News from the Lab 2021

July 16: Quantensimulator überflügelt Computer
16.07.2021 In der Fachzeitschrift Nature haben Innsbrucker Physiker um Andreas Läuchli gemeinsam mit Kollegen in Frankreich einen Quantensimulator für große Vielteilchensysteme präsentiert. Die Wissenschaftler konnten mit dem Simulator antiferromagnetische Materiezustände mit bis zu 200 Atomen erzeugen. Mit klassischen Simulationen lassen sich solche Festkörperphänomene kaum mehr untersuchen.
Jun. 17 at 10:15 online (Zoom): Group seminar
Speaker: Andre Gladbach | Title: Measurement interleaved quantum circuits | Abstract: A large class of many body models with local interactions can be modelled using random unitary quantum circuits. In this talk I first want to focus on a one-dimensional chaotic spin chain exposed to the evolution with random Haar matrices which is additionally substance to two different sets of projective measurements. This hybrid model shows an entanglement phase transition dependent on the frequency of the measurements. To further probe the entanglement properties, the system is simulated using a Clifford stabilizer circuit where much larger system sizes according to the Gottesman-Knill theorem can be achieved. Moreover, I will discuss different ideas and descriptions of the models focusing the entanglement phases. | See you all on Zoom!
May 20 at 10:15 online (Zoom): Group seminar
Speaker: Pol Ureta Canellas | Title: Introduction to Automatic Differentiation and some of its applications in Condensed-Matter Physics | Abstract: For the last 35 years, a new way to compute derivatives, called Automatic Differentiation, has taken the Machine Learning community by storm, and allowed it to flourish until its current state. In a recent work by Xie et al. [PhysRevB.101.245139], this tool has been extended to a ED method and applied to study the 1D TFIM phase transition. In this talk I will first introduce Automatic Differentiation and how it can be used to efficiently compute derivatives of the results obtained by a Dominant Eigensolver method. Then I will show how can one use it in Condensed-Matter Physics, in particular to compute Fidelity Susceptibility maps for some simple spin models. | See you online!
May 08 at 10:15 online (Zoom): Group seminar
Speaker: Patrick Wilhelm | Title: Cython: Python on steroids | Abstract: Despite its many advantages regarding flexibility and ease of use, Python is infamous for being slow. Especially with regard to applications in HPC, at some point the developer is either forced to resort to a different programming language, potentially having to rewrite existing code, or try his luck by digging through the repository in order to find a package containing an optimized implementation. Building on last week's talk by Alex, I will give you a gentle introduction to Cython, a project aiming to fix these shortcomings in a rather elegant, yet powerfull way. The talk will be mostly based on personal experience from a recent project and will be organized in levels of increasing complexity, covering both the incorporation of existing Python and C++ code. | See you online!
Apr. 29 at 10:15 online (Zoom): Group seminar
Speaker: Alexander Eberharter, ITP | Title: Overview and Introduction to Python and Conda on Compute Clusters | Abstract: Python is an easy to learn and well structured programming language. While it is not compiled to native code but rather interpreted by a Python runtime it still allows you write very fast code by the use of the Python scientific stack, bringing together simplicity and speed. In a short talk I will introduce you to the package manager conda which is used to install Python itself and many important Python modules. Furthermore I will show how to set up a fully functional Python environment on a remote compute cluster and talk about my recommendations for best practices. | See you online!
Apr. 22 at 10:15 online (Zoom): Group seminar
Speaker: Giuliano Giudici | Title: Quantum phases of matter in Rydberg atom arrays | Abstract: Quantum simulators based on Rydberg atom arrays are ideal platforms where the study of strongly correlated quantum matter is not hindered by the exponential complexity of many-body wave functions. The van der Waals interaction between Rydberg states naturally induces a constraint on the system, the so-called Rydberg blockade, with remarkable consequences on equilibrium properties and real-time dynamics. I will first focus on the one-dimensional setting and briefly discuss the rich ground state phase diagram of the constrained Rydberg atom chain. I will then move to two dimensions, where the lattice geometry can be tuned at will. Using large-scale exact diagonalization and tensor network methods, I will investigate the possibility of realizing exotic phases of matter in this experimentally controlled setup. | Everybody is welcome to join!
Mar. 17: Simon Graf "From Space-Time to Entanglement and back again" (Defensio Master via Zoom)
Supervisor: Univ.-Prof. Dr. Andreas Läuchli
Feb. 25 at 16:00 online (Zoom): Group seminar
Speaker: Ian Timms, UT Dallas | Title: Quantized Floquet topology with temporal noise | Abstract: Previous studies on two dimensional periodically driven Floquet systems have demonstrated a novel topological phase known as the anomalous Floquet insulator (AFI). The AFI has quantized, non-adiabatic charge pumping, carried by the chiral edge states of the system. Unlike a Chern insulator, the AFI is able to be localized in the bulk, and this topological response is robust to adding spatial disorder. We consider a more disruptive perturbation, adding temporal noise to each of the Floquet cycles to break the time periodicity. We solve this system numerically in a cylindrical geometry starting from a half-filled state and calculating the net charge pumped around the cylinder during each Floquet period. Surprisingly, we see that the quantization remains for a finite window of temporal disorder up to a time that increases as a power law in system size. We connect the eventual loss of quantization to diffusion of the charge front, which eventually depopulates the topological edge state. We further show initial connections to the Floquet superoperator formalism by analytically averaging over temporal disorder, where a transition is seen in the absence of spatial disorder. This work provides an important insight to how topological Floquet phases might behave in real materials, where noise from the bath is inevitable. | See you online!
Feb. 18 at 16:00 online (Zoom): Group seminar
Speaker: Lukas Sieberer | Title: Localization Counteracts Decoherence in Noisy Floquet Topological Chains | Abstract: The topological phases of periodically driven, or Floquet systems, rely on a perfectly periodic modulation of system parameters in time. Even the smallest deviation from periodicity leads to decoherence, causing the boundary (end) states to leak into the bulk of the system. In my talk, I will show that in one dimension this decay of topologically protected end states depends fundamentally on the nature of the bulk states: a dispersive bulk results in an exponential decay, while a localized bulk slows the decay down to a diffusive process. The localization can be due to disorder, which remarkably counteracts decoherence even when it breaks the symmetry responsible for the topological protection. As I will discuss, this result can be derived analytically by using a noise-averaged Floquet superoperator to describe the stroboscopic time evolution of the system. The eigenvectors and -values of this superoperator generalize the familiar concepts of Floquet states and quasienergies and allow us to describe decoherence due to noise efficiently. Our results are particularly relevant for experiments, where disorder can be tailored to protect Floquet topological phases from decoherence. | See you online!
Jan. 21 at 10:15 online (Zoom): Group seminar
Speaker: Walter Hahn (IQOQI Innsbruck) | Title: Long-lived coherence in driven spin systems: from two to infinite spatial dimensions | Abstract: Long-lived coherences, emerging under periodic pulse driving in the disordered ensembles of strongly interacting spins, offer immense advantages for future quantum technologies, but the physical origin and the key properties of this phenomenon remain poorly understood. In this talk, I will first give a general introduction to long-lived coherences in driven spin system and then present the results of our theoretical investigation of this effect in ensembles of different dimensionality. Our results imply the existence of the long-lived coherences in two-dimensional and infinite-dimensional (where every spin is coupled to all others) systems, which are of particular importance for quantum sensing and quantum information processing. We explore the transition from two to infinite dimensions, and show that the long-time coherence dynamics in all dimensionalities is qualitatively similar, although the short-time behavior is drastically different, exhibiting dimensionality-dependent singularity. Our study establishes the common physical origin of the long-lived coherences in different dimensionalities, and suggests that this effect is a generic feature of the strongly coupled spin systems with positional disorder. Our results lay out foundation for utilizing the long-lived coherences in a range of application, from quantum sensing with two-dimensional spin ensembles, to quantum information processing with the infinitely-dimensional spin systems in the cavity-QED settings | See you online!
Jan. 14 at 10:15 online (Zoom): Group seminar
Speaker: Artem Rakcheev | Title: Estimating Heating Times in Periodically Driven Quantum Many-Body Systems via Avoided Crossing Spectroscopy | Abstract: Periodically driven, also known as Floquet, quantum many-body systems have attracted a lot of attention in recent years, due to the possibility of realize Hamiltonians with otherwise inaccessible interactions and phases. For instance topological models such as the Harper-Hofstadter and Haldane models have been realized in ultracold atoms in optical lattices with periodic driving. Furthermore, genuine non-equilibrium phases (so called time crystals) can emerge in this setting. However, these systems can only be engineered approximately and persist only up to certain time scales, after which the system heats up to featureless states. Although the problem of heating is well appreciated, there are very few approaches to compute heating times for a concrete system. In this talk I will present a numerical method to estimate such heating times based on exact computations with small systems (https://arxiv.org/abs/2011.06017). The method relies on a quantitative avoided crossing spectroscopy - roughly speaking an identification of processes which can exchange energy efficiently at a given driving frequency. We relate this method to the Fermi Golden Rule approach introduced recently and study a driven ergodic spin chain, which was shown to feature a heating "threshold" behavior. Furthermore, we discuss how our methods is applicable in important regimes beyond the Fermi Golden Rule, for instance in settings with frequency depending couplings and close to a discrete time crystal. | See you online!

News from the Department

Further news are available on the German website