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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 (Renzo Kapust, Viktoria Noel, Jonas Wessely and Fabian Zhou). We are supported by the SFB 1225 ISOQUANT. Upcoming Talks Summer Semester 2024 Schedule
Past talks 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. 10.12.2024 {$\hspace{0.5cm}$} Frederick del Pozo (CPhT Ecole Polytechnique) Title: "Investigating interaction effects and disorder in coupled Kitaev wires: from DMRG to bosonization" Abstract: Topological superconductors offer a promising route to fault-tolerant quantum computation platforms at the hard-ware level. One proposal for realising a quantum computing platform is based on an array of topological superconducting wires, w,,hich can be idealised to some extent by the "Kitaev model" describing a tight-binding Hamiltonian of spineless fermions with p-wave superconducting pairing. The wire hosts Majorana fermions at its edges, however, the effects of interactions, disorder and couplings between individual wires can have non-trivial effects on the topological phases. This motivates us to develop novel techniques to investigate the phase diagram of coupled Kitaev wires in the presence of strong interactions and disorder. We focused in particular on ground state properties such as correlation functions and entanglement entropy, and, by introducing a real-space formulation of a topological invariant, we reveal the stabilising effects against disorder that strong repulsive interactions have. Additionally, an analytical approach around the quantum phase transition using the boson/fermion duality in 1D (bosonization) leads to a comprehensive set of renormalization group equations. We show that gapless critical modes emerging at the phase transition are also stabilised against certain disorder fluctuations of the chemical potential by large repulsive interactions, potentially offering a new avenue for applications in the future. 03.12.2024 {$\hspace{0.5cm}$} Valerio Pagni (ETH Zürich) Title: "Out of equilibrium critical dynamics of O(N) models." Abstract: "In this talk, I will introduce the equilibrium expansion, an approach that bridges equilibrium and non-equilibrium physics by enabling the computation of universal non-equilibrium quantities directly from equilibrium data. To showcase this framework, I will present the computation of the initial slip exponent in Model A, which characterizes short-time critical behavior following a sudden temperature quench. This analysis is conducted using the MSRJD formalism, specifically through the non-perturbative functional renormalization group (fRG) at the LPA' level. The equilibrium expansion also allows for the computation of static exponents and those associated with equilibrium dynamics. Notably, this method converges at n-th order when calculating n-point functions. I will discuss results for the slip exponent in general O(N) theories across dimensions d=2 to d=4, emphasizing their consistency with large-N analytical benchmarks. This approach offers new insights into universal behavior in critical non-equilibrium systems." 26.11.2024 {$\hspace{0.5cm}$} Chuang Huang (ITP Heidelberg) Title: Four-quark scatterings and pion PDA from Functional QCD Abstract: In this talk, I will discuss recent progress in developing the functional renormalization group (fRG) approach to first-principles QCD, focusing on four-quark scatterings and their application to calculating parton distribution amplitude (PDA) for pion. Specifically, we investigate dynamical chiral symmetry breaking and the emergence of mesonic bound states driven by the infrared dynamics of four-quark interactions. The flows of Fierz-complete four-quark interactions are computed coupled to the flow of the quark two-point function. This framework can be understood as the fRG analogue to the complete Bethe-Salpeter equations and the quark gap equation. The three-dimensional s,t,u-channel momentum approximation is employed for the Fierz-complete four-quark vertices. Using this approach, we have calculated key qualities, including the pion pole mass, pion decay constant, Bethe-Salpeter amplitudes, quark mass function, and quark wave function. From the Bethe-Salpeter amplitudes and the quark mass function, we can further compute the pion valence-quark quasi-PDA on the frame of large momentum effective theory. 19.11.2024 {$\hspace{0.5cm}$} Yuepeng Guan (Jilin University) Title: Ladder top-quark condensation imprints in supercooled electroweak phase transition Abstract: The electroweak (EW) phase transition in the early Universe might be supercooled due to the presence of the classical scale invariance involving Beyond the Standard Model (BSM) sectors and the supercooling could persist down till a later epoch around which the QCD chiral phase transition is supposed to take place. Since this supercooling period keeps masslessness for all the six SM quarks, it has simply been argued that the QCD phase transition is the first order, and so is the EW one. However, not only the QCD coupling but also the top Yukawa and the Higgs quartic couplings get strong at around the QCD scale due to the renormalization group running, hence this scenario is potentially subject to a rigorous nonperturbative analysis. In this work, we employ the ladder Schwinger-Dyson (LSD) analysis based on the Cornwall-Jackiw-Tomboulis formalism at the two-loop level in such a gauge-Higgs-Yukawa system. We show that the chiral broken QCD vacuum emerges with the nonperturbative top condensate and the lightness of all six quarks is guaranteed due to the accidental U(1) axial symmetry presented in the top-Higgs sector. We employ a quark-meson model-like description in the mean field approximation to address the impact on the EW phase transition arising due to the top quark condensation at the QCD phase transition epoch. In the model, the LSD results are encoded to constrain the model parameter space. We then observe the cosmological phase transition of the first-order type and discuss the induced gravitational wave (GW) productions. We find that in addition to the conventional GW signals sourced from an expected BSM at around or over the TeV scale, the dynamical topponium-Higgs system can yield another power spectrum sensitive to the BBO, LISA, and DECIGO, etc. 12.11.2024 {$\hspace{0.5cm}$} Nepomuk Ritz (LMU Munich) {$\hspace{0.2cm}$} Slides Title: Real-frequency correlation functions of correlated electronic systems from quantum field theory Abstract: A major challenge in the field of correlated electrons is the computation of dynamical correlation functions. For comparisons with experiments, one is interested in their real-frequency dependence. This is difficult to compute because imaginary-frequency data from the Matsubara formalism require analytic continuation, a numerically ill-posed problem. In this talk, I will show how to apply quantum field theory to the single-impurity Anderson model using the Keldysh instead of the Matsubara formalism with direct access to the self-energy and dynamical susceptibilities on the real-frequency axis. I will present results from the functional renormalization group (fRG) at the one-loop level and from solving the self-consistent parquet equations in the parquet approximation. For the first time using Keldysh fRG, we employ a parametrization of the four-point vertex, which captures its full dependence on three real-frequency arguments. We compare our results to benchmark data obtained with the numerical renormalization group and to second-order perturbation theory. We find that capturing the full frequency dependence of the four-point vertex significantly improves the fRG results compared with previous implementations and that solving the parquet equations yields the best agreement with the numerical renormalization group benchmark data but is only feasible up to moderate interaction strengths. In the final part of the talk, I will sketch how we plan to build on those results by outlining a combination of the non-perturbative but local dynamical mean-field theory (DMFT) with the diagrammatic methods from before to compute dynamical and non-local correlation functions. This will require a significant compression of the four-point vertex, and I will introduce the most promising candidate technique for that purpose, the quantics tensor cross interpolation. 29.10.2024 {$\hspace{0.5cm}$} Finn Temmen (FZ Jülich) Title: Overcoming Ergodicity Problems of Hybrid Monte Carlo using Radial Updates Abstract: Despite its many advantages, the sensible application of the Hybrid Monte Carlo (HMC) method is often hindered by the presence of large - or even infinite - potential barriers. These potential barriers partition the configuration space into distinct sectors, which leads to ergodicity violations and biased measurements of observables. In this work, we address this problem by augmenting the HMC method with a multiplicative Metropolis-Hastings update in a so-called "radial direction" of the fields, which enables jumps over the aforementioned potential barriers at comparably low computational cost. The effectiveness of this approach is demonstrated for the Hubbard model, formulated in a non-compact space by means of a continuous Hubbard-Stratonovich transformation. Our numerical results show that the radial updates successfully resolve the ergodicity violation, while simultaneously reducing autocorrelations. 16.07.2024 {$\hspace{0.5cm}$} Jean-Paul Blaizot (SPhT Saclay) Title: Emergence of hydrodynamics in expanding plasmas: attractors and fixed points. Abstract: One of the important features that emerges from the analysis of ultra-relativistic heavy ion collisions is that the produced matter can be remarkably well described by relativistic fluid dynamics. Much work has been devoted recently towards understanding why such a description works so well, in particular at early times where the produced matter is supposed to be far from equilibrium. In this talk, I shall address such issues within the framework of kinetics theory. I shall consider a simple set of equations that govern the expansion of boost-invariant plasmas of massless particles. These equations describe the early time, collisionless regime, and the transition to hydrodynamics at late time. These two regimes are associated to two fixed points of the underlying dynamical equations, which are connected by the so-called "hydrodynamic attractor". I shall argue that the success of second order hydrodynamics à la Israel Stewart has nothing to do with an "improvement" of hydrodynamics at early time, but is due to a subtle property of the Israel-Stewart equations that effectively mimic the collisionless regime. 02.07.2024 {$\hspace{0.5cm}$} Laura Batini (ITP Heidelberg) Title: Tunneling in string breaking and implications for hadronization Abstract: We study the pair production, string breaking, and hadronization of a receding electron-positron pair using the bosonized version of the massive Schwinger model in quantum electrodynamics in 1+1 space-time dimensions. Specifically, we study the dynamics of the electric field in Bjorken coordinates by splitting it into a coherent field and its Gaussian fluctuations. We find that the electric field shows damped oscillations, reflecting pair production. Interestingly, the computation of the asymptotic total particle density per rapidity interval for large masses can be fitted using a Boltzmann factor, where the temperature can be related to the hadronization temperature in QCD. Lastly, we discuss the possibility of an analog quantum simulation of the massive Schwinger model using ultracold atoms, explicitly matching the potential of the Schwinger model to the effective potential for the relative phase of two linearly coupled Bose-Einstein condensates. 25.06.2024 {$\hspace{0.5cm}$} Sipaz Sharma (Universität Bielefeld) Title: Charm degrees of freedom in hot matter from lattice QCD Abstract: I will talk about our recent results on the nature of charm degrees of freedom in hot strong interaction matter based on lattice QCD calculations of the second and fourth-order cumulants of charm fluctuations, and their correlations with net baryon number, electric charge, and strangeness fluctuations. We show that below the chiral crossover temperature thermodynamics of charm can be very well understood in terms of charmed hadrons. Above the chiral transition charm quarks show up as new degrees of freedom contributing to the partial charm pressure. However, up to temperatures as high as 175 MeV charmed hadron-like excitations provide a significant contribution to the partial charm pressure. Additionally, we find evidence for the not-yet-discovered charmed hadrons below the chiral crossover. 18.06.2024 {$\hspace{0.5cm}$} Andreas Geissel (TU Darmstadt) Title: Pressure and speed of sound in two-flavor color-superconducting quark matter Abstract: We investigate the thermodynamic properties of color-superconducting two-flavor quark matter at high densities and zero temperature at next-to-leading order (NLO) in the strong coupling and the gap. Assuming that the ground state of dense quark matter is a color superconductor, we calculate the pressure and the speed of sound for two massless quark flavors. Our results show that the NLO correction is comparable to the leading-order effects of the gap. In particular, we find that gap-induced corrections become increasingly relevant for both the pressure and the speed of sound. Finally, we provide a parameterization of the speed of sound and discuss generalizations of our results to three-flavor quark matter relevant to neutron stars. 11.06.2024 {$\hspace{0.5cm}$} Cedric Quint (MPIK) Title: How to Reconstruct the Axion DM Velocity Spectrum from CASPEr Abstract: We investigate the effect of the dark matter (DM) velocity distribution on the cosmic axion spin precession experiment (CASPEr). By exploring the dynamics of the experiment, we find that the characteristic scales allow for a linearized treatment of time evolution. We use this approach to reconstruct the velocity spectrum from numerically generated data by solving a linear inverse problem. Additionally, we establish bounds on the parameter space where this reconstruction method is applicable. 21.05.2024 {$\hspace{0.5cm}$} Eugen Dizer (ITP Heidelberg) Title: Spectral properties of ultracold Fermi gases Abstract: In this talk, we discuss a method to calculate non-perturbative spectral functions of ultracold Fermi gases directly in real frequencies, without the need for numerical reconstruction methods. The spectral functions provide access to transport and excitation properties of the ultracold gas, and can be used to obtain various physical observables which are measured in experiments. Our approach offers a wide range of applications, including the ab initio calculation of transport and spectral properties in the superfluid phase of the BCS-BEC crossover. 14.05.2024 {$\hspace{0.5cm}$} Xin Chen (ITP Heidelberg) Title: Unitary burning states, a lattice simulation of false vacuum Abstract: In this talk, we propose an array of optical cavities coupled with a 3-state spin. The ground state |g> and metastable state |m> of the spin are coupled with the excited state |e> with 2 photon absorption and single photon absorption respectively. The initial state is prepared in metastable state for each cavity, and a single photon in one cavity leads to a massive generation of photons across the cavity array. We study the evolving of massive photon generation process numerically and we analytically calculate the Lieb-Robinson bound with a simplified 4-state theory, which is in good consistence with the numerical results. Our model of the cavity array simulates the false vacuum tunneling back to real vacuum. 30.04.2024 {$\hspace{0.5cm}$} Yannis Arck (KIP Heidelberg University) Title: From Polar Bear Selfies to Quantum Science Abstract: What does a selfie with a polar bear have to do with applied quantum optical science? If you cannot think of a link immediately, then come and join this talk! I will take you on a wonderful journey into the Arctic Ocean and present some interesting research there. We will dive right into the physical oceanography of this unique place on Earth and I will provide some background information on environmental physics. The scientific focus will be on water samples from different areas of the Arctic Ocean and special emphasis is given on the water age and ventilation processes. And here comes the quantum science into play. With the Atom Trap Trace Analysis (ATTA) Method it is possible to date ocean water in a novel way. For the time scale of ocean ventilation, the radioactive argon isotope 39Ar is favorable because of its half-life of 268 years. The problem is the relative abundance of 39Ar to all argon which is only in the order of 10-16. ATTA tackles this issue by selectively catching and counting single 39Ar atoms. After a plasma source discharge the desired 39Ar atoms are slowed down in a Zeeman slower by laser cooling and are eventually caught in a magneto-optical trap. This method is superior to decay counting because it requires a much smaller sample size and processes it quicker. This talk will show some new results in the Arctic Ocean using ATTA. Of course, nice pictures and witty anecdotes about the corresponding research cruise from summer 2021 is presented as well. 23.04.2024 {$\hspace{0.5cm}$} Julian Mayr (KIP Heidelberg University) Title: Complex Langevin Methods for Analysis of the Spin-1 Bose gas Abstract: Complex Langevin as an approach to finite density lattice QCD has been known for a long time, but has seen a resurgence due to advances in theory, computational power and numerical methods. Here, we explore the application of this method to the analysis of Bose Einstein condensates. While they are typically well described by semiclassical Bogoliubov theory, in situations not well described by mean field theory, or when doing precision measurements, higher order corrections should be taken into account. We calculate the nonuniversal shift of the polar to easy plane transition in the spin-1 Bose gas due to quantum corrections. Additionally, we extract entanglement measures at the critical point using field theoretical methods. To this end, we adapt methods known from Hamiltonian Monte Carlo to complex Langevin, specifically Fourier acceleration, histogram reweighting and the replica trick. We demonstrate their applicability and show that they greatly enhance the effectiveness of complex Langevin for the task at hand. 06.02.2024 {$\hspace{0.5cm}$} Eduardo Ferreira (Universität Graz) Title: Towards TMDs with contour deformations Abstract: Hadrons are strongly interacting particles composed of quarks and gluons and described by Quantum Chromodynamics (QCD). Their internal structure can be described in terms of structure functions that encode, for example, the momentum and spin distributions of their constituents. Parton distribution functions (PDFs) and Transverse Momentum Distributions (TMDs), for example, describe the quark and gluon momentum distributions inside a hadron. These distribution functions are, however, not easy to calculate, because they are defined on the light front, whereas most hadron calculations are performed in a Euclidean metric. The main problem is then to go from Euclidean onto the light front. We are developing a new method to compute the parton distributions (TMDs and PDFs) from hadronic matrix elements using contour deformations. We will illustrate the method for a simple system of two interacting scalar particles of equal mass, using an handbag approximation to the matrix element, that includes the two-body Bethe-Salpeter amplitude as input (calculated from its Bethe-Salpeter Equation). Afterwards, the projection onto the light front is done through a combination of contour deformations and analytic continuation methods. We then explore ways of extending the handbag approximation by adding "quark-quark" interactions via the introduction of the four-point function in the diagram, which, in turn, is calculated self-consistently, from its own scattering equation. 30.01.2024 {$\hspace{0.5cm}$} Jan Philipp Klinger (Goethe University Frankfurt) Title: QCDs thermal phase transition: About massless many-flavour QCD from the lattice Abstract: The talk addresses the chiral phase transition of QCD with massless quarks. This is a challenging problem for lattice computations as the chiral limit of QCD is not directly simulable. Its study, however, provides constraints on the phase diagram of QCD with physical masses. We show that the chiral limit is approached via tricritical scaling relations which let us determine the order and temperature of the phase transition as a function of the number of quark flavours $N_f$. Based on simulations of lattice QCD with standard staggered fermions, it was found that QCD with massless quarks has a second order phase transition for $N_f \leq 7$. Additionally, we confirm an expected decrease in the critical temperature for increasing number of flavours. Running simulations on finer lattices and larger $N_f$ will allow us to resolve the question whether massless QCD approaches the conformal window by a first or a second order phase transition. 16.01.2024 {$\hspace{0.5cm}$} Friederike Ihssen (Heidelberg University) Title: Flowing fields and optimal RG-flows Abstract: Renormalisation group approaches are tailor made for resolving the scale-dependence of quantum and statistical systems, and hence their phase structure and critical physics. Usually this advantage comes at the price of having to truncate the full theory at hand, which asks for optimal expansion schemes. In the present talk I introduce a functional renormalisation group (fRG) approach for the effective action which includes general scale-dependent reparametrisations of the theory. This approach is used in an O(N)-theory to set up adaptive RG-flows that correspond to an optimal systematic expansion of the theory about the ground state or rather its full covariance or propagator. These parametrisations are induced by flowing fields that encode the differential reparametrisation steps. The approach is put to work for an investigation of the thermal phase transition in the O(4)- theory in view of applications to QCD. This talk is based on [arXiv:2305.00816], but I will also discuss other possible applications. 09.01.2024 {$\hspace{0.5cm}$} Yadikaer Maitiniyazi (Jilin University) Title: Irreversible vierbein postulate: Emergence of spacetime from quantum phase transition Abstract: We formulate a model for quantum gravity based on the local-Lorentz symmetry and diffeomorphism. A key idea is the irreversible vierbein postulate that a tree-level action for the model at a certain energy scale does not contain inverse vierbein. Under this postulate, only the spinor becomes a dynamical field and no gravitational background field is not introduced in a tree-level action. In this paper, after explaining the transformation rules of the local-Lorentz and diffeomorphism transformation in detail, a tree-level action is defined. We show that fermionic fluctuations induce a non-vanishing gravitational background field. |
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This page was last modified on April 17, 2025, at 12:26 PM