Higher-Dimensional Algebra II: 2-Hilbert Spaces

TL;DR

This paper introduces 2-Hilbert spaces as categories with Hilbert space-like structures, explores their monoidal variants, and establishes their relation to representations of supergroupoids, extending classical harmonic analysis to higher categories.

Contribution

It defines 2-Hilbert spaces and their monoidal forms, proves a categorified Doplicher-Roberts theorem, and constructs a higher Fourier transform, advancing higher category theory and quantum algebra.

Findings

Every symmetric 2-H*-algebra is equivalent to Rep(G) for some compact supergroupoid G.

Constructed a categorified Fourier transform for compact abelian groups.

Rep(U(n)) is the free connected symmetric 2-H*-algebra on one n-dimensional object.

Abstract

A 2-Hilbert space is a category with structures and properties analogous to those of a Hilbert space. More precisely, we define a 2-Hilbert space to be an abelian category enriched over Hilb with a *-structure, conjugate-linear on the hom-sets, satisfying <fg,h> = <g,f*h> = <f,hg*>. We also define monoidal, braided monoidal, and symmetric monoidal versions of 2-Hilbert spaces, which we call 2-H*-algebras, braided 2-H*-algebras, and symmetric 2-H*-algebras, and we describe the relation between these and tangles in 2, 3, and 4 dimensions, respectively. We prove a generalized Doplicher-Roberts theorem stating that every symmetric 2-H*-algebra is equivalent to the category Rep(G) of continuous unitary finite-dimensional representations of some compact supergroupoid G. The equivalence is given by a categorified version of the Gelfand transform; we also construct a categorified version of the Fourier transform when G is a compact abelian group. Finally, we characterize Rep(G) by its universal properties when G is a compact classical group. For example, Rep(U(n)) is the free connected symmetric 2-H*-algebra on one even object of dimension n.