strength. I will show that the p-wave trimer can in fact be stabilised by the presence of the Fermi sea, much like the situation in the Cooper pair problem. Finally, I will discuss how these transitions might be observed in cold-atom experiments.

Program Summer Semester 2010
Program Winter Semester 2009/2010