SEMINARS

Our group seminar features talks on current research topics from the field of Quantum Information and Quantum Many-Body Physics, and in particular Tensor Networks, given both by group members and by external guest speakers.

If you are interested in receiving seminar announcements, please send an informal e-mail to schuch-office.quantum[at]univie.ac.at to be added to our mailing list.

› For info on past seminars click here.

2025 Winter Semester

During the 2025 winter semester, the seminar generally takesplace on Mondays at 11:30 in the Erwin Schrödinger lecture hall (Boltzmanngasse 5, 5th floor). Occasionally, there might be additional seminars out of schedule, or seminars given online, as announced here.

Shuhei Oyama (University of Vienna)

Higher Structures on Boundary Conformal Manifolds: Higher Berry Phase and Boundary Conformal Field Theory
Abstract: The space of conformal field theories, known as the conformal manifold, has a rich geometric structure. Locally, it is parametrized by exactly marginal operators, and its two-point functions endow it with a Riemannian metric. The existence of the conformal manifold places significant constraints on CFT data and plays an important role in string theory and holography. There are analogous spaces associated with boundary and defect exactly marginal deformations, known as the boundary and defect conformal manifolds. We discuss novel geometric structures—gerbe structures—on the boundary conformal manifold. Our approach is based on the notion of multi-wavefunction overlap, which was first proposed in the context of tensor networks to define a many-body generalization of the Berry phase, i.e., higher Berry phases. In this talk, I will first give a brief review of the formulation of the higher Berry phase in lattice systems. I will then focus on the similarity between tensor network states and boundary CFT states and use it to define a gerbe structure on the boundary conformal manifold. This talk is based on the preprint arXiv:2507.12525.

(date/time/location: 13.10.2025, 11:30, Erwin Schrödinger lecture hall, Boltzmanngasse 5, 5th floor)

Anna Francuz (University of Vienna)

Topological Order in the Rydberg Blockade on the Kagome Lattice with PEPS
Abstract: Topological order is notoriously difficult to detect—both theoretically and experimentally—due to the absence of local observables capable of distinguishing it from other long-range entangled phases. This challenge has motivated a shift from real materials toward quantum simulation platforms, particularly programmable Rydberg atom arrays, which offer promising avenues for realizing topological order. A notable example is the Kagome lattice quantum simulator demonstrated in Ref. [1]. In this work, we use two-dimensional Projected Entangled Pair States (PEPS) to variationally optimize the PXP model on the Kagome lattice. Our study also benchmarks a new algorithm for ground-state optimization with automatic differentiation (AD), as well as an algorithm for extracting F-symbols and modular data from topologically ordered PEPS. In my talk, I will introduce the concept of automatic differentiation and show the speedup achieved through implicit function differentiation, rather than direct differentiation of the fixed-point equations for both the VUMPS and symmetric CTM algorithms. Finally, I will present preliminary results from our PEPS optimization of the PXP model using the VUMPS contraction scheme.

[1] G. Semeghini, H. Levine, A. Keesling, S. Ebadi, T. T. Wang, D. Bluvstein, R. Verresen, H. Pichler, M. Kalinowski, R. Samajdar, A. Omran, S. Sachdev, A. Vishwanath, M. Greiner, V. Vuletic, and M. D. Lukin,
Science 374, 1242 (2021).

(date/time/location: 20.10.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

Kevin Tam (University of Vienna)

Conformal Field Theory and Parton Approaches to SU(n)_k Chiral Spin Liquids
Abstract: Chiral spin liquids (CSLs) constitute bosonic analogues of the celebrated fractional quantum Hall states. Such examples of chiral topological order break time-reversal symmetry and exhibit gapless edge modes described by chiral conformal field theories (CFTs). In this seminar, I will first review some basic CFT [1]. I will then discuss some recent results from Liu et al. relating chiral correlators to parton states [2], two common approaches to constructing model wavefunctions for chiral systems.

[1] P. Di Francesco, P. Mathieu, and D. Sénéchal, Conformal Field Theory (1997).
[2] T. Liu, Y.-H. Wu, H.-H. Tu, and T. Xiang, “Bridging conformal field theory and parton approaches to SU(n)_k chiral spin liquids,” Physical Review. B. (2025). doi.org/10.48550/arXiv.2501.09567

(date/time/location: 27.10.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

David Blanik (University of Vienna)

Categorical gauging and emergent quantum double models
Abstract: It was recently observed [1] that iterated gauging of one-dimensional quantum many-body states invariant under the action of a finite abelian symmetry group yields ground states of abelian quantum double models. In this talk, I will give a pedagogical introduction to a recently proposed generalization [2] of the classic gauging procedure [3] to arbitrary fusion-categorical symmetries, and show that quantum double models also emerge in the non-abelian setting. This talk is based on ongoing work with José Garre-Rubio.

[1] Garre-Rubio. Emergent (2+1)D Topological Orders from Iterative (1+1)D Gauging. arXiv:2403.07575
[2] Vancraeynest-De Cuiper et al. From Gauging to Duality in One-Dimensional Quantum Lattice Models. arXiv: 2509.22051
[3] Haegeman et al. Gauging Quantum States: From Global to Local Symmetries in Many-Body Systems. arXiv:1407.1025

(date/time/location:03.11.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

András Molnár (University of Vienna)

Exact Tensor Network eigenstates of local Hamiltonians
Abstract: In this talk I characterize exact tensor network (TN) eigenstates of local Hamiltonians. The characterization is a local equation involving the Hamiltonian terms and the tensors defining the TN state. We illustrate the statement for matrix product states (MPS). This equation has far-reaching applications such as finding an MPS path that satisfies the Schrödinger equation with a (time- dependent) local Hamiltonian path, characterizing exact scar states, or even characterizing exactly solvable models.

(date/time/location: 10.11.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

Paul Brehmer (University of Vienna)

Anatomy of a PEPS code
Abstract: In this seminar talk, I will explore the software modalities of modern tensor network numerics, with a focus on a growing ecosystem of open-source Julia packages initiated by colleagues in the Quantum Group at Ghent University [1, 2]. This suite of tools ranges from low-level tensor primitives to high-level algorithms, supporting performant and reproducible computational workflows in research. As a concrete case study, I will discuss the ongoing development of projected entangled-pair state (PEPS) algorithms [3], highlighting the challenges of managing complexity and ensuring interoperability in large-scale tensor network simulations.

[1] L. Devos and J. Haegeman, TensorKit.jl: A Julia package for large-scale tensor computations, with a hint of category theory, arXiv:2508.10076 (2025)
[2] QuantumKitHub, github.com/QuantumKitHub
[3] P. Brehmer, L. Burgelman, Z. Yue and L. Devos, PEPSKit.jl: A Julia package for projected entangled-pair state simulations (in preparation), https://github.com/QuantumKitHub/PEPSKit.jl

(date/time/location: 17.11.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

Refik Mansuroglu (University of Vienna)

Simulation Complexity of Gaussian Bosonic Hamiltonians
Abstract: The simulation of bosonic states under Gaussian Bosonic Hamiltonians can be very easy or very hard depending on whether squeezing is allowed or not. If squeezing is not allowed, certain moments of position and momentum observables can be efficiently simulated by a quantum computer. If it is allowed, the simulation becomes PostBQP-hard, meaning it would require arbitrary post-selection. I present a subclass of simulable Hamiltonians and prove hardness for the rest.

(date/time/location: 24.11.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)

Andreas Klingler (University of Vienna)

Ground-State Degeneracy in 1D Frustration-Free Hamiltonians: Thresholds, Jumps, and Undecidability
Abstract: In this talk, I will discuss how the ground-state degeneracy of 1D frustration-free Hamiltonians with nearest-neighbor interactions behaves as the system size grows. The main question we explore is: if we know the ground-state degeneracy for small systems, what can we conclude about larger systems? I will first show that for commuting Hamiltonians, there is a clear threshold beyond which the degeneracy remains fixed. Moving to non-commuting Hamiltonians, we see that small-system behavior can be misleading, with degeneracy sometimes jumping unexpectedly at arbitrary size. Finally, we will see that in general, it is even undecidable to predict whether the degeneracy collapses or grows in the large-system limit.

(date/time/location: 01.12.2025, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)