- Speaker:
**Tomoki Ozawa**(AIMR, Tohoku University) - Title: Topological band structure in quantum and classical systems
- Date/Time: 16:00-17:30, Friday, November 27, 2020

- Speaker:
**Nobuyuki Okuma**(YITP, Kyoto University) - Title: Topology and anomaly in non-Hermitian skin effects: from index theorem to monopole catalysis
- Date/Time: 16:00-17:30, Friday, December 4, 2020

- Speaker:
**Yosuke Kubota**(Shinshu University) - Title: Coarse geometry and operator algebra in condensed-matter physics
- Date/Time: 16:00-17:30, Friday, January 8, 2021

- Speaker:
**Yuan Yao**(RIKEN) - Title: Critical parafermionic systems - from bosonization to parafermionization
- Date/Time: 16:00-17:30, Friday, March 26, 2021
- Abstract: In this talk, we will discuss a Zk parafermionization and bosonization scheme for one-dimensional lattice models and field theories on a torus, starting from the conventional bosonization for k=2. The Zk parafermionization enables us to investigate the critical theories of parafermionic chains whose fundamental degrees of freedom are parafermionic, and we find that their modular covariance properties are unconventional in that their partition functions on a torus transform differently from any conformal field theory when k>2. Several concrete examples of lattice models will be discussed where parafermionic statistics can be manifested. This talk is based on the preprint: arxiv:2012.07529.

- Speaker:
**Mayuko Yamashita**(RIMS, Kyoto University) - Title: The classification problem of non-topological invertible QFT's and a physically motivated model for the Anderson duals
- Date/Time: 16:00-17:30, Friday, April 23, 2021
- Abstract: Freed and Hopkins conjectured that the deformation classes of non-topological invertible quantum field theories are classified by a generalized cohomology theory called the Anderson dual of bordism theories. The main difficulty of this problem lies in the fact that we do not have the axioms for QFT's. In this talk, I will explain the ongoing work to give a new approach to this conjecture. We construct a new, physically motivated model for the Anderson duals. This model is constructed so that it abstracts a certain property of invertible QFT's which physicists believe to hold in general. I will start from basic motivations for the classification problem, report the progress of our work and explain future directions. This is the joint work with Yosuke Morita (Kyoto, math) and Kazuya Yonekura (Kyushu, physics).

- Speaker:
**Hosho Katsura**(the University of Tokyo) - Title: The SU(N) Hubbard model: some rigorous results
- Date/Time: 16:00-17:30, Friday, May 21, 2021

- Speaker:
**Ryohei Kobayashi**(ISSP, the University of Tokyo) - Title: Anomalies in (2+1)d fermionic topological phases and (3+1)d path integral state sums for fermionic SPT phases
- Date/Time: 16:00-17:30, Friday, June 25, 2021

- Speaker:
**Minkyu Kim**(the University of Tokyo) - Title: Toric code induced by bicommutative Hopf algebras
- Date/Time: 16:00-17:30, Friday, July 30, 2021

- Speaker:
**Yoshimasa Hidaka**(KEK) - Title: Higher group symmetry in axion electrodynamics
- Date/Time: 16:00-17:30, Friday, September 17, 2021

- Speaker:
**Shuhei Oyama**(YITP, Kyoto University) - Title: Generalized Thouless Pump and the Matrix Product State (video)
- Date/Time: 16:00-17:30, Friday, November 19, 2021

- Speaker:
**Chris Bourne**(Tohoku University) - Title: Fermionic gapped ground states and topological phases in the infinite volume limit
- Date/Time: 16:00-17:30, Friday, December 17, 2021

- Speaker:
**Kazuya Yonekura**(Tohoku University) - Title: Classification of topologically nontrivial terms in action
- Date/Time: 16:30-18:00, Friday, January 21, 2022

- Speaker:
**Shingo Kobayashi**(RIKEN) - Title: Fragile topological insulators protected by rotation symmetry without spin-orbit coupling
- Date/Time: 16:00-17:30, Friday, March 25, 2022

- Speaker:
**Shin Hayashi**(Tohoku University) - Title: Topological invariants related to corner states via index theory and matrix factorizations
- Date/Time: 16:00-17:30, Friday, May 20, 2022

- Speaker:
**Yuya Tanizaki**(Yukawa Institute for Theoretical Physics, Kyoto University) - Title: Semiclassics with 't Hooft flux and confinement of 4d gauge theories via center vortices
- Date/Time: 16:30-18:00, Friday, June 10, 2022
- Abstract:Confinement of 4d gauge theories is usually the strong-coupling problem, and it is a difficult task to understand even its qualitative features. We are developing its semiclassical understanding based on the idea of volume independence or adiabatic continuity. We conjecture that the strong-coupling regime of many 4d gauge theories is continuously connected to the weak-coupling theories on small R2xT2 with the nontrivial ”Ęt Hooft flux. As a partial justification, we explicitly confirm that the dilute gas of center vortices can describe the fractional theta periodicity for pure YM theory and the chiral Lagrangian for QCD in this small T2 regime. We also uncover why this is possible in view of the 't Hooft anomaly matching condition.

- Speaker:
**Kansei Inamura**(ISSP, the University of Tokyo) - Title: Construction of 2d bosonic topological field theories with fusion category symmetries and their fermionization
- Date/Time: 16:00-17:30, Friday, July 22, 2022

- Speaker:
**Tsuneya Yoshida**(University of Tsukuba) - Title: Effects of symmetry and interactions on exceptional points
- Date/Time: 16:30-18:00, Friday, September 30, 2022

- Speaker:
**Seishiro Ono**(the University of Tokyo) - Title: High-throughput calculations of topological and nodal superconductivity in materials
- Date/Time: 16:30-18:00, Friday, October 21, 2022
- Abstract: Symmetry-transformation properties of Cooper pairs under spatial symmetries, dubbed pairing symmetry, are closely related to how electrons pair up. However, pairing symmetries for the majority of materials are not known. Sidestepping this issue, we tackle an alternative problem: we predict topological and nodal superconductivity in materials for each possible pairing symmetry. Our results could be compared with experimental results and reduce the candidates of pairing symmetries. In this talk, after reviewing our diagnostic scheme, we apply our method to some recently discovered superconductors. This talk is based on the following paper: Feng Tang*, Seishiro Ono*, Xiangang Wan, and Haruki Watanabe, Phys. Rev. Lett. 129, 027001 (2022).

- Speaker:
**Yuji Hirono**(Asia Pacific Center for Theoretical Physics) - Title: Structures and functions of chemical reaction networks
- Date/Time: 16:00-17:30, Friday, November 25, 2022
- Abstract: Chemical reactions form a large web of networks inside living cells and they are responsible for physiological functions. Understanding the behavior of complex reaction networks is a challenging and interesting problem. In this talk, I would like to illustrate how the techniques used in algebraic topology can shed light on the relationship between structures and functions of chemical reaction systems. In particular, we will discuss the following two problems: (1) response of reaction systems to external perturbations and (2) simplification of complex reaction networks without altering the behavior of the system.

- Speaker:
**Ching-Kai Chiu**(RIKEN) - Title: The connections between twisted bilayer physics and topological semimetals
- Date/Time: 16:00-17:30, Friday, December 23, 2022

- Speaker:
**Chris Bourne**(Tohoku University) - Title: Index theory and topological phases for symmetric quantum walks
- Date/Time: 16:00-17:30, Friday, January 20, 2023

- Speaker:
**Masahiko G. Yamada**(Gakushuin University) - Title: Matrix product renormalization group
- Date/Time: 16:00-17:30, Friday, February 24, 2023

- Speaker:
**Masaki Natori**(the University of Tokyo) - Title: An alternative proof of Bott periodicity theorem using configuration spaces and bulk-edge correspondence
- Date/Time: 16:00-17:30, Friday, May 26, 2023

- Speaker:
**Yuto Moriwaki**(RIKEN) - Title: Bootstrap hypothesis in two-dimension and the operad structure on moduli spaces
- Date/Time: 16:00-17:30, Friday, June 9, 2023
- Abstract: Conformal field theory is believed to be consistent (well-defined) if it satisfies the bootstrap equation. This claim is called the bootstrap hypothesis and is used, for example, in the numerical calculation of critical exponents for the 3-D Ising model. In this talk, we will formulate the bootstrap hypothesis mathematically in the two-dimensional case and give a proof under certain conditions. The consistency of 2d conformal field theory comes down to the representation theory of chiral conformal field theory (vertex operator algebra), and the important step in the proof is to show that the operad of the moduli spaces of punctured Riemann spheres acts topologically on the representation category of the vertex operator algebra. From this point of view, the bootstrap equation is a generator of a certain groupoid operad, and the consistency follows. This result is inspired by Huang-Kong's full field algebra, but we believe that our approach makes their result more conceptually comprehensible.

- Speaker:
**Shoto Aoki**(Osaka University) - Title: Curved domain-wall fermion and its anomaly inflow
- Date/Time: 16:00-17:30, Friday, July 7, 2023

- Speaker:
**Hiromi Ebisu**(YITP, Kyoto University) - Title: 2+1d Foliated gauge theories and symmetry algebra
- Date/Time: 16:30-18:00, Friday, September 8, 2023

- Speaker:
**Mitsunobu Tsutaya**(Kyushu University) - Title: Homotopy type of the space of finite propagation unitary operators on Z
- Date/Time: 16:00-17:30, Friday, October 6, 2023

- Speaker:
**Masataka Watanabe**(Nagoya University) - Title: Physics of the Split Property
- Date/Time: 16:00-17:30, Friday, November 10, 2023
- Abstract: I will give a concrete physical procedure which obtains the intermediate Type I factor of von Neumann algebras of quantum field theories, used in defining the reduced density matrix and in computing the entanglement entropy.

- Speaker:
**Philip Boyle Smith**(Kavli IPMU) - Title: Duality webs of chiral fermionic CFTs with c ≤ 16
- Date/Time: 17:00-18:30, Friday, December 22, 2023
- Abstract: We classified all chiral fermionic CFTs of central charge ≤ 16 using Kac's theorem and bosonisation. This talk will discuss the derivation of this result, its application to non-supersymmetric heterotic strings, and how such theories fall into duality webs under topological manipulations involving the spin structure. The latter part will involve addressing some often-overlooked subtleties of bosonisation in gravitationally-anomalous theories, which we do from various perspectives involving anomalies, topological phases, and TFTs.

- Speaker:
**Ryuichi Shindou**(Peking University) - Title: Effect of weak band topology in metal-insulator transition in chiral symmetry classes
- Date/Time: 16:30-18:00, Friday, February 9, 2024 (
)**Note that the time slot is different from the usual one.** - Abstract: An interplay between disorder and a weak band topology in chiral symmetry classes leads to an intermediate phase between metal and Anderson localized phases, dubbed as "quasi-localized" phase, where wave function along a spatial direction with the weak topological index is delocalized but exponentially localized along the other direction(s). In this talk, I will introduce our recent studies on this topics.

- Speaker:
**Motohiko Ezawa**(the University of Tokyo) - Title: Topological-universal hybrid quantum computation based on Majorana fermions
- Date/Time: 16:00-17:30, Friday, April 5, 2024
- Abstract: We start with reviews on quantum computation and braiding of Majorana fermions. Then, we show that only the braiding of Majorana fermions is not enough for universal quantum computation due to the embedding problem, where certain gates including the CNOT gate cannot be embedded in larger qubits. This problem is solved by introducing many-body Majorana operators [1]. We also derive analytic formula for short Kitaev chain describing low-energy theory [2], which will be useful for recent experimental realization of Majorana fermions in quantum dots [3]. Finally, we show that an N-connected honeycomb chain of the Kitaev spin liquid model acts as 3N+2 qubits [4] and discuss how to braid and execute fusion based on local spin correlation operators.
[1] M. Ezawa, Systematic construction of topological-nontopological hybrid universal quantum gates based on many-body Majorana fermion interactions arXiv:2304.06260

[2] M. Ezawa, Robustness of Majorana edge states of short-length Kitaev chains coupled with environment, arXiv:2310.18083

[3] Tom Dvir, et. al. Realization of a minimal Kitaev chain in coupled quantum dots, Nature 614, 445 (2023).

[4] M. Ezawa, Braiding and fusion of Majorana fermions in minimal Kitaev spin liquid on a single hexagon with 5 qubits, arXiv:2401.01051

- Speaker:
**Tetsuo Deguchi**(Ochanomizu University) - Title: Exact results on the elasticity of polymer networks derived through homology in topology
- Date/Time: 16:00-17:30, Friday, May 24, 2024
- Abstract: The elastic theory of polymer networks such as phantom networks is reformulated and generalized in terms of homology in topology, so that useful exact results are derived not only for the Gaussian networks but also for polymer networks of chains with nonlinear potentials. For an illustration, the fractional power scaling behavior is shown in the chain-density dependence of the elastic modulus for polymer networks in melts, by making use of the results of phantom networks. Furthermore, a systematic method is presented for exactly calculating the nonlinear elasticity of such polymer networks that consist of finitely extensible ideal chains. It leads to an exact expression of the nonlinear elastic stress for any given lattice-type network. It is emphasized that spontaneous symmetry breaking plays a significant role in the elasticity of polymer networks, although it is usually not mentioned explicitly. Some of the derived results seem to be consistent with molecular dynamical simulations, scattering experiments and stress measurement of gels. In the introduction of this talk the standard theory of phantom networks is briefly reviewed and its potential difficulties are argued. The present talk is based on research results of the JST CREST project (2019 JPMJCR19T4) with the title: "Construction of homological topology theory on the elasticity of polymer networks and creation of devices through mixing ring polymers in polymeric materials" and in collaboration with Jason Cantarella, Clayton Shonkwiler and Erica Uehara.

- Speaker:
**Kan Kitamura**(RIKEN iTHEMS) - Title: Quantum symmetries on simple C*-algebras
- Date/Time: 16:00-17:30, Friday, June 21, 2024