Vienna Theory Lunch Seminar

by Christopher Lepenik (UV), Maximillian Löschner (UV), Alexander Soloviev (TU)
and David Toneian (TU)

Tuesdays 12:15-13:30

held alternately at:

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

University of Vienna (UV): Boltzmanngasse 5, 5th floor, Schrödinger Lecture Hall

We thank our kind sponsors:

Dean of physics, TU

Faculty of Physics, UV

Daniel Grumiller, TU

Idee:

Idea:

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 Beginn des Masterstudiums dem Vortrag folgen kann. BachelorstudentInnen können besonders von dem Seminar profitieren, da es ihnen ermöglicht einen Eindruck in die Forschungsarbeit beider Universitäten zu erhalten. Die Vortragenden werden dabei auch ermutigt darüber zu sprechen, warum sie ein gewisses Forschungsgebiet gewählt haben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden und es ist nicht notwendig einen Vortrag über eine "perfekte", abgeschlossene Arbeit zu halten.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza) 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. Undergraduate 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).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailingliste eintragen.

Just attend! To receive informations via email register for the Mailinglist.

Mar 21 2017

UV

Jan Rosseel
(Mathematical Physics, University of Vienna)

Non-relativistic field theories in arbitrary backgrounds

Abstract: Coupling field theories to arbitrary background space-time metrics is often useful, as it allows one to easily define e.g. the energy-momentum tensor and correlators thereof. In the relativistic case, it is well known how field theories can be coupled to arbitrary background geometries via e.g. minimal coupling. In this talk, I will review some aspects of non-relativistic geometry and show what the analogue of minimal coupling is for non-relativistic field theories. I will then show how this prescription can be easily obtained via a particular non-relativistic limit.

Mar 28 2017

TU

Ferdinand Horvath
(University of Vienna)

Mathur's Inequality and the Black Hole Information Paradox

Abstract: Ever since Stephen Hawking discovered that black holes emit radiation, the physics community has been trying to accommodate the effects of this phenomenon. One of its consequences is the so-called information paradox. This paradox arises once a black hole evaporates through the emission of Hawking radiation, when those parts of the radiation that left the black hole can't be described as entangled with the hole anymore. While the theory assumes a pure initial state and hence full information about the particles in the hole and those emitted, information is lost once the hole is gone. This implies a loss of unitarity. Several ways to avoid this prospect are conceivable but few of them seem favourable. One such resort is the supposition that Hawking radiation has been treated too superficially since higher order corrections of its state are usually neglected. Their contribution could destroy the particles' entanglement, thus resolving the entire paradox. This presentation investigates Samir Mathur's research, who tried to disprove this proposal. Mathur shows that as long as these corrections to the Hawking state are assumed to be small, they cannot affect the first order entropy in a decisive way. Mathur's assumptions are examined in greater detail and his results are revised to conform to Hawking's results. We refine the entropy inequalities he proposed and attempt to directly compute the entanglement entropy of the Hawking radiation.

Apr 4 2017

UV

Georg Stettinger
(TU Wien)

Twisted Warped Entanglement Entropy

Abstract: The aim of this talk is to calculate the entanglement entropy of an interval in different two-dimensional warped conformal field theories (WCFT). The result by Castro et al. is generalized to a second WCFT with a different symmetry algebra. This is done in two ways: First using the Rindler method and second using the replica trick. The new WCFT is particularly interesting because it appears as holographic dual of a boosted Rindler-spacetime. On the gravitational side, entanglement entropy is much easier to compute and I show that the results agree if one locates the field theory on the horizon at r=0 rather than at r --> ∞.
This statement is shown to be also true for the slightly more involved case of boosted Rindler-AdS.

Apr 11-18 2017

No Lunch Seminar

Osterferien - Easter Break

Apr 25 2017

TU

Prof. Gerhard Hensler
(Astrophysics Institute, UV)

The Cosmic Matter Circuit - how to understand the Evolution of Galaxies

Abstract: Most galaxies like our Milky Way started to form soon after the Big Bang. Fed with gas from inter-galactic streams in the Cosmic Web they started to form stars and since then send their light through the universe. Since stars gain their energy from nuclear fusion processes in their cores, they synthesize heavier elements but also exhaust their fuel and have to "die". The heavier a star is, the shorter is its lifetime and the more energetic its death. Already during their lives stars heat their surrounding interstellar medium (ISM) by their radiation and continuous mass loss. These stellar winds and final explosions not only drive the dynamics of the ISM but also restitute processed gas and by this enrich it with heavier elements. This chemical evolution can be traced in galaxies by the determination of element abundances in stars of different ages compared with those in the present ISM.

This stellar matter cycle happens locally within galaxies and couples with the dynamics of gas and stars on a larger galactic scale. While the gas mixes and homogenizes on this scale and loses its memory dissipatively, stars with longer lifetiimes move accordingly to their velocity dispersion, only affected by gravitation, and retain their memory on their origin much easier.

Galaxies exist, however, not in isolation but are surrounded by intergalactic gas and other galaxies of different sizes so that they couple to their environment in the form that intergalactic gas can be further accreted and interactions with other galaxies occur even leading to merging. Vice versa, also gas and stars can be dissolved from a galaxy by various processes, by this, contributing to a large-scale matter cycle.

In this talk most of the physical processes determining the cosmic matter circuit on various scales will be described and composed to the global picture of dynamical and chemical galaxy evolution.

May 2 2017

UV

Olaf Krüger
(University of Vienna)

Generating functions - Methods of counting

Abstract: I present a toolbox called „generating functions“ - whenever you are confronted with some sort of counting problem, you can reach into that box and find something useful. For example, given a complicated recurrence formula for the coefficients of some sequence, a generating function can provide an exact formula. One might also find further recurrences, which might be simpler etc. etc.

There is a huge amount of counting problems and each can be tackled differently. However, most of them can be solved within the framework of generating functions. Due to the variety of problems, I present the talk via a collection of examples for which I use ordinary, exponential and Dirichlet power series generating functions.

May 9 2017

TU

Prof. Ivette Fuentes
(Vienna Center for Quantum Science and Technology, UV)

Gravity in the quantum lab

Abstract: Quantum experiments are reaching relativistic regimes. Quantum communication protocols have been demonstrated at long lenghts scales and experiments are underway to distribute entanglement between Earth and Satellite-based links. At these regimes the Global Positioning System requieres relativistic corrections. Therefore, it is necessary to understand how does motion and gravity will affect long-range quantum experiments. Interestingly, relativistic effects can also be observed at small lengths scales. Some effects have been demonstrated in superconducting circuits involving boundary conditions moving at relativistic speeds and quantum clocks have been used to measure time dilation in table-top experiments. In this talk I will present a formalism for the study of gravitational effects on quantum technologies. This formalism is also applicable in the development of new quantum technologies that can be used to deepen our understanding of physics in the overlap of quantum theory and relativity. Examples include accelerometers, gravitational wave detectors and spacetime probes underpinned by quantum field theory in curved spacetime.

May 16 2017

UV

Lukas Semmelrock
(HEPHY, Vienna)

How can dark matter influence structure formation?

Abstract: Lambda Cold Dark Matter is, to this date, the standard model of cosmology for describing the formation and evolution of structures in the universe. In this model the universe is made up of visible matter, dark matter and dark energy. Cosmic Microwave Background measurements show that dark matter is approximately five times as abundant as visible matter suggesting that the properties of dark matter can strongly influence the formation of structures. Simulations show that the distribution of matter in the universe can be very well modelled on large scales with dark matter only interacting gravitationally. On small scales, however, discrepancies between simulations and observations arise. These discrepancies can be resolved by introducing a dark matter self-interaction. If the mediator particle of such interactions is light enough, it can be emitted as "dark bremsstrahlung" in dark matter collisions. My research deals with such inelastic collisions of self-interacting dark matter particles and its implications on structure formation in the universe. For this purpose I introduce and analyse various generic dark matter models.

May 23 2017

TU

Sukhwinder Singh
Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences

Tensor networks as new tools for quantum gravity?

Tensor networks are practical tools, developed in the last two decades, to efficiently simulate a large class quantum many-body systems at low temperatures, on a classical computer. A popular example of a tensor network is "Matrix Product States", which form the basis of DMRG---a breakthrough simulation algorithm for one dimensional quantum lattice systems, commonly used in condensed matter physics and quantum chemistry.
In this talk, I will introduce a recent conjecture that certain other tensor networks (namely, the MERA), which are naturally suited for simulating lattice models described by conformal field theories, may be a realization of the holographic principle (more specifically, the AdS/CFT correspondence) of quantum gravity on a lattice.
Suggested readings:
1) Roman Orus, "A practical introduction to tensor networks", arXiv:1306.2164 (2013).
2) "Holographic principle", e.g., see Wikipedia.
3) Guifre Vidal, “Entanglement Renormalization”, Physical Review Letters 99, 220405 (2007).
4) Brian Swingle, “Entanglement renormalization and holography”, Physical Review D 86, 065007 (2012), arXiv:0905.1317.
5) SS, arXiv:1701.04778
6) SS, N. McMahon, G. Brennen, arXiv:1702.00392

May 30 2017

UV

Giacomo Guarnieri
(University of Olomouc)

Characterization of heat in non-Markovian open quantum systems

Abstract: We characterize the time behavior of the heat exchange between an open quantum system and its environment in a non-Markovian dynamical regime.
We begin by studying the time behavior of its mean value by means of full-counting formalism showing that, at variance with what happens in the Born-Markov semigroup limiting case, heat can backflow from the environment to the system. After providing a general condition for the occurrence of such phenomenon and a quantifier for its amount, we showed two explicit applications to two paradigmatic examples of open quantum systems, i.e. the spin-boson and the quantum Brownian motion. Results on the one hand allow for the identification of parameter regions where the heat backflow is absent or maximum. On the other hand the study of the relationship with suitable estimators of non-Markovianity highlights how the occurrence of heat backflow represents a stricter condition than the latter.
We then move to an environmental-assisted erasure protocol scenario, the framework of application of the famous Landauer’s principle. Once again relying on full-counting statistics techniques, we find a new family of asymptotically-tight lower bounds to the mean dissipated heat, and we explicitly show in an interesting quantum system how this family of quantum thermodynamical lower bounds can outperform Landauer's.

Jun 6 2017

No Lunch Seminar

Pfingsten - Pentecost

Jun 13 2017

TU

Nicolai Friis
(Institute for Quantum Optics and Quantum Information, UV)

Gaussian Quantum Thermodynamics

Abstract: One of the most fundamental tasks in quantum thermodynamics is extracting energy from one quantum system and subsequently storing this energy in an appropriate battery. Both of these steps, work extraction and charging, can be viewed as cyclic Hamiltonian processes realized by unitary transformations acting locally on a quantum system. While there exist so-called passive states whose average energy cannot be lowered by unitary transformations, it is safe to assume that the energy of any not-fully charged (quantum) battery may be increased unitarily. Nonetheless, unitaries raising the average energy by the same amount may differ in qualities such as their precision, fluctuations, and charging power, which one wishes to optimize. However, while work may be extracted from non-passive states in principle and optimal ways may be found of charging any specific battery, the required unitaries may be complicated and extremely difficult to realize in practice. It is hence of crucial importance to understand the qualities that can be expected from practically implementable transformations. In this talk, I will discuss the limitations for work extraction and battery charging when restricting to the feasibly realizable family of Gaussian unitaries.

Jun 20 2017

UV

Hamed Barzegar
(Gravitational Physics Group, UV)

GLOBAL QUANTITIES IN GENERAL RELATIVITY - Generalization of ADM & Komar Quantities

Abstract: One of the most important quantities of a physical system is the energy (or mass) of that system. But, the energy and mass in General Relativity have no unique definition, compared against the Special Relativity where the mass (and energy) is well-defined. As it turns out, there is no local notion of mass and energy in General Relativity. However, we can attribute global quantities, defined at large distances, to some spacetimes. ADM formalism provides a way to calculate such quantities. There is also the notion of Komar quantities which are defined in spacetimes with some symmetries. In this talk, I will briefly review the 3+1 formalism which is the mathematical framework of the ADM Hamiltonian formalism. Then, I will talk about the ADM Hamiltonian formalism and asymptotic flatness in order to define the global quantities such as ADM quantities and Komar quantities. At the end, I will present the ideas of my master's thesis, which is concerned with the derivation of the ADM quantities within the geometric Hamiltonian formalism of Kijowski, Tulczyjew and Chrusciel. Based on this formalism, I will present the generalizations of the ADM and Komar quantities to n+1-dimensional spacetimes. Further generalizations are in progress.

Jun 27 2017

TU

Raphaela Wutte
(TU Wien)

Near Horizon Boundary Conditions for Spin-3 Gravity in Flat Space

Abstract: In field theories the physical content of the theory is given by the field equations and the boundary conditions. While it is common practice in non-gravitational theories to demand that the fields asymptotically vanish at the boundary of spacetime, boundary conditions for theories of gravity are quite subtle. Recently (2015/2016), three different groups (Donnay et al, Afshar et al and Hawking et al) asked the question whether boundary conditions can be formulated for the near horizon region of non-extremal black holes in a sensible way. In this talk I review the extension of the boundary conditions of Afshar et al to higher-spin gravity in three-dimensional flat space. We find that the near horizon symmetries are governed by a surprisingly simple algebra and that there seems to be a universal expression for the entropy of (higher-spin) black holes/flat space cosmologies in terms of the near horizon charges in three dimensions.