# David Reutter

Emmy Noether Junior Research Group Leader

Universität Hamburg, Fachbereich MathematikBundesstraße 55, 20146 Hamburgemail: [firstname.lastname]@uni-hamburg.deI am leading the independent Emmy Noether research group "Topological quantum field theory beyond three dimensions" at the Department of Mathematics of the University of Hamburg.

If you're interested in my research and want to join the group as a PhD or master student, do not hesitate to get in touch!

## Research interests

I am interested in applications of higher category theory to — and its interplay with — low-dimensional topology, homotopy theory, representation theory and mathematical physics.

My research interests include:

higher category theory and homotopy theory

topological quantum field theory

quantum algebra (fusion categories, subfactors, quantum groups, . . .) and its higher dimensional/categorical variants

## Biography

Previously, I was a postdoctoral fellow at the **Max Planck Institute for Mathematics** in Bonn. Before that, I was a DPhil student at the University of Oxford under the supervision of Jamie Vicary and the Strauch postdoctoral fellow at the **Mathematical Sciences Research Institute**** **(MSRI) in Berkeley for the spring 2020 programs Higher categories and categorification and Quantum symmetries. My DPhil thesis on `Higher linear algebra in topology and quantum information theory' can be found here.

A brief version of my CV can be found here.

## Publications

### 15) Semisimple field theories detect stable diffeomorphism

With Christopher Schommer-Pries

Extending the work of the first author, we introduce a notion of semisimple topological field theory in arbitrary even dimension and show that such field theories necessarily lead to stable diffeomorphism invariants. The main result of this paper is a proof that this 'upper bound' is optimal: To this end, we introduce and study a class of semisimple topological field theories, generalizing the well known finite gauge theories constructed by Dijkgraaf-Witten, Freed and Quinn. We show that manifolds satisfying a certain finiteness condition -- including 4-manifolds with finite fundamental group -- are indistinguishable to these field theories if and only if they are stably diffeomorphic. Hence, such generalized Dijkgraaf-Witten theories provide the strongest semisimple TFT invariants possible. These results hold for a large class of ambient tangential structures.

We discuss a number of applications, including the constructions of unoriented 4-dimensional semisimple field theories which can distinguish unoriented smooth structure and oriented higher-dimensional semisimple field theories which can distinguish certain exotic spheres.

Along the way, we show that dimensional reductions of generalized Dijkgraaf-Witten theories are again generalized Dijkgraaf-Witten theories, we utilize ambidexterity in the rational setting, and we develop techniques related to the ∞-categorical Möbius inversion principle of Gálvez-Carrillo--Kock--Tonks.

### 14) Computads for weak ω-categories as an inductive type

With Christopher Dean, Eric Finster, Ioannis Markakis and Jamie Vicary.

We give a new description of computads for weak globular ω-categories by giving an explicit inductive definition of the free words. This yields a new understanding of computads, and allows a new definition of ω-category that avoids the technology of globular operads. Our framework permits direct proofs of important results via structural induction, and we use this to give new proofs that every ω-category is equivalent to a free one, and that the category of computads with variable-to-variable maps is a presheaf topos, giving a direct description of the index category. We prove that our resulting definition of ω-category agrees with that of Batanin and Leinster and that the induced notion of cofibrant replacement for ω-categories coincides with that of Garner.

### 13) Zigzag normalisation for associative n-categories

With Lukas Heidemann and Jamie Vicary

The theory of associative n-categories has recently been proposed as a strictly associative and unital approach to higher category theory. As a foundation for a proof assistant, this is potentially attractive, since it has the potential to allow simple formal proofs of complex high-dimensional algebraic phenomena. However, the theory relies on an implicit term normalisation procedure to recognize correct composites, with no recursive method available for computing it.

Here we describe a new approach to term normalisation in associative n-categories, based on the categorical zigzag construction. This radically simplifies the theory, and yields a recursive algorithm for normalisation, which we prove is correct. Our use of categorical lifting properties allows us to give efficient proofs of our results. This normalisation algorithm forms a core component of the proof assistant homotopy.io, and we illustrate our scheme with worked examples.

**Proceedings of the 37th Annual ACM/IEEE Symposium on Logic in Computer Science(LICS 2022)**

### 12) Minimal nondegenerate extensions

With Theo Johnson-Freyd

We prove that every slightly degenerate braided fusion category admits a minimal nondegenerate extension. As a corollary, every pseudounitary super modular tensor category admits a minimal modular extension. This completes the program of characterizing minimal nondegenerate extensions of braided fusion categories.

Our proof relies on the new subject of fusion 2-categories. We study in detail the Drinfel'd centre Z(Mod-B) of the fusion 2-category Mod-B of module categories of a braided fusion 1-category B. We show that minimal nondegenerate extensions of B correspond to certain trivializations of Z(Mod-B). In the slightly degenerate case, such trivializations are obstructed by a class in H^5 (K (Z2 , 2); k^x) and we use a numerical invariant -- defined by evaluating a certain two-dimensional topological field theory on a Klein bottle -- to prove that this obstruction always vanishes.

Along the way, we develop techniques to explicitly compute in braided fusion 2-categories which we expect will be of independent interest. In addition to the known model of Z(Mod-B) in terms of braided B-module categories, we introduce a new computationally useful model in terms of certain algebra objects in B. We construct an S-matrix pairing for any braided fusion 2-category, and show that it is nondegenerate for Z(Mod-B). As a corollary, we identify components of Z(Mod-B) with blocks in the annular category of B and with the homomorphisms from the Grothendieck ring of the Müger centre of B to the ground field.

### 11) A 3-categorical perspective on G-crossed braided categories

With Corey Jones and David Penneys

A braided monoidal category may be considered a 3-category with one object and one 1-morphism. In this paper, we show that, more generally, 3-categories with one object and 1-morphisms given by elements of a group G correspond to G-crossed braided categories, certain mathematical structures which have emerged as important invariants of low-dimensional quantum field theories. More precisely, we show that the 4-category of 3-categories C equipped with a 3-functor BG→C which is essentially surjective on objects and 1-morphisms is equivalent to the 2-category of G-crossed braided categories. This provides a uniform approach to various constructions of G-crossed braided categories.

### 10) A type theory for strictly unital ∞-categories

With Eric Finster and Jamie Vicary

We present a type theory for strictly unital ∞-categories, in which a term computes to its strictly unital normal form. Using this as a toy model, we argue that it illustrates important unresolved questions in the foundations of type theory, which we explore. Furthermore, our type theory leads to a new definition of strictly unital ∞-category, which we claim is stronger than any previously described in the literature.

### 9) Semisimple 4-dimensional topological field theories cannot detect exotic smooth structure

We prove that semisimple 4-dimensional oriented topological field theories lead to stable diffeomorphism invariants and can therefore not distinguish homeomorphic closed oriented smooth 4-manifolds and homotopy equivalent simply connected closed oriented smooth 4-manifolds. We show that all currently known 4-dimensional field theories are semisimple, including unitary field theories, and once-extended field theories which assign algebras or linear categories to 2-manifolds. As an application, we compute the value of a semisimple field theory on a simply connected closed oriented 4-manifold in terms of its Euler characteristic and signature. Moreover, we show that a semisimple 4-dimensional field theory is invariant under ℂP^2-stable diffeomorphisms if and only if the Gluck twist acts trivially. This may be interpreted as the absence of fermions amongst the `point particles' of the field theory. Such fermion-free field theories cannot distinguish homotopy equivalent 4-manifolds. Throughout, we illustrate our results with the Crane-Yetter-Kauffman field theory associated to a ribbon fusion category. As an algebraic corollary of our results applied to this field theory, we show that a ribbon fusion category contains a fermionic object if and only if its Gauss sums vanish.

### 8) Uniqueness of unitary structure for unitarizable fusion categories

A unitary fusion category is a fusion category over the complex numbers with a compatible positive dagger structure. Such unitary structures naturally arise in many applications and constructions of fusion categories, most notably in the context of operator algebras, subfactor theory, and mathematical physics. Not every fusion category admits a unitary structure, and a fusion category could, in principle, have more than one unitary structure. Especially in light of the recent powerful operator algebraic classification techniques of unitary fusion categories, the question of uniqueness of unitary structure has become a significant open problem. In this paper, we prove that every unitarizable fusion category, and more generally every semisimple C*-tensor category, admits a unique unitary structure. Our proof is based on a categorified polar decomposition theorem for monoidal equivalences between such categories.

### 7) High-level methods for homotopy constructions in associative n-categories

With Jamie Vicary.

The theory of associative n-categories (ANC) is a model for semistrict higher categories, recently developed by **Christoph Dorn**, **Christopher Douglas**, and **Jamie Vicary**, and based on a combinatorial representation of higher string diagrams. In this work, we continue the development of ANCs and introduce the proof assistant **homotopy.io**. We develop a general contraction principle in ANCs which forms the mathematical heart of **homotopy.io**, allowing string diagram manipulations in arbitrary dimensions.

**Proceedings of the 34th Annual ACM/IEEE Symposium on Logic in Computer Science, 2019 (LICS 2019) **

### 6) Fusion 2-categories and a state-sum invariant for 4-manifolds

With Christopher Douglas.

One of the early successes of quantum topology was the construction of a three-dimensional topological field theory by Turaev-Viro, and its generalization by Barrett-Westbury, starting from the data of a spherical fusion category. In this work, we introduce spherical fusion 2-categories and construct a state-sum invariant of oriented singular piecewise-linear 4-manifolds, which categorifies the Turaev-Viro-Barrett-Westbury invariant. There are many examples of fusion 2-categories, including the 2-category of module categories of a braided fusion category, the 2-category of 2-representations of a finite 2-group and twisted linearizations of finite 2-groups.

### 5) The Morita theory of quantum graph isomorphisms

With Benjamin Musto and Dominic Verdon.

Using the machinery developed in `A compositional approach to quantum functions', we classify instances of quantum pseudo-telepathy in the graph isomorphism game by showing that graphs quantum isomorphic to a graph G correspond to certain Frobenius algebras in a monoidal category of quantum graph automorphisms of G. In certain cases, this classification can be expressed in group-theoretical terms.

**Communications in Mathematical Physics, 365(2):797-845, 2019**

### 4) A compositional approach to quantum functions

With Benjamin Musto and Dominic Verdon.

Pseudo-telepathy is a phenomenon in quantum information, where two non-communicating parties can use pre-shared entanglement to perform a task classically impossible without communication. In this work, we uncover a connection between pseudo-telepathy and quantum symmetry, expressed in the language of fusion categories. We use this connection to classify instances of pseudo-telepathy in the `graph isomorphism game'. In this case, the relevant categories are (co)representation categories of `quantum automorphisms groups' of graphs, studied in compact quantum group theory. Along the way, we develop a 2-categorical framework for finite quantum set theory.

### 3) Shaded tangles for the design and verification of quantum programs

With Jamie Vicary.

We give a tangle-based graphical language for the description of quantum circuits such that isotopic tangles yield equivalent programs. This leads to several new protocols and topological insight into known phenomena such as error correcting codes and local inversion of cluster states. We analyze 11 known quantum procedures in this way -- including entanglement manipulation, error correction and teleportation -- and in each case present a fully-topological formal verification, yielding generalized procedures in some cases.

**Proceedings of the Royal Society A, 475(2224):20180338, 2019**

### 2) A classical groupoid model for quantum networks

With Jamie Vicary.

We describe a certain combinatorial 2-category and use it to give a mathematical analysis of a new type of classical computer network architecture, intended as a model of a new technology that has recently been proposed in industry. Our approach is based on groubits, generalizations of classical bits based on groupoids. This network architecture allows the direct execution of a number of protocols that are usually associated with quantum networks, including teleportation, dense coding and secure key distribution.

### 1) Biunitary constructions in quantum information theory

With Jamie Vicary.

Biunitary connections are central tools in the study and classification of subfactors. At the same time, several important quantum informatic quantities such as Hadamard matrices, unitary error bases and quantum Latin squares can be expressed as special cases of biunitaries. This leads to a range of applications of biunitaries in various 2-categories to quantum and classical information theory. In this paper, we use the interpretation of Hadamard matrices, unitary error bases and quantum Latin squares as biunitaries to obtain many new construction methods for these quantities. An extended abstract can be found **here**.

## Talks

**Upcoming**

**September 27, 2022.**"Fusion 2-categories and their homotopy hypergroups" at the workshop "Higher categories and topological order" at the American Institute of Mathematics.

**Past**

**September 16, 2022.**"Semisimple field theories detect stable diffeomorphism" at the annual meeting of the German Mathematical Society in Berlin.**August 17, 2022.**"Higher S-matrices and a form of Poincaré duality for anomalous TQFTs" at the workshop "Higher structures and field theory" at the Erwin Schrödinger Institut, Wien. (slides.)**June****9****, 2022.**"Higher S-matrices and higher modular categories" at the workshop "Global categorical symmetries" at the Perimeter Institute, Waterloo. (slides, video)**March 24, 2022.**"Semisimple topological quantum field theories" at the workshop "Recent developments in representation theory and mathematical physics" at the Mathematical Research Institute Oberwolfach.**March 8, 2022.**"Fusion 2-categories, their Drinfeld centers, and the minimal modular extension conjecture" at the Wales MPPM seminar. (video. slides.)**December 1, 2021.**"Semisimple topological quantum field theories and exotic smooth structure" at the Oberseminar Algebra und Topologie, Goethe-Universität Frankfurt. (slides.)**November 30, 2021.**"Generalized Dijkgraaf-Witten theories and stable diffeomorphism" at the Mathematics of QFT seminar, Max Planck Institute Bonn. (slides.)**November 9, 2021.**"Semisimple field theories and stable diffeomorphisms" at the EPFL topology seminar. (slides.)**October 23-24, 2021.**"Higher fusion categories as quantum homotopy types" at the AMS western sectional meeting, special session on Tensor categories and applications.**July 5 - 7, 2021.**Workshop "Operads, Calculus, and Homotopy theory methods in Topology" at the University of Lille. (slides.)**June 24, 2021.**"Minimal nondegenerate extensions" at the**April 28, 2021. "**Semisimple topological quantum field theories and stable diffeomorphisms" at the Topology seminar, Purdue University.**April 23, 2021.**"Semisimple topological quantum field theories and stable diffeomorphisms" at the Algebra/Topology seminar, University of Copenhagen. (slides.)**April 12, 2021.**"On fusion 2-categories". Quantum Groups Online Seminar.**March 9, 2021.**"Intro to fusion 2-categories" at the AIM workshop Fusion categories and tensor networks. (video. slides.)**February 19, 2021.**Seminar talk "Semisimple topological quantum field theories in even dimensions" at the TQFT Club, University of Lisbon. (video. slides**.)****December 2020.**Seminar talk "From non-unital skein theory to modified traces and non-semisimple TQFTs" at the Hausdorff Institute for Mathematics, Bonn.**December 2020.**Seminar talk "Semisimple topological field theories and exotic smooth structure" at the IMJ-PRG topology seminar, Paris. (slides.)**September 2020.**Seminar talk "Semisimple field theories and exotic smooth structure" at the University of Notre Dame topology seminar.**August 2020.**Seminar talk "Semisimple field theories and exotic smooth structure" at the online seminar series University Quantum Symmetries Lectures.**August 2020.**Invited talk "On fusion 2-categories" at the workshop Topological Orders and Higher Structures at the ESI Vienna. A video of the talk can be found here.**June 2020.**Seminar talk "Semisimple topological quantum field theories and exotic smooth structure" at the Seminar on Algebra, Geometry and Physics, Max-Planck Institute.**May 2020.**Seminar talk "On fusion 2-categories" at MSRI. A video of the talk can be found here.**March 2020.**Invited talk "Semisimple 4-dimensional topological field theories cannot detect exotic smooth structure" at the MSRI workshop on Tensor categories and topological quantum field theories.**November 2019.**Seminar talk "From fusion 2-categories to 4-manifold invariants" at the Oberseminar Globale Analysis at the Universität Regensburg.**October 2019.**Invited talk "From quantum teleportation to biunitary connections (and back)" at the Oberwolfach workshop on Subfactors and Applications.**August 2019.**Invited talk "The Morita theory of quantum graph isomorphisms" at the workshop Focused research group on noncommutative mathematics and quantum information at the University of Bristol.**June 2019.**Contributed talk "High-level methods for homotopy construction in associative n-categories" at the conference Logic in Computer Science (LICS) in Vancouver.**June 2019.**Joint mini-course with Jamie Vicary on "Higher categories and quantum structures" at the Summer research program on quamtum symmetries at the Ohio State University.**October 2018.**Invited talk "From quantum automorphisms to pseudo-telepathy" at the workshop Cohomology of quantum groups and quantum automorphism groups of finite graphs, at the University of Saarbrücken.**September 2018.**Contributed talk "A compositional approach to quantum functions" at the First Symposium on Compositional Structures (SYCO 1) at the University of Birmingham.**May 2018.**Invited talk "Pseudo-telepathy via graphs and fusion categories" at the IOP's nonlinear and complex physics group's spring meeting on graph theory and physics at Imperial College, London.**April 2018.**Invited talk "Hopf algebras and 3-categories" at the University of Cambridge's junior geometry seminar.**March 2018.**Invited talk "Quantum functions and the Morita theory of quantum graph isomorphisms" at the workshop Combining viewpoints in quantum theory at the ICMS Edinburgh. A video of the talk can be found here.**March 2018.**Invited talk "Higher algebra in quantum information theory" at Stanford University's quantum information seminar.**August 2017.**Invited talk "Frobenius algebras, Hopf algebras and 3-categories" at the conference Hopf algebras in Kitaev's quantum double models at the Perimeter Institute, Canada. A video of my talk can be found here.**July 2017.**Contributed talks "Biunitary constructions in quantum information" and "Shaded tangles for the design and verification of quantum programs" at QPL 2017. Videos of my talks can be found here and here.**June 2017.**Contributed talks "A 2-categorical approach to composing quantum structures" and "A classical groupoid model for quantum networks" at CALCO 2017.**January 2017.**Contributed talk "Biunitary constructions in quantum information" at QIP 2017, the top international quantum information conference. Slides and video of the talk can be found here and here.