The Cold Quantum Coffee brings together research students of the institute to discuss topics revolving around gauge theories, quantum gravity, cold quantum gases, solid state systems, and everything in between. The seminar is organized by students. In each seminar we have a talk of a member of the institute or an invited speaker. For further questions or in case you want to give a talk, please contact one of the organizers (Bruno Faigle-Cedzich, Nicolas Wink and Felix Ziegler).
We are supported by the SFB 1225 ISOQUANT.
Date: Tuesday 16:15
Location: Institute for Theoretical Physics, Philosophenweg 16, seminar room
|16. 07. 2019||
Kevin Geier (Heidelberg University)
Analog reheating of the early universe in the laboratory
The early universe has undergone a transition from a super-cooled state after cosmic inflation to a hot, thermal state. We propose an analog experimental implementation of this cosmic reheating using an ultra-cold Bose gas. In our mapping, a Bose-Einstein condensate plays the role of the inflaton field, which describes the state of the universe after inflation. The expansion of the universe as well as the dynamics of the inflaton field are encoded in the time-dependence of the atomic interaction, which can be tuned via Feshbach resonances. We illustrate by means of classical-statistical simulations that the dynamics of the system involves the known stages of reheating. At early times, parametric instabilities lead to the production of Bogoliubov quasi-particles as excitations on top of the condensate, mimicking cosmological particle production by the decaying inflaton field. At later times, the system develops a turbulent cascade transporting energy to higher momenta in a self-similar way. The final stage of the dynamics, where the system relaxes to thermal equilibrium, is dominated by quantum fluctuations and therefore not captured by the classical-statistical approximation, which motivates an experimental study of this process using a quantum simulator.
|09. 07. 2019||
Paul Wittmer (Heidelberg University)
Non-Equilibrium Dynamics in a Holographic Superfluid
We use holographic methods to study the real-time dynamics of two- and three-dimensional strongly correlated many-body quantum systems in a genuinely non-perturbative framework. A superfluid is described holographically in terms of a higher-dimensional gravitational system with an Abelian scalar Higgs model. A fast numerical implementation of the bulk equations of motion in the probe limit is used to evolve the systems on large grids. The two-dimensional system is put in a far-from-equilibrium state by preparing topological vortex defects as quench-like initial conditions for the superfluid’s dynamics. We focus in particular on the dynamics of vortex—anti-vortex annihilation processes. The results are discussed in terms of known equations of motion for vortices in Gross-Pitaevskii theory and compared to numerical results of such. The second part of this talk is focused on the three-dimensional system. The differences between the two- and three-dimensional systems are highlighted before we present and discuss how for the first time in a holographic superfluid the creation and evolution of vortex rings is observed. As for the two-dimensional system, we compare our findings to known results from Gross-Pitaevskii theory.
03. 07. 2019 - 15:15
(Note the special date)
Benjamin Knorr (Radboud University, Nijmegen)
Form Factors in Quantum Gravity and Matter
Form factors are a central ingredient of the effective action, since they store the full momentum dependence of interactions. In particular in asymmetric configurations, where simple scale identifications might give a too naive picture, they are crucial to obtain correct predictions. In this talk I will give a classification of potentially relevant form factors in quantum gravity coupled to matter, and present some results obtained with non-perturbative renormalisation group techniques.
|02. 07. 2019||
Konstantin Otto (University of Gießen)
Compact Objects from the Functional Renormalization Group
In recent years, the study of neutron stars has experienced a surge of interest. One major goal of modern research is to probe the equation of state (EoS) of neutron star matter with macroscopic observations such as masses and radii and thereby learn about its microscopic structure. This leads to yet unanswered questions like the possible existence of stars with a quark matter core and the unknown role of strangeness. In this talk we address these problems by employing EoS from the two- and three-flavor quark-meson models obtained from the functional renormalization group in local potential approximation. We study the influence of quantum fluctuations by comparing the results to mean-field calculations. Furthermore, we satisfy the beta equilibrium and charge neutrality conditions which naturally follow from the electroweak sector.
|18. 06. 2019||
Emilio Torres (University of Cologne) Slides
Compatible orders in Dirac materials: symmetries and phase diagrams
Chiral symmetry breaking patterns in Dirac systems can be realized by different interactions that can moreover appear simultaneously. In the simplest setting, where the gapless, semimetallic Dirac phase is adjacent to two gapped phases, the system exhibits a multicritical point (MCP) where all three phases meet. It is well known that the presence of Dirac fermions alters the critical behaviour of phase transitions in a way that is not accounted for by mere Landau Ginzburg theory, so it is natural to ask to what extent this MCP and the related phase diagram is affected by the gapless fermions. In this talk we address this question by functional renormalization group methods. We obtain the phase diagrams associated to this MCPs for interactions that induce different long range orders, and show that even in the absence of topological terms, the Dirac fermions have a symmetry-enhancing effect that extends to large portions of the phase diagram.
|21. 05. 2019||
Torsten Zache (Heidelberg University) Slides
Dynamical topological transitions in the massive Schwinger model
Analog quantum simulators and digital quantum computers have the exciting prospect to access physical phenomena that lie beyond the reach of classical simulations. In the context of lattice gauge theories, the implementation of simple toy models, such as the massive Schwinger model (QED in one spatial dimension), is possible with current technology. Motivated by anomalous and topological properties of QCD, we consider the real-time dynamics of the massive Schwinger model with a topological theta term. Following a quench of the theta term, we identify so-called dynamical quantum phase transitions (DQPT), which have previously been found in various condensed matter models. We establish a general dynamical topological order parameter based on two-time correlation functions that allows to detect these transitions in the presence of interactions. Our numerical simulations employing exact diagonalization indicate that the dynamical topological transitions persist beyond the weak-coupling regime and show robust signatures on small, coarse lattices. These findings make this phenomenon an ideal target for near-future quantum simulator experiments.
|14. 05. 2019||
Lukas Kades (Heidelberg University) Slides
Spectral Reconstruction with Deep Neural Networks
We explore artificial neural networks as a tool for the reconstruction of physical spectral functions from imaginary time Green's functions. We systematically investigate a reconstruction based on a straightforward supervised learning approach with feedforward neural networks. A detailed analysis of its performance on physically motivated mock data is provided along with a comparison to established methods of Bayesian inference. We find that the use of labelled training data in combination with an appropriate optimisation procedure can lead to a superior reconstruction accuracy, in particular at larger noise levels. The advantages and disadvantages of the supervised approach as well as potential improvements are discussed in detail.
|07. 05. 2019||
Dietrich Roscher (University of Cologne) Slides
Fractionalization in spin systems: an FRG perspective
In the last decades, a plethora of new phenomena and structures has been found in low-energy condensed matter systems that seem to defy the Landau paradigm of phase transitions. Oftentimes, effective theories with certain topological features and fractionalized degrees of freedom are best suited to describe the experimental findings. Unfortunately, writing down such an effective theory and understanding its relation to the accepted microscopic model for the actual material is usually not the same thing.
In this talk, I show how functional renormalization group can be employed to systematically construct the effective action of spin systems in a so-called spin liquid phase. Unexpectedly, the common toolbox of spontaneous symmetry breaking is well suited for this task and can be adapted to develop an understanding of phenomena that are often considered to be inherently "topological". Besides making predictions for hitherto poorly understood physical systems, functional RG thus provides a neat perspective on fractionalization and emergent gauge fields in strongly correlated spin systems.
|12. 02. 2019||
Lukas Barth (Heidelberg University)
A geometric framework to compare classical field theories
How to compare two physical theories? Is there a way to determine when they have something in common? Is it possible to "intersect" two theories to see if they share any structure? And could one transfer methods that are used to solve problems in one theory to another? In this talk, a mathematical framework is introduced that provides a possibility to answer the above questions for classical field theories (like electrodynamics, hydrodynamics, relativity theory, etc). In this framework the differential equations that usually describe the dynamics in a classical field theory are transformed into geometric objects (by understanding them as submanifolds of Jet Bundles). In this way, one can define a map between those submanifolds that can be interpreted as a correspondence under which two theories become comparable. In particular, this correspondence facilitates to define the intersection of theories as the intersection of two manifolds which is a well-defined notion under certain conditions. This intersection can then be investigated with geometric and cohomological methods to find out if it can be understood as a subtheory of the intersected theories. One can then show that solutions of this intersection can be transferred to solutions of both intersected theories which facilitates a transfer of methods.
|29. 01. 2019||
Natalia Sánchez-Kuntz (Heidelberg University)
Prelude to the reference frame interpretation
In this talk a revision of the main no-go theorems in Quantum Mechanics — namely those that deal with locality, contextuality and realism — is delivered. This revision aims at the possibility of constructing an interpretation of Quantum Mechanics that is consistent with the causal structure of Special Relativity.
We show firstly that Bell’s theorem (and the EPR paradox) cannot be posed in a factual scenario, which we define. We give several motivations to regard physics and the phenomena it describes as factual, and impose factuality in Quantum Mechanics (non-factuality would lead to non-contextuality, which is ruled out by the no-go theorems of contextuality).
Our second result is in the realms of the nature of the quantum state: is the quantum state an ontological description of the system or is it just an epistemological tool for prediction?
We show that in order to conclude that a certain quantum state is ontological —through the PBR theorem  — the precise quantum system whose state is concluded to be ontological must have undergone a measurement. So it is shown that what PBR demonstrate is that systems which have been measured are described by states which are ontological. What this implies is that, by means of the PBR theorem, one cannot conclude the ontology of a quantum state which describes a system that has not been measured.
Our revision has then a far-reaching consequence, which is what we call the reference frame interpretation. This interpretation is inspired by Bohr’s complementarity principle, and is still under construction. If time allows, the general structure of this interpretation will be given in the concluding part of the talk.
 Pusey, M. F., Barrett, J., Rudolph, T.: On the reality of the quantum state. Nature Physics. 8, 475 (2012)
|15. 01. 2019||
Kevin Keiler (University of Hamburg)
State engineering of impurities in a lattice by coupling to a Bose gas
After a brief introduction to the physics of ultracold atoms and our numerical method ML-MCTDHX, I will present the localization pattern of interacting impurities, which are trapped in a lattice potential and couple to a Bose gas. For small interspecies interaction strengths, the impurities populate the energetically lowest Bloch state or localize separately in different wells with one extra particle being delocalized over all the wells, depending on the lattice depth. In contrast, for large interspecies interaction strengths we find that due to the fractional filling of the lattice and the competition of the repulsive contact interaction between the impurities and the attractive interaction mediated by the Bose gas, the impurities localize either pairwise or completely in a single well. Tuning the lattice depth, the interspecies and intraspecies interaction strength correspondingly allows for a systematic control and engineering of the two localization patterns. The sharpness of the crossover between the two states as well as the broad region of their existence supports the robustness of the engineering. Moreover, we are able to manipulate the ground state's degeneracy in form of triplets, doublets and singlets by implementing different boundary conditions, such as periodic and hard wall boundary conditions.
|18. 12. 2018||
Alexander Schuckert (Technische Universität München)
Many body chaos near a second order thermal phase transition
Chaos is one way of seeing why classical systems thermalize: details about the initial state are effectively forgotten as trajectories diverge exponentially quickly. Recently, the study of out-of-time-ordered correlation functions (OTOCs) in quantum mechanical systems has generalized the notion of chaos to many-body systems.
Here, we study the classical analogue of OTOCs near the second order thermal phase transition of a real scalar field theory in two spatial dimensions. We show that even in this regime without well-defined quasi-particles, the OTOC exhibits a lightcone-like behaviour as has been previously found in quantum systems. We reveal a transition to chaos in the symmetric phase and a local maximum of the Lyapunov exponent (the "strength" of chaos) at the the phase transition. Both features are put into context to the fluctuations of the order parameter. Furthermore, the butterfly velocity (the speed at which chaos spreads through the system) has a global maximum at the phase transition. Lastly, a self-similar behaviour of the temporal fluctuations of the Lyapunov exponent is shown, with an exponent in agreement with the 2D KPZ universality class.
To conclude the talk, a short introduction will be given into how OTOCs have been studied in quantum field theory and how current short comings might be overcome in future studies.
|27. 11. 2018||
Ingolf Bischer (Max-Planck-Institut für Kernphysik)
New neutrino interactions: Theoretical motivation and experimental probes
I review the approach of general neutrino interactions as an effective parametrisation of new physics including aspects of their theoretical motivation. Furthermore, I discuss some prospects of upcoming experiments to test them.
|20. 11. 2018||
Stefanie Czischek (Heidelberg University)
Violating Bell’s inequality with Langevin dynamics in a deep belief network
A representation of quantum spin-1/2 states using artificial neural networks, specifically restricted Boltzmann machines, has been introduced in . This approach can be used to numerically simulate ground states and dynamics after sudden quenches in quantum many-body systems. We implemented this ansatz to benchmark the method on a one-dimensional transverse-field Ising model and found that the simulation of dynamics struggles in the vicinity of the quantum phase transition, where also other simulation methods based on matrix-product-states fail . To improve the neural network approach, we use a Langevin machine, which can be extended to deeper networks and enables the possibility to measure different bases using a deep belief network. We apply Langevin dynamics to sample the real parts of spin states from the network and add the complex phase via an additional reweighting approach. We show that we can violate Bell’s inequality using this network structure to represent a Bell-pair state. This suggests that the deep belief networks can be used to represent full quantum spin states, where the precision of the representation can be increased by going to deeper networks with more hidden layers, which is possible using the Langevin sampling.
 G. Carleo, M. Troyer, Science 355, 602-606 (2017)
 S. Czischek, M. Gärttner, T. Gasenzer, Phys. Rev. B 98, 024311 (2018)
|16. 10. 2018||
Martin Roelfs (KU Leuven)
Faddeev-Popov Matrix in Linear Covariant Gauge: First Numerical Results
In order to make meaningful continuum predictions for observables in gauge theories, gauge fixing is required. This is done using the Faddeev-Popov procedure, which introduces unphysical particles called ghosts whose sole purpose is related to fixing the gauge. A popular gauge is the Landau gauge, and there are a lot of analytical and numerical predictions available for various quantities in this gauge. Remember, although observables in gauge theories should be gauge invariant, there are many quantities which are not. For example, the gluon and ghost two-point functions are gauge dependent quantities. It is therefore interesting to look at the broader class of Linear Covariant Gauges (LCG), of which Landau is a special case, to study this gauge dependence. Although there have been some numerical studies of the gluon two-point function in LCG, reliable data on the ghost propagator has remained elusive. This is because it turns out to be hard to define a lattice equivalent of the Faddeev-Popov operator in LCG. In a recent preprint (https://arxiv.org/abs/1809.08224) we proposed a possible definition of the Faddeev-Popov operator on the lattice, which has all the properties we know and love about the continuum version, and some preliminary data for SU(2) and SU(3) Yang-Mills theories has been obtained.