VQSS is a small online seminar on quantum symmetries organized by David Penneys and Giovanni Ferrer at the Ohio State University.

Topics of interest

  • Tensor and fusion categories
  • Higher categories
  • Subfactors and operator algebras
  • Topological phases of matter
  • Quantum information

Meeting Time

We usually meet on Tuesdays 2:00 pm - 3:00 pm, US Eastern Time Zone. However, sometimes we meet in person at a possibly different hour. See future talks for the precise meeting time of a given talk.


Some of our previous talks can be found on the OSU Math youtube channel.

Future Talks

Identifying invertiblility of bimodule categories

Speaker: Jacob Bridgeman (Ghent University)

Date: Dec. 6, 2022

Abstract: Fusion categories, and their module categories, have many applications in both mathematics and physics. One notion of equivalence between fusion categories is Morita equivalence, which is witnessed by an invertible bimodule category. We provide a readily verifiable condition for deciding if a given bimodule category is invertible. This condition makes use of the skeletal data, and utilizes orthogonality of characters of annular algebras. We then extend this to generalized Schur orthogonality of matrix elements in this setting. Generalized Schur orthogonality in this setting has application in physics. We show that it is equivalent to the notion of MPO-injectivity, which is central to the study of topological orders with tensor networks. This closes an open question concerning tensor network representations for string-net models and plays a central role in the study of generalized symmetries. Based on arXiv:2211.01947 Work with Laurens Lootens and Frank Verstraete

Past Talks

Kumjian-Pask Fibrations and Their C*-Algebras

Speaker: Lydia de Wolf (Kansas State University)

Date: Nov. 29, 2022

Abstract: In this talk, we will present discrete Conduche fibrations and Kumjian- Pask fibrations, which were studied as a generalization of k-graphs. We will briefly summarize the basic results about their C*-algebras and path groupoids in general, and then demonstrate examples of KPfs in which specific choices allow for more exploration than the fully general case.

Discrete extensions of simple C*-algebras

Speaker: Roberto Hernandez Palomares (U Waterloo)

Date: Nov. 15, 2022

Abstract: A subfactor is a unital inclusion of simple von Neumann algebras, and their 'size' is measured by the Jones index. Vaughan Jones proved the striking Index Rigidity Theorem establishing the set of values the index takes is exactly \{4\cos^2(\pi/n)\}_{n\geq 3}\cup [4, \infty]. This discovery started the modern theory of subfactors and their classification program. Discrete subfactors conform to an ample and well-behaved class resembling the properties of discrete groups. These subfactors are studied through the Standard Invariant --which we view as an action of a unitary tensor category together with a chosen generator-- and can be reconstructed from it. In this talk, I will define discreteness for inclusions of C*-algebras and establish a theorem stating that --similarly to their von Neumann counterparts,-- these can be reconstructed from a Standard Invariant. This is work in progress, joint with Brent Nelson and Matthew Lorentz.

Two non Gamma factors with non isomorphic ultrapowers

Speaker: Srivatsav Kunnawalkam Elayavalli (UCLA/IPAM)

Date: Oct. 25, 2022

Abstract: I will show you how to construct a non Gamma factor such that it and L(F_2) have non isomorphic ultrapowers. This settles a fundamental open problem in the classification of II_1 factors. This is joint work with Adrian Ioana and Ionut Chifan.

Unitary anchored planar algebras II: the non-unitary correspondence

Speaker: David Penneys (OSU)

Date: Oct. 18, 2022

Abstract: We'll continue our series of lectures on unitary anchored planar algebras. In this second talk, we'll discuss the correspondence between anchored planar algebras and pointed module tensor categories. No knowledge from the first talk will be assumed. Both talks I and II will be used for talk III.

Unitary anchored planar algebras I: unitary adjunction

Speaker: David Penneys (OSU)

Date: Oct. 11, 2022

Abstract: I'll give the first talk in a series on a new result with Andre Henriques on unitarity for anchored planar algebras. This first talk will focus on unitary adjunction between 2-Hilbert spaces.

Anomalous symmetries on operator algebras

Speaker: Sergio Girón Pacheco (University of Oxford)

Date: Oct. 4, 2022

Abstract: An anomalous action on an operator algebra A is a mapping from a group G to the automorphism group of A, which is multiplicative up to inner automorphisms of A. Anomalous symmetries can be rephrased as actions of pointed fusion categories on A. Starting from the basics, I will introduce anomalous actions and discuss some history of their study in the literature. I will then discuss their existence and classification on simple C*-algebras.

An algebraic quantum field theoretic approach to toric code with boundary

Speaker: Daniel Wallick (OSU)

Date: Sep. 20, 2022

Abstract: Topologically ordered quantum spin systems have become an area of great interest, as they may provide a fault-tolerant means of quantum computation. One of the simplest examples of such a spin system is Kitaev's toric code. Naaijkens made mathematically rigorous the treatment of toric code on an infinite planar lattice (the thermodynamic limit), using an operator algebraic approach via algebraic quantum field theory. We adapt his methods to study the case of toric code with boundary. In particular, we recover the condensation results described in Kitaev and Kong and show that the boundary theory is a module tensor category over the bulk, as expected.

Braided tensor categories from von Neumann algebras

Speaker: Quan Chen (OSU)

Date: Sep. 13, 2022

Abstract: Given a W*-category C, we construct a unitary braided tensor category End_loc(C) of local endofunctors on C, which is a new construction of a braided tensor category associated with an arbitrary W*-category. For the W*-category of finitely generated projective modules over a von Neumann algebra M, this yields a unitary braiding on Popa's χ~(M), which extends Connes' χ(M) and Jone's kappa invariant. Given a finite depth inclusion M_0\subset M_1 of non-Gamma II1 factors, we show that χ~(M_\infty) is equivalent to the Drinfeld center of the standard invariant, where M_infty is the inductive limit of the Jones tower of basic construction.

K-theoretical classification of inductive limit actions of unitary fusion categories on AF-algebras

Speaker: Quan Chen (OSU)

Date: Jul. 26, 2022

Abstract: We introduce a K-theoretic invariant for actions of unitary fusion categories on unital C*-algebras. We show that this is a complete invariant for inductive limits of finite dimensional actions of fusion categories on unital AF-algebras. In particular, this gives a complete invariant for inductive limit actions of finite groups on AF-algebras. We apply our results to obtain a classification of finite depth, strongly AF-inclusions of unital AF-algebras. This is joint work with Roberto Hernandez Palomares and Corey Jones.

The sheaf theory of classical and virtual knots

Speaker: Micah Chrisman (OSU)

Date: Jul. 19, 2022

Abstract: Virtual knots are typically defined combinatorially. They are collection of planar diagrams that are considered equivalent up to a finite sequence of moves. By contrast, knots in the 3-sphere can be defined geometrically. They are the points of a space of knots. The space has a topology so that equivalent knots lie in the same path component. Here we will give geometric construction of virtual knots using sheaf theory. We define a site $(VK,J_{VK})$ so that the category $Sh(VK,J_{VK})$ of sheaves on this site can be naturally interpreted as the “space of virtual knots”. The points of this Grothendieck topos correspond exactly to virtual knots. An equivalence of virtual knots corresponds to a path in this space, or more precisely, a geometric morphism $Sh([0,1]) \to Sh(VK,J_{VK})$. Many other combinatorial concepts in virtual knot theory can likewise be given a geometric reformulation using the language of sheaves.

From C*-categories to W*-categories

Speaker: Giovanni Ferrer (OSU)

Date: Jun. 28, 2022

Abstract: In this talk, we will construct the free W*-category generated by a C*-category. This leads us to prove the Sherman-Takeda theorem along the way, which states that the double-dual of a C*-category agrees with the bicommutant of its universal representation.

Notes: PDF file available

Tanaka Duality (Part II)

Speaker: David Green (OSU)

Date: Jun. 21, 2022

Composing topological domain walls and anyon mobility

Speaker: David Penneys (OSU)

Date: Jun. 14, 2022

Abstract: We study the concatenations of topological domain walls and their decompositions into superselection sectors. Our approach uses a description of particle mobility across domain walls in terms of tunneling operators. These are formalized in a 3-category of (2+1)D topological orders with a fixed anomaly described by a unitary modular tensor category A, algebraically characterized by the 3-category of A-enriched unitary fusion categories. This is joint work with Fiona Burnell, Peter Huston, and Corey Jones.

Gray-categories model algebraic tricategories (Part II)

Speaker: Giovanni Ferrer (OSU)

Date: May 4, 2022 (Wednesday, OUTSIDE!)

Abstract: In this talk, we will adapt the technique of Gurski, Johnson, and Osorno to show the localization of GrayCat at the weak equivalences is equivalent to the category of algebraic tricategories and pseudo-natural equivalence classes of weak 3-functors.

Small at infinity compactification of a von Neumann algebra

Speaker: Srivatsav Kunnawalkam Elayavalli (Vanderbilt)

Date: May 10, 2022

Abstract: We will discuss recent work with C. Ding and J. Peterson where we develop the theory of the small at infinity compactification of a von Neumann algebra. An important feature of this theory is that it can be seen as an application of the theory of operator bimodules to the classification of von Neumann algebras, thereby building on the program initiated by Connes, Haagerup and various other mathematicians in the era of 1970's to 2010's wherein ideas from operator system/space theory such as injectivity and so on were applied to the classification of von Neumann algebras with great success. We use this to define the notion of proper proximality for von Neumann algebras, and find several applications including the solution of a question of Popa asking if L(G) where G is an inner amenable group can embed into L(F_2); the equivalence between the Haagerup property and the compact approximation property for II_1 factors settling an open problem from 1995; solid ergodicity for Gaussian actions without any mixing assumptions improving on results of Boutonnet, Chifan-Ioana.

Some of our previous talks can be found on the OSU Math youtube channel.

Gray-categories model algebraic tricategories (Part I)

Speaker: Giovanni Ferrer (OSU)

Date: Apr. 26, 2022 (OUTSIDE!)

Abstract: Lack described a Quillen model structure on the category GrayCat of Gray-categories and Gray-functors, for which the weak equivalences are the weak 3-equivalences. In this talk, we will go over basic facts about 3-dimensional category theory and model structures.

Some basics on the basic construction

Speaker: Daniel Wallick (OSU)

Date: Apr. 19, 2022

Abstract: Given a unital inclusion of tracial von Neumann algebras A ⊆ B, Vaughan Jones’ basic construction gives a von Neumann algebra ⟨B,e_A⟩ with B⊆⟨B,e_A⟩ a unital inclusion. When A and B are II_1 factors and A ⊆ B is finite index, then ⟨B,e_A⟩ is a II_1 factor, with the unique trace satisfying a Markov property. However, if B is an arbitrary tracial von Neumann algebra, then there may not exist a trace on ⟨B,e_A⟩ satisfying this Markov property, even if ⟨B,e_A⟩ is finite. We will give a necessary and sufficient condition for there to exist a Markov trace on ⟨B,e_A⟩ when B is finite-dimensional. The results in this talk are from Jones' seminal 1983 paper "Index for Subfactors."

Q-system realization and applications

Speaker: Quan Chen (OSU)

Date: Apr. 12, 2022

Abstract: Q-systems are unitary versions of Frobenius algebra objects which appeared in the theory of subfactors. I will discuss higher unitary idempotent completion for C*/W* 2-categories called Q-system completion and its inverse 2-functor called realization. We will explain several applications of Q-system realization, including the coend construction to describe the inductive limit II_1 factor of a Jones tower and induced actions of unitary tensor categories on C*-algebras. Time permitting, we will explain the relationship between the equivariant K-theory and the K-theory of the realization.

Graded extensions of generalized Haagerup categories

Speaker: Pinhas Grossman (UNSW)

Date: Mar. 29, 2022

Abstract: There is an obstruction theory for graded extensions of fusion categories due to Etingof, Nikshych, and Ostrik. However, computing the obstructions can be difficult in concrete examples. In this talk we will discuss a direct construction of graded extensions of generalized Haagerup categories using operator algebras, which leads to a number of new examples of fusion categories. This is joint work with Masaki Izumi and Noah Snyder.

Filtered Frobenius algebras in monoidal categories

Speaker: Harshit Yadav (Rice University)

Date: Mar. 22, 2022

Abstract: We develop filtered-graded techniques for algebras in monoidal categories with the goal of establishing a categorical version of Bongale's 1967 result: A filtered deformation of a Frobenius algebra over a field is Frobenius as well. Towards the goal, we construct a monoidal associated graded functor, building on prior works of Ardizzoni-Menini, of Galatius et al., and of Gwillian-Pavlov. We then produce equivalent conditions for an algebra in a rigid monoidal category to be Frobenius in terms of the existence of categorical Frobenius form. These two results of independent interest are used to achieve our goal. As an application of our main result, we show that any exact module category over a symmetric finite tensor category is represented by a Frobenius algebra in it. This is joint work with Dr. Chelsea Walton (Rice University).

Braided quantum symmetries of graph C*-algebras

Speaker: Sutanu Roy (National Institute of Science Education and Research (NISER) Bhubaneswar)

Date: Mar. 8, 2022

Abstract: In this talk, we will explain the existence of a universal braided compact quantum group acting on a graph C*-algebra in the twisted monoidal category of C*-algebras equipped with an action of the circle group. To achieve this we construct a braided version of the free unitary quantum group. Finally, we will compute this universal braided compact quantum group for the Cuntz algebra. This is a joint work in progress with Suvrajit Bhattacharjee and Soumalya Joardar.

From 3-manifolds to modular data

Speaker: Yang Qiu (UC Santa Barbara)

Date: Jan. 18, 2022

Abstract: The progress of TQFT has revealed connections between the algebraic world of tensor categories and the topological world of 3-manifolds, such as Reshetikhin-Turaev and Turaev-Viro theories. Motivated by M-theory in physics, Cho-Gang-Kim recently proposed another relation by outlining a program to construct modular data from certain classes of closed oriented 3-manifolds. In this talk, I will talk about our mathematical exploration of this program. This talk is based on the joint works: [Cui-Qiu-Wang, arXiv: 2101.01674], [Cui-Gustafson-Qiu-Zhang, arXiv: 2106.01959].

Microscopic definitions of anyon data

Speaker: Kyle Kawagoe (University of Chicago)

Date: Jan. 11, 2022

Abstract: The theory of anyons represents an enormously successful collaboration between the worlds of mathematics and physics. Although unified by topological quantum field theory, this topic is discussed very differently in these two fields. In math, we describe these theories by unitary modular braided fusion categories. In physics, these theories are described by Hilbert spaces with gapped Hamiltonians and ground states with long range entanglement. These differences raise a fundamental question: How can we understand the mathematical data in an anyon theory in a physical context? In this talk, we will give precise definitions of these data for any given microscopic model. These definitions are also operational in that they come with a method for calculating these data. We will also give a brief overview of how this formalism can be extended to other problems in the mathematical physics of topological phases of matter.

Fusion Categories over Non–Algebraically Closed Fields

Speaker: Sean Sanford (Indiana University)

Date: Dec. 7, 2021

Abstract: Much of the early work on Fusion Categories was inspired by physicists desire for rigorous foundations of topological quantum field theory. One effect of this was that base fields other than the complex numbers were rarely considered, if at all. The relevant features of $\mathbb{C}$ that make the theory work are the fact that it is characteristic zero, and algebraically closed.

This talk will focus on the interesting things that can be found when the algebraically closed requirement is removed. The content will start with lots of examples, and slowly accelerate into higher categorical implications.

Milnor's $\bar{\mu}$-invariants for knots in thickened surfaces, virtual links, and welded links.

Speaker: Micah Chrisman (OSU)

Date: Nov. 9, 2021

Abstract: Milnor's $\bar{\mu}$-invariants are concordance invariants of links in $S^3$. For knots in $S^3$, the invariants are always vanishing. Here we construct a non-trivial extension of Milnor's $\bar{\mu}$-invariants to knots (and links) in thickened surfaces $\Sigma \times [0,1]$, where $\Sigma$ is closed and oriented. The extended Milnor invariants are invariant under concordance and vanish on homologically trivial trivial knots in $\Sigma \times [0,1]$. They are always stronger as slice obstructions than the generalized Alexander polynomial. There are examples in which the extended Milnor invariants provide stronger slice obstructions than the graded genus, writhe polynomial, Rasmussen invariant, and parity projection. As an application, we show that the concordance group of long virtual knots is not abelian. This answers a question posed in 2008 by Turaev.

Spin model subfactors

Speaker: Michael Montgomery (Vanderbilt)

Date: Nov. 2, 2021

Abstract: Complex Hadamard matrices generate a class of irreducible hyperfinite subfactors with integer Jones index coming from spin model commuting squares. I will prove a theorem that establishes a criterion implying that these subfactors have infinite depth. I then show that Paley type II and Petrescu's continuous family of Hadamard matrices yield infinite depth subfactors. Furthermore, infinite depth subfactors are a generic feature of continuous families of complex Hadamard matrices. In this talk I will present an outline for the proof of these results.

Strong 1-boundedness for von Neumann algebras and Property T

Speaker: Srivatsav Kunnawalkam Elayavalli (Vanderbilt)

Date: Oct. 26, 2021

Abstract: Strong 1-boundedness is a notion introduced by Kenley Jung which captures (among Connes-embeddable von Neumann algebras) the property of having "a small amount of matrix approximations". Some examples are diffuse hyperfinite von Neumann algebras, vNa’s with Property Gamma, vNa’s that are non prime, vNa’s that have a diffuse hyperfinite regular subalgebra and so on. This is a von Neumann algebra invariant, and was used to prove in a unified approach remarkable rigidity theorems. One such is the following: A non trivial free product of von Neumann algebras can never be generated by two strongly 1-bounded von Neumann subalgebras with diffuse intersection. This result cannot be recovered by any other methods for even hyperfinite subalgebras. In this talk, I will present joint work with David Jekel and Ben Hayes, settling an open question from 2005, whether Property (T) II_1 factors are strongly 1-bounded, in the affirmative. Time permitting, I will discuss some insights and future directions.

Spectral bounds for chromatic number of quantum graphs

Speaker: Priyanga Ganesan (Texas A&M)

Date: Oct. 5, 2021

Abstract: Quantum graphs are an operator space generalization of classical graphs that have appeared in different branches of mathematics including operator systems theory, non-commutative topology and quantum information theory. In this talk, I will review the different perspectives to quantum graphs and introduce a chromatic number for quantum graphs using a non-local game with quantum inputs and classical outputs. I will then show that many spectral lower bounds for chromatic numbers in the classical case (such as Hoffman’s bound) also hold in the setting of quantum graphs. This is achieved using an algebraic formulation of quantum graph coloring and tools from linear algebra.

A categorical Connes' $\chi(M)$

Speaker: David Penneys (OSU)

Date: Sep. 21, 2021

Abstract: We consider the W* tensor category $\widetilde{\chi}(M)$ of approximately inner and centrally trivial bimodules over a $\rm II_1$ factor $M$, which generalizes the usual notions for automorphisms in Connes' definition of $\chi(M)$. We construct a unitary braiding on $\widetilde{\chi}(M)$ extending Jones' $\kappa$ invariant on $\chi(M)$, solving a problem posed by Popa from 1994. For a non-Gamma finite depth $\rm II_1$ subfactor $N\subset M$, we prove that the unitary braided tensor category $\widetilde{\chi}_{\operatorname{fus}}(M_\infty)\subset \tilde{\chi}(M_\infty)$ of finite depth objects is the Drinfeld center $Z(\mathcal{C})$, where $\mathcal{C}$ is the standard invariant of $N\subset M$, and $M_\infty$ is the inductive limit $\rm II_1$ factor of $N\subset M$. This is joint work with Corey Jones and Quan Chen.



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  • Tensor categories.
  • Subfactors and operator algebras.
  • Higher Categories.
  • Topological phases of matter.


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  • Sagittis adipiscing.
  • Felis enim feugiat.


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  3. Felis enim feugiat.
  4. Dolor pulvinar etiam.
  5. Etiam vel felis lorem.
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Name Description Price
Item One Ante turpis integer aliquet porttitor. 29.99
Item Two Vis ac commodo adipiscing arcu aliquet. 19.99
Item Three Morbi faucibus arcu accumsan lorem. 29.99
Item Four Vitae integer tempus condimentum. 19.99
Item Five Ante turpis integer aliquet porttitor. 29.99


Name Description Price
Item One Ante turpis integer aliquet porttitor. 29.99
Item Two Vis ac commodo adipiscing arcu aliquet. 19.99
Item Three Morbi faucibus arcu accumsan lorem. 29.99
Item Four Vitae integer tempus condimentum. 19.99
Item Five Ante turpis integer aliquet porttitor. 29.99


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