
Vienna Theory Lunch Seminarby Florian Ecker (TU), Christopher Lieberum (UV), Florian Lindenbauer (TU) and Maximilian Ofner (UV) Tuesdays 12:3013:45held alternately
at:
We thank our kind sponsors:

Idee: 
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. 
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. 
Wie kann ich teilnehmen? 
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 UV 
Thomas Mieling 
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. Hithertodeveloped descriptions of such experiments are based mainly on semiclassical analyses or drastically simplified models based on quantum field theory in curved spacetime. After discussing the motivation behind such efforts and reviewing previous theoretical work on this subject, this talk presents a model of singlephoton interferometry in curved spacetimes, 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 spacetime, together with perturbative calculations on fiber optics in noninertial systems. 
Oct 10 2023 
Paul Worm 
Hightemperature superconductivity and where to find it I will give a theoretical perspective on hightemperature superconductors, with a special focus on quasitwodimensional compounds like cuprates and nickelates. Specifically, I will review their electronic structure, how to construct effective lowenergy Hamiltonians and approximate solutions of those. Implications for superconductivity are discussed along the way. 
Oct 17 2023 
Christoph Regner 
Beyond the NarrowWidth Limit for OffShell 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 newphysics searches. Studies concerning precise theoretical predictions of the top production and its decay are commonly based on the narrowwidth (NW) limit of the top quark propagator or on full offshell 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 offshell effects. Our approach generalizes results known from semileptonic Bdecays 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 
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 
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 nonconservation 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 fluxbalance 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 TU 
Ritankar Chatterjee 
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 TU 
Lukas Rachbauer 
Micromanipulation, quantum metrology and vacuum forces: a unified perspective based on the scattering matrix We introduce the quantum WignerSmith (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. 
Nov 21 2023 UV 
David Blanik 
Using Matrix Product States to Classify Quantum Phases in (1+1)D Understanding the phase diagram of correlated quantum manybody 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 symmetryprotected topological (SPT) phases in (1+1)D. 
Nov 28 2023 TU 
Aleksi Kurkela 
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 heavyion 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 neutronstar observations in combination with stateoftheart 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 UV 
Paul Grosskopf 
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 ddimensional TQFTs as functors from the dbordism 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 Jalgebras. 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 TU 
Srinath Bulusu 
Finite element methods for gauge theories 
Jan 9 2024 TU 
Christian Käding 
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 FeynmanVernon 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 UV 
Jake Xuereb 
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. 
Jan 23 2024 TU 
Jonas Mager 
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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