Quantum Navigator
- Home Page
- Past Talks (2020-2024)
- Past Talks (2015-2019)
- Past Talks (2010-2014)
- Cold Quantum Coffee Art
Interna:
|
Cold Quantum Coffee The Cold Quantum Coffee brings together research students at the Institute for Theoretical Physics to discuss topics revolving around phenomenology, quantum gravity, cold quantum gases, solid state systems, and everything in between. The seminar is organised by students, for students. For further questions or in case you want to give a talk, please contact one of the organisers (Maruice Beringuier, Zois Gyftopoulos, Renzo Kapust, and Fabian Zhou). We are supported by the SFB 1225 ISOQUANT. Upcoming Talks Summer Semester 2024 Schedule
Past talks 01.07.2025 {$\hspace{0.5cm}$} Eugen Dizer (Heidelberg University) Title: Mass-gap description of heavy impurities in Fermi gases Abstract: Single impurities immersed in a degenerate Fermi gas exhibit fascinating many-body phenomena, such as the polaron-to-molecule transition and Anderson's orthogonality catastrophe (OC). It is known that mobile impurities of finite mass can be described as quasiparticles, so called Fermi polarons. However, Anderson showed in 1967 that the ground state of a static, infinitely heavy impurity in a Fermi sea is orthogonal to the ground state of the system without impurity - a hallmark of the OC and a fundamentally non-perturbative effect. As a result, conventional variational approaches or path integral methods fail to capture the OC accurately. Despite decades of research, a unified approach connecting the quasiparticle description of Fermi polarons with Anderson’s OC has remained elusive. In this work, we present a theoretical framework for finite-mass impurities in a Fermi sea that combines Anderson’s OC, the polaron-to-molecule transition and the quasiparticle picture. Our theory provides new insights into the nature of impurity physics and many-body correlations, describing how quasiparticle behavior emerges from the OC. 24.06.2025 {$\hspace{0.5cm}$} Mireia Tolosa Simeón (Ruhr-Universität Bochum) Title: Quantum critical Dirac semimetals and finite-temperature effects Abstract: The chiral Ising-, XY-, and Heisenberg models serve as effective descriptions of Dirac semimetals undergoing a quantum phase transition into a symmetry-broken ordered state. Interestingly, their quantum critical points govern the physical behavior of the system in the vicinity of the transition even at finite temperatures. In this project, we explore the chiral models at zero and finite temperature, both in the Dirac phase as well as in the symmetry-broken phases. To that end, we set up a functional renormalization group approach, which allows us to systematically track (1) the phenomenon of semimetallic precondensation, (2) the manifestation of the Mermin-Wagner- Hohenberg theorem due to pseudo-Goldstone fluctuations at finite temperatures, and (3) the quantitative behavior of the system in the quantum critical fan, for instance, by calculating the fermionic quasiparticle weight. Our work aims at a more holistic understanding of chiral models near their quantum critical point, including, for example, the description of non-Dirac liquid behavior, in analogy to the non-Fermi liquid behavior in metallic quantum critical points, and the signatures of Berezinskii-Kosterlitz-Thouless (BKT) physics in the chiral XY model, connecting our work to topological phase transitons. 27.05.2025 {$\hspace{0.5cm}$} Martina Zündel (LPMMC, Grenoble) Title: Driven-dissipative bosons in one dimension Abstract: The trace‐preserving, completely positive time evolution of an open quantum system is governed by the Lindblad master equation. Numerically probing bosons in such systems is particularly challenging because of their infinite‐dimensional Hilbert space. In this talk, we first investigate the spectral properties of the one‐dimensional driven‐dissipative Bose–Hubbard model with one-body incoherent pumping and loss. We compare these open-system spectra with (i) the ground-state excitation branches (Lieb I and II) and (ii) finite-temperature spectral functions. In the driven-dissipative regime, we find a nontrivial renormalization of the decay rate of the retarded Green's function at large hopping amplitudes and strong interactions. Next, employing random-matrix theory, we demonstrate that the XX model (hard-core bosons on a lattice)— its continuum limit, the Tonks-Girardeau gas, known for its integrability even at finite temperature and under quenched time evolution—becomes non-integrable upon the introduction of homogeneous Markovian one-body pumping and loss. We then use perturbation theory to reveal emergent structures in the full Lindblad spectrum. Finally, we show how tensor-network methods enable access to larger system sizes and higher bosonic occupations. By adding two-body loss to the driven-dissipative Bose–Hubbard model, a quasi-condensate can emerge whose phase fluctuations in the one-dimensional thermodynamic limit, depending on the parameter regime, fall into the Kardar–Parisi–Zhang universality class. We complement the picture by investigating the spectrum of the full quantum evolution at strong but finite interactions. 11.02.2025 {$\hspace{0.5cm}$} Mathieu Kaltschmidt (Zaragoza University) Title: Spectrum of Global Strings and the Axion Dark Matter Mass Abstract: Cold dark matter axions produced in the post-inflationary scenario serve as clear targets for their experimental detection, since it is in principle possible to give a sharp prediction for their mass once we understand precisely how they are produced from the decay of global cosmic strings in the early Universe. We performed a dedicated analysis of the spectrum of axions radiated from strings based on large scale numerical simulations of the cosmological evolution of the Peccei-Quinn field on a static lattice. It turns out that there are several systematic effects that have been overlooked in previous works, such as the dependence on the initial conditions, contamination due to oscillations in the spectrum, and discretisation effects, some of which could explain the discrepancy in the literature. We confirmed the trend that the spectral index q of the axion emission spectrum increases with the string tension, but did not find a clear evidence of whether it continues to increase or saturates to a constant at larger values of the string tension due to the severe discretisation effects. Taking this uncertainty into account and performing the extrapolation to realistic string tensions with a simple power law assumption on the spectrum, we found that the dark matter mass is predicted in the range of $m_a \approx 95-450 \mu$eV. 04.02.2025 {$\hspace{0.5cm}$} Leon Sieke (Giessen University) Title: Critical dynamics of non-equilibrium phase transitions Abstract: In context of the search for the QCD critical point in heavy-ion collisions, a deep understanding of the out-of-equilibrium dynamics of the system is necessary to make well-grounded predictions for signatures in final states. To this end, we study the critical dynamics of a scalar field theory in the same static universality class with classical-statistical lattice simulations. In particular, I will present results for the non-equilibrium behavior of the system under a quench protocol in which the symmetry-breaking external field is changed at a constant rate through the critical point. I discuss the connection to the Kibble-Zurek mechanism and present universal non-equilibrium scaling functions of cumulants of the order parameter up to fourth order. These are highly sensitive to the correlation length and therefore of special interest as possible signatures of a critical point. 28.01.2025 {$\hspace{0.5cm}$} Matthias Carosi (TU Munich) Title: False Vacuum Decay beyond the quadratic approximation. Abstract: Computing the decay rate of a metastable vacuum in QFT relies on a semi-classical method that hinges on instantons. In the presence of a radiatively broken classical symmetry, the traditional instanton method yields an IR-divergent result, which is naturally regulated when including quantum corrections. In this talk, I show how the instanton method can be cast into the language of the effective action and how we can use this, together with the 2PI effective action formalism, to go beyond the quadratic approximation to include quantum corrections systematically. The key technical difficulty is the resummation of non-local self-energies in a spherically symmetric background. I will show explicitly how this is achieved, and finally, I will discuss some numerical results to show how the conventional Hartree approximation is generally not justified. 21.01.2025 {$\hspace{0.5cm}$} Anka Van de Walle (LMU and University of Ghent) Title: Reimagining quantum many-body problems with neural networks Abstract: The recent success of machine learning inspires the use of neural networks to address complex problems in many-body physics. In this talk, we will explore how neural networks can represent quantum systems without relying on external data, where neural quantum states (NQS) offer new tools for tackling longstanding challenges. Topics include methods for ground-state search, quantum state reconstruction from data, and the simulation of dynamics in interesting physical systems. A particular emphasis will be placed on the time-dependent neural quantum states (t-NQS) framework; a ChatGPT-inspired neural network architecture designed to capture the time evolution of quantum systems. We will delve into the challenges and successes of applying these methods, highlighting their implications for understanding and simulating quantum phenomena across diverse settings. This talk aims to provide both a broad overview and a focused examination of how neural networks are reshaping the landscape of many-body physics. 14.01.2025 {$\hspace{0.5cm}$} Shi Yin (Giessen University) Title: The QCD moat regime and its real-time properties Abstract: Dense QCD matter may exhibit crystalline phases. Their existence is reflected in a moat regime, where mesonic correlations feature spatial modulations. We study the real-time properties of pions at finite temperature and density in QCD in order to elucidate the nature of this regime. We show that the moat regime arises from particle-hole-like fluctuations near the Fermi surface. This gives rise to a characteristic peak in the spectral function of the pion at nonzero spacelike momentum. This peak can be interpreted as a new quasi particle, the moaton. In addition, our framework also allows us to directly test the stability of the homogeneous chiral phase against the formation of an inhomogeneous condensate in QCD. We find that the formation of such a phase is highly unlikely for baryon chemical potentials \mu_B≤630 MeV. Meanwhile, to investigate the origin of the moat behavior in the chiral phase transition, we perform an analytical calculation of the pion two-point correlation function within the mean-field quark-meson model. We find that Friedel oscillations may arise in the moat regime. Such oscillations have already been observed in condensed matter physics. |
|||||||||||||||||||||||||||||||
This page was last modified on September 07, 2025, at 02:50 PM