Extension Theory and the Classification of Fermionic Strongly Fusion 2-Categories (with an Appendix by Thibault Didier D'ecoppet and Theo Johnson-Freyd)

Kernkonzepte

This paper presents a homotopy theoretic classification of fermionic strongly fusion 2-categories using the concept of group graded extensions.

Zusammenfassung

• Bibliographic Information: D´ecoppet, T.D. (2024). Extension Theory and Fermionic Strongly Fusion 2-Categories (with an Appendix by Thibault Didier D'ecoppet and Theo Johnson-Freyd). Symmetry, Integrability and Geometry: Methods and Applications, 20(2024), 092, 20 pages. https://doi.org/10.3842/SIGMA.2024.092

• Research Objective: This paper aims to classify fermionic strongly fusion 2-categories, motivated by their role in the broader classification of topological orders in (3+1) dimensions.

• Methodology: The paper utilizes the framework of group graded extension theory for fusion 2-categories. It leverages the existence of the relative 2-Deligne tensor product and analyzes the Brauer–Picard space associated with the fusion 2-category 2SVect.

• Key Findings:

  • The paper establishes that faithfully G-graded extensions of a fusion 2-category C are classified by homotopy classes of maps from BG to BBrPic(C), the delooping of the Brauer–Picard space of C.
  • It demonstrates that fermionic strongly fusion 2-categories equipped with a faithful grading are classified by a finite group G, a class ϖ in H2(BG; Z/2), and a class π in SH4+ϖ(BG), where SH denotes twisted supercohomology.

• Main Conclusions: The paper provides a comprehensive classification of fermionic strongly fusion 2-categories, building upon previous work on bosonic strongly fusion 2-categories and invertible fusion 2-categories.

• Significance: This research contributes significantly to the understanding of fusion 2-categories, particularly in the context of topological quantum field theories and the classification of topological orders.

• Limitations and Future Research: The paper primarily focuses on fermionic strongly fusion 2-categories with a faithful grading. Further research could explore the classification of such categories without this restriction or investigate the connection with Tambara–Yamagami 2-categories in the context of extension theory.

Statistiken

Zitate

Wichtige Erkenntnisse aus

Extension Theory and Fermionic Strongly Fusion 2-Categories (with an Appendix by Thibault Didier D\'ecoppet and Theo Johnson-Freyd)

by Thib... um arxiv.org 10-18-2024

https://arxiv.org/pdf/2403.03211.pdf

Tiefere Fragen

How does the classification of fermionic strongly fusion 2-categories extend to the more general setting of arbitrary fusion 2-categories?

While the provided text focuses on classifying strongly fusion 2-categories (those with either Vect or SVect as their braided fusion 1-category of endomorphisms of the unit object), extending this to arbitrary fusion 2-categories is a significant leap in complexity. Here's a breakdown of the challenges and potential approaches:
Challenges:

Richer Structure: Arbitrary fusion 2-categories can have much more complicated braided fusion 1-categories of endomorphisms of the unit object (ΩC). This means the Brauer-Picard space, which classifies extensions, becomes significantly harder to compute.
Non-invertible Simples: Unlike strongly fusion 2-categories, general fusion 2-categories can have non-invertible simple objects. This means they are not simply group graded extensions of a base 2-category like 2Vect or 2SVect.
Lack of General Tools: The classification of fermionic strongly fusion 2-categories heavily relies on the specific structure of 2SVect and tools from homotopy theory. These tools might not directly generalize to arbitrary fusion 2-categories.
Potential Approaches:

Morita Equivalence: As mentioned in the text, every fusion 2-category is Morita equivalent to a 2-Deligne tensor product of a strongly fusion 2-category and an invertible fusion 2-category. One could try to leverage this decomposition. Understanding the interplay between these components under extensions could be a path forward.
Categorical Group Cohomology: The classification of fermionic strongly fusion 2-categories uses group cohomology implicitly through homotopy theory. Developing a more explicit theory of "categorical group cohomology" for higher categories might provide a framework for studying extensions of general fusion 2-categories.
Representation Theory: Fusion 2-categories are intimately connected to the representation theory of certain algebraic structures. Exploring these connections might offer insights into the structure of general fusion 2-categories and their extensions.
In summary, classifying arbitrary fusion 2-categories is a challenging open problem. It requires developing new tools and insights beyond those used for the strongly fusion case. The Morita equivalence decomposition and the potential for a categorical group cohomology theory offer promising avenues for future research.

Could there be alternative approaches to classifying fermionic strongly fusion 2-categories that do not rely on homotopy theory?

While the provided text utilizes homotopy theory extensively, exploring alternative approaches to classifying fermionic strongly fusion 2-categories is an interesting prospect. Here are some possibilities:
Direct Categorical Constructions:

Generators and Relations: One could attempt to classify fermionic strongly fusion 2-categories by explicitly constructing them from generators and relations. This would involve identifying a minimal set of generating objects and morphisms, along with relations they satisfy. This approach might be more algebraic and less topological.
Universal Properties: Characterizing fermionic strongly fusion 2-categories through universal properties could provide a classification without relying on homotopy theory. This would involve identifying specific categorical constructions for which these 2-categories are universal solutions.
Connections to Other Structures:

Conformal Field Theory: Fermionic structures naturally arise in conformal field theory (CFT). Investigating the relationship between fermionic strongly fusion 2-categories and certain classes of CFTs might lead to a classification based on CFT data.
Higher Gauge Theory: Fusion 2-categories are related to higher gauge theory, a generalization of gauge theory involving higher-dimensional objects. Exploring this connection, particularly in the context of fermionic structures, could offer a different perspective on classification.
Challenges of Alternative Approaches:

Complexity: Even with alternative approaches, the inherent complexity of fermionic strongly fusion 2-categories might make a complete classification quite challenging.
Lack of Existing Framework: Developing these alternative approaches would require building new frameworks and tools, which could be a significant undertaking.
In conclusion, while homotopy theory provides a powerful framework for classifying fermionic strongly fusion 2-categories, exploring alternative approaches based on direct categorical constructions or connections to other areas like CFT or higher gauge theory could be fruitful. However, these alternative approaches also come with their own set of challenges and might require significant effort to develop.

What are the implications of this classification for the development of topological quantum computing platforms?

The classification of fermionic strongly fusion 2-categories, particularly those with non-trivial twists, has significant implications for topological quantum computing:
New Types of Topological Order:

Beyond Abelian Anyons: Fermionic strongly fusion 2-categories can describe topological orders beyond those characterized by abelian anyons, which are the focus of many existing topological quantum computing proposals. This opens up the possibility of richer and potentially more powerful quantum computing platforms.
Non-trivial Exchange Statistics: The presence of fermions introduces non-trivial exchange statistics, going beyond the anyonic statistics found in purely bosonic systems. This could lead to new braiding-based quantum gates and computational resources.
Fault-Tolerance and Error Correction:

Symmetry-Protected Topological Order: The twist in the classification corresponds to a symmetry-protected topological (SPT) order. SPT orders are believed to have enhanced protection against certain types of errors, which is crucial for building robust quantum computers.
New Error Correction Codes: Understanding the structure of fermionic strongly fusion 2-categories could lead to the development of novel error correction codes tailored to these exotic topological orders.
Material Realization and Experimental Platforms:

Guidance for Material Search: The classification provides a theoretical framework for identifying and characterizing materials that could potentially host these exotic fermionic topological orders.
Design of Experimental Setups: A deeper understanding of the properties of these 2-categories can guide the design of experiments to detect and manipulate the associated anyonic excitations.
Challenges and Future Directions:

Physical Realization: While the classification provides a theoretical foundation, realizing these exotic topological orders in physical systems remains a significant experimental challenge.
Control and Manipulation: Developing techniques to control and manipulate the anyonic excitations in these systems is crucial for building functional quantum computers.
In summary, the classification of fermionic strongly fusion 2-categories offers a rich landscape of possibilities for topological quantum computing. It paves the way for exploring new types of topological order, potentially leading to more robust and powerful quantum computing platforms. However, significant experimental and theoretical challenges remain in realizing and harnessing the potential of these exotic topological phases.