
Vienna Theory Lunch Seminar
by
Theodor Adaktylos, Andreas Gabriel, Niklas
Johansson, Christoph Spengler and NilsOle Walliser
Tuesdays 12:1513:30 held alternatively
at: TU Wien ( Wiedner Hauptstr. 810, yellow area,
10th floor, seminar
room E136 ) Uni Wien ( Boltzmanngasse 5, 5th floor, Schrödinger HS / large seminar room ) We thank our kind
sponsors: 
Idee: Idea: 
Wie auf vielen Universitäten
praktiziert wollen wir ein LunchSeminar 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. Prediploma 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 hep.itp.tuwien.ac.at oder an christoph.spengler at univie.ac.at 
Just attend! To receive informations via email go to the Mailinglist or drop an email to dburke at hep.itp.tuwien.ac.at or christoph.spengler at univie.ac.at 
5 Oct 2010 Uni Wien 
Maximilian Attems
(TU Wien) 
Instabilities in Quark Gluon Plasma Abstract:
The quarkgluon plasma (QGP) is a phase at extremely high temperatures formed
by deconfined quarks and gluons as predicted by quantum chromodynamics (QCD).
NonAbelian plasma instabilities play a crucial role in the nonequilibrium
dynamics of a weakly coupled QGP. The evolution of ChromoWeibel instabilties
can be carried out thanks to realtime lattice simulations. The developed
hardexpandingloop (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 discplane 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 nbody calculations with Newtonian gravity (and Dark
Matter halos) or deepMOND 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 axisymmetric case, which is then nothing else than a particlemechanics 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 socalled 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 BoseEinstein 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 discretetime 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 quantumwalk 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 2010TU Wien 
XuGuang 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
Bfield respectively for strange quark matter, which arise due to the
nonleptonic weak processes u + s <> u + d. As an application, we
discuss the effects of these bulk viscosities on the rmode 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 rmode 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 quarkgluon
plasma one observes in heavyion 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 backreactions to the metric. As the electric field
and the induced current cause a net energy inflow to the system, the
plasma is continually heated up and the corresponding gravity solution has
an expanding horizon. After proposing a consistent latetime expansion
scheme, we present analytic solutions in the scheme up to nextleading
order, and our solutions are new timedependent solutions of 5D asymptotic
AdS EinsteinMaxwell(ChernSimons) theory. To extract dual CFT stress
tensor and U(1) current from the solutions, we perform a rigorous
holographic renormalization of EinsteinMaxwellChernSimons theory
including full backreactions, which can in itself be an interesting
addition to literatures. As byproducts, we obtain interesting
modifications of energymomentum/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 nonperturbative 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 finitedepth 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 twolevel 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 socalled 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 weakcoupling 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 hardthermalloop 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 23T_c. This is a nontrivial 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 spindown impurity atom interacting attractively with a spinup 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 pwave 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 coldatom experiments. 
Program Summer Semester
2010
Program Winter Semester
2009/2010