Vienna Theory Lunch Seminar

by Florian Ecker (TU), Christopher Lieberum (UV), Florian Lindenbauer (TU) and Maximilian Ofner (UV)

Tuesdays 12:30-13:45

held alternately at:

TU Wien (TU): Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room DB10E11

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

We thank our kind sponsors:

Faculty of Physics, TU

Faculty of Physics, UV



Nach pandemiebedingter Pause wollen wir das Vienna theory lunch seminar wiedererwecken, 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.

After a break due to the pandemic we want to revive the Vienna theory lunch seminar. 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.

Oct 3 2023


Thomas Mieling
(Uni Wien)

Theoretical Aspects of Gravitational Quantum Optics

To study the interplay of gravity and quantum physics, experiments are under development that aim at measuring phase shifts induced by Earth’s gravitational field in single photons or entangled photon pairs. Hitherto-developed descriptions of such experiments are based mainly on semi-classical analyses or drastically simplified models based on quantum field theory in curved space-time. After discussing the motivation behind such efforts and reviewing previous theoretical work on this subject, this talk presents a model of single-photon interferometry in curved space-times, specifically developed to describe setups as considered in current experimental proposals. This comprises a quantization of the electromagnetic field in dielectric media, located in curved space-time, together with perturbative calculations on fiber optics in non-inertial systems.

Oct 10 2023


Paul Worm
(TU Wien)

High-temperature superconductivity and where to find it

I will give a theoretical perspective on high-temperature superconductors, with a special focus on quasi-two-dimensional compounds like cuprates and nickelates. Specifically, I will review their electronic structure, how to construct effective low-energy Hamiltonians and approximate solutions of those. Implications for superconductivity are discussed along the way.

Oct 17 2023


Christoph Regner
(Uni Wien)

Beyond the Narrow-Width Limit for Off-Shell and Boosted Top Quark Decays

Due to its large mass the top quark plays an important role in consistency checks of the Standard Model and new-physics searches. Studies concerning precise theoretical predictions of the top production and its decay are commonly based on the narrow-width (NW) limit of the top quark propagator or on full off-shell computations. Starting with a short introduction to effective field theories, I will present a novel approach for boosted top quarks that allows to combine the properties of the NW limit and off-shell effects. Our approach generalizes results known from semileptonic B-decays and allows to derive a factorization theorem that accounts for boosted top quark production, subsequent top decay as well as finite lifetime effects.

Oct 24 2023


Finnian Gray
(Uni Wien)

When does separation of variables work? Applications to black hole spacetimes.

Separation of variables is one of the most common ansaetze to study partial differential equations, particularly in physics. However, it is less common to ask when and why this is possible. I will discuss the geometric characterization of the separation of variables and the relation to conserved quantities due to explicit and hidden symmetries. I will then discuss how this applies to and has advanced our understanding of the physics of particles and fields in black hole spacetimes.

Oct 31 2023


Adrien Fiorucci
(TU Wien)

The Role of Sources in Flat Space Holography

The aim of this talk is to review the main obstacles to the construction of flat space holography on the basis of the celebrated AdS/CFT correspondence – namely the null nature of the conformal boundary and the non-conservation of gravitational charges in the presence of radiation – and to discuss how to deal with them. It will be argued that the putative holographic dual theory is a Carrollian conformal field theory, coupled to external sources that account for the radiation reaching the boundary. To formalise this coupling, a generalised concept of variational symmetries must be introduced, for which Noether’s first theorem yields flux-balance laws instead of conservation laws. Some implications of this new formalism will be discussed, both in classical mechanics and in quantum field theory.

Nov 7 2023


Ritankar Chatterjee
(IIT Kanpur)

Adventures in the Tensionless Corner of String Theory

Tensionless string theory is a candidate for the ultra high energy limit of string theory. In this talk I discuss some of the scenarios when a string becomes tensionless based on our earlier work. Thereafter I revisit the formulation of tensionless closed bosonic string theory following earlier works where it will be revealed that there can be three different quantum theories for tensionless strings. Afterwards I discuss our latest work where we have studied all the three quantum theories in a compactified target spacetime to study the impact of compactification on all these theories.

Nov 14 2023


Lukas Rachbauer
(TU Wien)

Micromanipulation, quantum metrology and vacuum forces: a unified perspective based on the scattering matrix

We introduce the quantum Wigner-Smith (QWS) operator, a Hermitian operator describing the interaction between the spatial as well as the quantum degrees of freedom of light and a local classical parameter of a linear, but otherwise arbitrarily complex scattering medium through which the light propagates. The QWS operator builds a bridge between quantum micromanipulation, vacuum forces and quantum metrology on the one side, and the formalism of classical scattering matrices, which are experimentally measurable in a noninvasive manner, on the other side.
The QWS operator can be used to describe generalized forces (momentum transfer, angular momentum transfer, pressure) that quantum light exerts on classical target objects. From the classical far-field scattering matrix and its dependence on the corresponding local parameter, the effect of quantum light in the near-field (in the vicinity of the target object) can be inferred. Our formalism makes it possible to identify quantum states of light that have an optimal effect (largest or smallest possible force, least possible quantum noise in the force) on the target object. If the light field is in the vacuum state, the formalism naturally provides the vacuum contributions to the forces, also known as Casimir forces.
Another application of the QWS operator lies in quantum metrology. The variance of the QWS operator is proportional to the quantum Fisher information (QFI), which in turn provides a measure on how precisely a parameter of the scattering system (e.g. the position of a scatterer or its orientation) can be measured. The optimization of the QFI determines --- even in complex, open scattering systems --- how the spatial structure and the quantum degrees of freedom of the light must be designed in order to achieve the physically best possible measurement precision.

Nov 21 2023


David Blanik
(Uni Wien)

Using Matrix Product States to Classify Quantum Phases in (1+1)D

Understanding the phase diagram of correlated quantum many-body systems is among the most important and most challenging tasks towards a comprehensive understanding of such systems. In this talk I will give a brief introduction to tensor networks and explain how to use tensor network methods, in particular Matrix Product States (MPS), to classify gapped quantum phases. Specifically, we will derive the well known classification of symmetry-protected topological (SPT) phases in (1+1)D.

Nov 28 2023


Aleksi Kurkela
(University of Stavanger)

QCD in the cores of neutron stars

Neutron stars are the densest astrophysical objects in our universe, reaching densities as high as those realized in ultrarelativistic heavy-ion collisions at the LHC. In these collisions ordinary nuclear matter melts into a new phase of elementary particle matter, quark matter. This naturally raises the question: does quark matter also exist inside neutron stars? The rapid advancement in neutron-star observations in combination with state-of-the-art QCD calculations is providing us with an unprecedented view of the extreme matter deep in the cores of the stars. In my talk, I describe how recent advancements in theory of superdense matter inform us about what lies in the centers of neutron stars and how different constraints point to the existence of quark matter cores in large neutron stars.

Dez 5 2023


Paul Grosskopf
(Université libre de Bruxelles)

TQFTs, HQFTs and other category theoretical approaches to quantum field theory

Topological Quantum Field Theories (short: TQFTs) are mathematical toy models for quantum field theory. Coming from physics they quickly sparked interest with mathematicians, not only because they lead to new topological invariants for manifolds or knots. Going back to Atiyah the most common formulation uses category theory to describe d-dimensional TQFTs as functors from the d-bordism category to some symmetric monoidal category, most noteably the category of vector spaces and linear maps. Many variations of these have developed over the years, such as Homotopy Quantum Field Theories (HQFTs), open/closed TQFTs or defect TQFTs. One main question is the classification of these various notions of TQFTs by means of algebraic structures, such as Frobenius algebras and J-algebras. This talk explores the basics of the field as well as gives an impression on the recent work on classifications in dimension 2.

Dez 12 2023


Srinath Bulusu
(TU Wien)

Finite element methods for gauge theories

The variational principle lies in the heart of classical physics: given an action $S[\phi]$ which depends on degrees of freedom $\phi$ the evolution of the physical system follows the equations of motion which can be obtained by means of variation $\delta_{\phi} S = 0$. This in combination with the theory of Partial Differential Equations (PDEs) naturally leads to the so-called Finite Element Method (FEM) - a practial approach to numerical simulations that has established itself in the fields of structural mechanics, fluid mechanics and electrodynamics. Further FEM research has paved the way towards numerical discretizations that hold the potential to faithfully represent symmetries and nuances of the continuous theory. In this lecture, we will develop the basic notions to talk about FEM and the so-called Exterior Finite Elements with the application of electrodynamics in mind. Towards the end I will deliver a few insights into a recent research project, in which we aim to construct a FEM formulation for non-Abelian gauge theories.

Jan 9 2024


Christian Käding
(TU Wien)

Probing new physics with open quantum systems

The field theory of open quantum systems has ample applications in areas like particle or nuclear physics, cosmology, and quantum gravity. In this talk, I will introduce open quantum systems and their description via the Feynman-Vernon influence functional in field theory. Subsequently, I will discuss applications of this formalism to the search for new physics beyond the standard models of particles and cosmology.

Jan 16 2024


Jake Xuereb
(TU Wien)

Quantum Information Processing with Finite Thermodynamic Resources

Rolf Landauer once remarked “Information is not an abstract entity but exists only through a physical representation, thus tying it to all the restrictions and possibilities of our real physical universe.” In this talk I will review how this thought manifests itself in various ways in quantum information processing.

To begin with, keeping track of the passage of time requires the production of entropy and any unitary is generated by Hamiltonian which is enacted by an agent for some time. So how does an agent's ability to keep time impact their ability to carry out quantum computation? In [1] we make use of ideas from the field of quantum clocks to bound the achievable fidelity of a class of quantum circuits depending on their circuit complexity and the quality of the clock the agent has access to.

Secondly, the compression of messages of pure quantum states is a paradigmatic protocol in quantum information processing consisting of agents measuring, enacting unitaries and tracing out and appending qubits. How does the temperature of thermal states the agents have access to and the quality of their clocks impact their ability to compress quantum information? In [2] we present a number of bounds relating the achievable fidelity to various quantities which can be bounded by the entropy produced by the agents in a cooling protocol enacted to improve their free states.

[1] - J. Xuereb, P. Erker, F. Meier, M. T. Mitchison, and M. Huber, Impact of imperfect Timekeeping on quantum control, Physical Review Letters 131, 160204 (2023).

[2] - J. Xuereb, T. Debarba, M. Huber, P.l Erker, Quantum Coding with Finite Thermodynamic Resources, arXiv:2311.14561 (2023).

Jan 23 2024


Jonas Mager
(TU Wien)

Applications of holographic QCD and the role of anomalous symmetries

This talk will present applications of holographic models of QCD to interesting processes in hadron physics. In some way or another most of these processes involve anomalous symmetries. Holographic models of QCD are often particularly good at reproducing the symmetries of the dual theory and their anomalies, hence the predictions for these observables are of special interest. In particular, I will show different ways the U(1)_A anomaly can be realised in holographic models and subsequently broken by quark masses, during which we will encounter interesting concepts such as superconnections and tachyon condensation. I will present numerical results for observables, such as hadronic light by light scattering, double photon decay of mesons and comment on possible improvements, as well as the limits of these gravity models. On the more theoretical side QCD and its cousins have some interesting and subtle (higher form) symmetries. The descriptions of these symmetries in the gravity dual are intriguing, and expectations from QCD can be compared to results from holography.

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