nLab dagger-compact category



Monoidal categories

monoidal categories

With symmetry

With duals for objects

With duals for morphisms

With traces

Closed structure

Special sorts of products



Internal monoids



In higher category theory



A \dagger-compact category is a category which is a

and a

in a compatible way. So, notably, it is a monoidal category in which

  • every object has a dual;

  • every morphism has an \dagger-adjoint.


A category CC that is equipped with the structure of a symmetric monoidal †-category and is compact closed is \dagger-compact if the dagger-operation takes units of dual objects to counits in that for every object AA of CC we have

AA * ϵ A I σ A×A * η A A *A. \array{ && A \otimes A^* \\ & {}^{\epsilon_A^\dagger}\nearrow \\ I && \downarrow^{\mathrlap{\sigma_{A \times A^*}}} \\ & {}_{\eta_A}\searrow \\ && A^* \otimes A } \,.



Relation to self-duality

If each object XX of a compact closed category is equipped with a self-duality structure XX *X \simeq X^\ast, then sending morphisms to their dual morphisms but with these identifications pre- and postcomposed

() :(XfY)(YY *f *X *X) (-)^\dagger \;\colon\; (X \stackrel{f}{\longrightarrow} Y) \mapsto (Y \stackrel{\simeq}{\to} Y^\ast \stackrel{f^\ast}{\longrightarrow} X^\ast \stackrel{\simeq}{\to} X)

constitutes a dagger-compact category structure.

See for instance (Selinger, remark 4.5).

Applied for instance to the category of finite-dimensional inner product spaces this dagger-operation sends matrices to their transpose?.


The concept was introduced in

with an expanded version in

under the name “strongly compact” and used for finite quantum mechanics in terms of dagger-compact categories. The topic was taken up

  • Peter Selinger, Dagger compact closed categories and completely positive maps, in Proceedings of the 3rd International Workshop on Quantum Programming Languages (QPL 2005), ENTCS 170 (2007), 139–163.

    (web, pdf)

where the alternative terminology “dagger-compact” was proposed, and used for the abstract characterization of quantum operations (completely positive maps on Bloch regions of density matrices).

The examples induced from self-duality-structure are discussed abstractly in

  • Peter Selinger, Autonomous categories in which AA *A \simeq A^\ast, talk at QPL 2010 (pdf)

That finite-dimensional Hilbert spaces are “complete for dagger-compactness” is shown in

Last revised on June 12, 2019 at 02:26:30. See the history of this page for a list of all contributions to it.