SEMINARS

Our weekly 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@univie.ac.at to be put on our mailing list.

Winter Semester 2021/2022

During the winter semester 2021/2022, the seminar generally takes place Fridays at 11:15 in the Kurt Gödel lecture hall (Strudlhofgasse 4, Room 3E63). Occasionally, there might be additional seminars out of schedule, or seminars given online, as announced here.

 Seminar calendar for the Winter Semester 2021/2022

DATE TIME SPEAKER & TITLE
15.10. 11:15 (internal meeting)
22.10. 11:15 (internal meeting)
29.10. 11:15 (internal meeting)
05.11. 13:30 Kade Head-Marsden (Harvard):
Quantum algorithms for non-unitary time evolution of quantum systems

Location: Ernst Mach lecture hall (Strudlhofgasse 4, 2nd floor, room 3248)
12.11. 11:15 Alex Turzillo (Max Planck Institute of Quantum Optics, Munich)
Symmetry-protected topological order in open quantum systems

Location: Kurt Gödel lecture hall (Strudlhofgasse 4, ground floor, room 3E63)
19.11. 11:15 Bram Vanhecke (University of Vienna):
Frustrated systems and tensor networks

Location: Kurt Gödel lecture hall (Strudlhofgasse 4, ground floor, room 3E63)
26.11. 11:15 Ilya Kull (University of Vienna):
An area law for 2D frustration-free spin systems

Location: online
03.12. 09:30 David Blanik (University of Vienna):
An informal introduction to (co)homology and applications in physics

Location: online
10.12. 11:15 Wen-Tao Xu (University of Vienna):
Matrix product operator algebras, non-abelian anyons and their splitting

Location: online
17.12. 11:15 José Garre Rubio (University of Vienna):
Classifying non-onsite symmetries with tensor networks

Location: online
14.01. 11:15 Mingru Yang (University of Vienna):
Weak ergodicity breaking, quantum many-body scars, and Hilbert space fragmentation

Location: online
21.01. 11:15 Andras Molnar (University of Vienna)
28.01. 11:15 Manuel Mekonnen (University of Vienna)

Kade Head-Marsden (Harvard University):

05.11.2021, 13:30 CET, Ernst Mach lecture hall, Strudlhofgasse 4, 2nd floor, room 3248

Quantum algorithms for non-unitary time evolution of quantum systems
Abstract: Open quantum system evolution in the presence of an environment is crucial to understanding and improving many processes including the communication of quantum information and the transfer of energy. Quantum computing platforms have emerged as a promising route to modelling and predicting the behaviour of such systems. However, mapping inherently non-unitary dynamics into the unitary framework of gate-based quantum algorithms is a challenging task. Here, I will discuss two different density matrix gate-based quantum algorithms to predict the dynamics of open quantum systems. The first algorithm is dilation based where the Hilbert space of interest is expanded to map non-unitary evolution into a unitary framework [1,2]. The second algorithm is based on the decomposition of a non-unitary matrix into Hermitian and anti-Hermitian components [3]. I will discuss the theory behind these algorithms, their extension from the Markovian to the non-Markovian regime, and applications relevant in chemistry and physics [4].

[1] Z. Hu, R. Xia, and S. Kais, Sci. Rep., 10, 3301 (2020)
[2] K. Head-Marsden, S. Krastanov, D. A. Mazziotti, and P. Narang, Phys. Rev. Res., 3 (1), 013182 (2021)
[3] A.W. Schlimgen, K. Head-Marsden, L. Sager, P. Narang, and D. A. Mazziotti (submitted 2021) arXiv:2106.12588
[4] Z. Hu, K. Head-Marsden, D. A. Mazziotti, P. Narang, and S. Kais (submitted 2021) arXiv:2101.05287v2

Alex Turzillo (Max Planck Institute of Quantum Optics, Munich):

12.11.2021, 11:15 CET, lecture hall: Kurt Gödel lecture hall (Strudlhofgasse 4, ground floor, room 3E63)

Symmetry-protected topological order in open quantum systems
Abstract: This talk will discuss the robustness of symmetry protected topological (SPT) order to evolution by noisy channels. By studying the evolution of non-local string order parameters, we find that one-dimensional SPT order is destabilized by generic couplings to the environment as well as by couplings satisfying a weak symmetry condition that directly generalizes from closed systems. We introduce a stronger symmetry condition on channels that ensures SPT order is preserved.

Bram Vanhecke (University of Vienna):

19.11.2021, 11:15 CET, lecture hall: Kurt Gödel lecture hall (Strudlhofgasse 4, ground floor, room 3E63)

Frustrated systems and tensor networks
Abstract: Classical frustrated systems are characterised by an extensive ground state degeneracy. As such, they could still have interesting physics at zero temperature. Generally these systems are hard to study numerically, and certainly characterising the ground state ensemble can be challenging as there are no thermal fluctuations. We show how tensor network methods may be applied to frustrated systems in two and three dimensions with roughly the same ease as they may be applied to unfrustrated classical systems. In particular we will show how to study the ground state ensemble directly, which requires circomventing a sort of sign-problem in tensor networks.

Ilya Kull (University of Vienna):

26.11.2021, 11:15 CET, online

An area law for 2D frustration-free spin systems
Abstract: I will try to give an overview of a recent proof of an entanglement entropy area law in the ground states of 2D frustration-free systems (2103.02492). For the most part, I will be talking about previous results in the same vein (1301.1162 and 1905.11337). The proof techniques are very similar in all three papers (the 2D problem is reduced to 1D) and employ the same arsenal of tools and insights. Those tools will be the focus of the talk.

David Blanik (University of Vienna):

03.12.2021, 9:30 CET, online

An informal introduction to (co)homology and applications in physics
Abstract: I will give a very basic introduction to the theory of homology and cohomology groups. In regards to their applications in physics, I will focus mainly on the topics of group cohomology and the BRST construction.

Wen-Tao Xu (University of Vienna):

10.12.2021, 11:15 CET, online

Matrix product operator algebras, non-abelian anyons and their splitting
Abstract: Using Kitaev’s quantum double model as an example, I will introduce the matrix product operator (MPO) algebras, which describe the symmetry of topological tensor network states on the entanglement level. The anyonic excitations can be constructed from the irreps of the MPO algebras. From this construction, the internal degrees of freedom of non-Abelian anyons can be specified. Through boson condensation, the internal degrees of freedom of a non-Abelian anyon can split into Abelian anyons. I will also discuss the possible ways to detect splitting.

José Garre Rubio (University of Vienna):

17.12.2021, 11:15 CET, online

Classifying non-onsite symmetries with tensor networks
Abstract: In this talk I will give an introduction to the classification of non-onsite symmetries of spin chains. The approach is based on tensor networks: the symmetry operators are represented as matrix product operators and the states are matrix product states. I will show the structure of the invariants classifying these phases and the connection to symmetry protected topological phases. Finally, I will comment on the gauging of these phases and possible generalizations where the symmetry is not restricted to the group case.

Mingru Yang (University of Vienna):

17.12.2021, 11:15 CET, online

Weak ergodicity breaking, quantum many-body scars, and Hilbert space fragmentation
Abstract: It has been thought thermalization is the inevitable fate of many interacting quantum systems, whose dynamics allow them to fully explore the vast configuration space regardless of the initial state--the behavior known as quantum ergodicity, with the only two counterexamples being integrable systems and many-body localization. However, recent experiments in quantum simulators based on 51 Rydberg atoms found persistent revivals for certain initial states, revealing the existence of a set of special non-thermal eigenstates in the bulk of the many-body energy spectrum, dubbed "quantum many-body scars", which is a new mechanism of ergodicity breaking. In the presentation, I will give an introduction to this weak ergodicity breaking phenomenon and three formalisms to try to unify the towers of quantum many body scar states in different models. I will show how matrix product states and tangent space methods become useful tools in the analytical construction of these states and giving a semiclassical description. I will also point out some open questions that might be interesting for us to explore.

Past seminars