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.

2026 Summer Semester

During the 2026 summer semester, the seminar generally takes place on Thursday at 11:30 in the Erwin Schrödinger lecture hall (Boltzmanngasse 5, 5th floor). Occasionally, there might be additional seminars out of schedule as announced here.

Kshiti Sneh Rai (Leiden University)

Matrix product state approximations to quantum states of low energy-variance
Abstract: In this work, we show how to efficiently simulate pure quantum states in one dimensional systems that have both finite energy-density (i.e. in the bulk of the spectrum) and vanishingly small energy fluctuations. We analyze a tensor network algorithm that produces matrix product states whose energy-variance decreases as the bond dimension increases. Our results imply that variances as small as ∝ 1 / log N can be achieved with polynomial bond dimension. This establishes that there exist states with a very narrow support in the bulk of the spectrum that still have moderate entanglement entropy, in contrast with typical eigenstates that display a volume law. Our main technical tool is a Berry-Esseen theorem for spin systems, which strengthens central limit behavior for the energy distribution of product states.

(date/time/location: 03.03.2026, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, ground floor, Kurt Gödel lecture room)

Shuhei Oyama (University of Vienna)

Generalized Cluster States and Strip 2-Algebras
Abstract: The importance of symmetry in understanding the properties of physical systems is well recognized. In particular, the presence of symmetry can impose strong constraints on the low-energy behavior of a system, as captured by Lieb-Schultz-Mattis (LSM)-type theorems. Furthermore, when a system supports nontrivial defect structures, considering how symmetry acts on those defects can also constrain the interfaces and defects allowed in the system.

In our previous work [1], by studying defects in 1+1-dimensional SPT phases protected by non-invertible symmetry, we established a bulk-boundary correspondence and classified parameterized families. In recent work, we generalized these results to 2+1 dimensions [2,3]. In particular, we constructed the 2+1-dimensional cluster state and its generalization, the generalized cluster state, and classified defects by studying representations of the symmetry acting on them, namely the strip 2-algebra.

In this seminar, I will first explain the PEPS representation of the ground state　of the (generalized) cluster state and the PEPO representation of the non-invertible symmetry, and show how to construct the partial action of the PEPO on the PEPS using action tensors. I will then use these action tensors to construct symmetry operators acting on defects, and study their representation theory. Finally, I will discuss an application of this framework: the formulation and proof of the bulk-boundary correspondence for generalized cluster states. If time permits, I will also comment on further physical consequences, including symmetry-enforced degeneracy and applications to the classification of parameterized families.

This talk is based on the following papers:
[1] arXiv:2408.15960
[2] arXiv:2601.08615
[3] arXiv:2601.08616

(date/time/location: 19.03.2026, 11:30, Boltzmanngasse 5/Strudlhofgasse 4, 5th floor, Erwin Schrödinger lecture hall)