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 added to our mailing list. For info on past seminars click here.

2022 Winter Semester

During the 2022 winter semester, the seminar takes place on Thursdays at 11:00 a.m. in the Josef Stefan lecture room (Faculty of Physics, Strudlhofgasse 4, 3rd floor, room 3329), generally in person if the situation reg. the COVID pandemic allows this. Occasionally, there might be additional seminars out of schedule, or seminars given online, as announced here.

 Seminar calendar for the 2022 Winter Semester

DATE TIME SPEAKER & TITLE
27.10. 11.00 José Garre Rubio (University of Vienna):
Gauging quantum states with non-anomalous matrix product operator symmetries
03.11. 11.45 András Molnár (University of Vienna):
Tensor networks generating the same states
10.11. 11.00 Christopher Popp (University of Vienna):
Bound Entanglement of Bell Diagonal Pairs of Qudits
17.11. 11.00 Martin Renner (University of Vienna):
Simulating qubit correlations with classical communication
24.11. 11.00 Bram Vanhecke (University of Vienna):
Some scaling methods for infinite tensor network calculations
01.12. 11.00 no seminar
15.12. 11.00 David Blanik (University of Vienna):
title of talk: TBA
12.01. 11.00 Anna Francuz (University of Vienna):
title of talk: TBA
19.01. 11.00 Mingru Yang (University of Vienna):
title of talk: TBA
26.01. 11.00 Ilya Kull (University of Vienna):
title of talk: TBA

José Garre Rubio (University of Vienna)

Gauging quantum states with non-anomalous matrix product operator symmetries
Abstract: Gauging a global symmetry of a system amounts to introducing new degrees of freedom whose transformation rule makes the overall system observe a local symmetry. In quantum systems there can be obstructions to gauging a global symmetry. When this happens the symmetry is dubbed anomalous. Such obstructions are related to the fact that the global symmetry cannot be written as a tensor product of local operators. We study non-local symmetries that have an additional structure: they take the form of a matrix product operator (MPO). We exploit the tensor network structure of the MPOs to construct local operators from them satisfying the same group relations, that is, we are able to localize even anomalous MPOs. For non-anomalous MPOs, we use these local operators to explicitly gauge the MPO symmetry of a one-dimensional quantum state obtaining non-trivial gauged states. We show that our gauging procedure satisfies all the required properties as the standard on-site case does. We also show how this procedure is naturally represented in matrix product states protected by MPO symmetries. In the case of anomalous MPOs, we shed light on the obstructions to gauging these symmetries.

(date/time/location: 27.10.2022, 11:00, Josef Stefan lecture hall, Strudlhofgasse 4, 3rd floor, room 3329)

András Molnár (University of Vienna)

Tensor networks generating the same states
Abstract: In this talk I will review different scenarios where a state has two different natural tensor network descriptions. To understand the structure of these states, one has to be able to relate the two descriptions to each other. I will show how to derive such a relation for a restricted class of tensors.

(date/time/location: 03.11.2022, 11:45, Josef Stefan lecture hall, Strudlhofgasse 4, 3rd floor, room 3329)

Christopher Popp (University of Vienna)

Bound Entanglement of Bell Diagonal Pairs of Qudits
Abstract: For dimension d ≥ 3, a form of entanglement exists that is hard to detect and called bound entanglement due to the fact that such entangled states cannot be used for entanglement distillation. Up to this date, no efficient solution is known to differentiate bound entangled from separable states. We address and compare this problem named separability problem for a family of bipartite Bell diagonal qudits with special algebraic and geometric structures and applications in quantum information processing tasks in different dimensions. Using analytical and numerical methods, we successfully classify large shares of representative Bell diagonal PPT states for d = 3 and d=4. Via those representative states, we are able to estimate the volumes of separable and bound entangled states among PPT qudits. Comparing the structure of bound entangled states and their detectors, we find considerable differences in the detection capabilities for different dimensions and relate those to differences of the Euclidean geometry for qutrits (d = 3) and ququarts (d = 4). A detailed visual analysis of the set of separable Bell diagonal states in both dimensions allows a conjecture relating the group structure of Bell diagonal states of the analyzed family to necessary and sufficient mixing conditions for separable states.

Based on:
Almost complete solution for the NP-hard separability problem of Bell diagonal qutrits
(https://www.nature.com/articles/s41598-022-16225-z)
Bound Entanglement of Bell Diagonal Pairs of Qutrits and Ququarts: A Comparison
(https://arxiv.org/abs/2209.15267)

(date/time/location: 10.11.2022, 11:00, Josef Stefan lecture hall, Strudlhofgasse 4, 3rd floor, room 3329)

Martin Renner (University of Vienna)

Simulating qubit correlations with classical communication
Abstract: Bell's famous theorem shows that quantum correlations cannot be reproduced by local hidden variables. However, it is possible to simulate quantum entanglement when the parties are allowed to send some classical information. For the simplest case of projective measurements on two maximally entangled qubits, Toner and Bacon (Phys. Rev. Lett. 91, 187904, 2003) proved that already a single bit is sufficient. At the same time, all previously known protocols to simulate non-maximally entangled qubit pairs require more resources. Here, we improve these protocols and show that two weakly entangled qubits can be simulated with a single bit as well. We also study the case of positive operator-valued measurements (POVMs) and show that two bits are sufficient to simulate local POVMs on any entangled qubit pair. For the latter case, previously known protocols required an unbounded amount of communication in the worst case.

(date/time/location: 17.11.2022, 11:00, Josef Stefan lecture hall, Strudlhofgasse 4, 3rd floor, room 3329)

Bram Vanhecke (University of Vienna):

Some scaling methods for infinite tensor network calculations
Abstract: Finite size calculations for critical systems, be they performed by ED, MC or other, enjoy a generic and well understood scaling behaviour of the couplings and observables with respect to the system size. The ready availability and robustness of this 'scaling hypothesis' -with system size- has proven very practical and powerful. Tensor network calculations on the other hand may be performed in the thermodynamic limit, which is a nice feature but hinders direct application of the tried and tested finite size scaling methods. We will discuss some attempts at making an infinite tensor network equivalent of the old scaling hypothesis, for MPS, PEPS and bPEPS. We will show some of the benefits and shortcomings of this approach through some benchmark applications, and if time permits also an application to a critical QFT.

(date/time/location: 24.11.2022, 11:00, Josef Stefan lecture hall, Strudlhofgasse 4, 3rd floor, room 3329)