# nLab quantum information theory via dagger-compact categories

Contents

category theory

## Applications

#### Monoidal categories

monoidal categories

## In higher category theory

under construction

# Contents

## Idea

Central aspects of quantum mechanics with finite-dimensional spaces of quantum states — such as tensor products of qbit states of relevance in quantum information theory and quantum computation — follow from the formal properties of the category FinHilb of finite-dimensional Hilbert spaces. These properties are axiomatized by saying that FinHilb is an example of a †-compact category.

Conversely, much of finite-dimensional quantum mechanics and quantum computation can be formulated in any †-compact category, and general reasoning about †-compact categories themselves yields results about quantum mechanics and quantum computation.

A transparent string diagram calculus in †-compact categories as exposed in (Coecke, Kindergarten quantum mechanics) provides an intuitive and powerful tool for reasoning in $\dagger$-compact categories.

## Quantum mechanical concepts in $\dagger$-compact categories

Let $(C,\otimes,I, \dagger)$ be a †-compact category.

We list various concepts in quantum mechanics and their corresponding incarnation in terms of structures in $C$.

### Classical measurement outcomes

An observable in quantum mechanics formulated on a Hilbert space is modeled by a self-adjoint operator, and the classical measurement outcomes of this operator provide, at least under some assumptions, an orthogonal basis on the Hilbert space.

That, more abstractly, the notion of orthogonal basis of an object can be phrased intrinsically inside any suitable $\dagger$-compact category is the point made in (CoeckePavlovicVicary).

### Complex phases

The underlying “algebra of quantum amplitudes” of the corresponding quantum mechanical system is the endomorphism monoid of the tensor unit

$\mathbb{C}_C = End_C(I) \,.$

In (Vicary) it is shown that in $\dagger$-compact categories with all finite limits over certain “tree-like” diagrams compatible with the $\dagger$-structure, this $\mathbb{C}_C$ has the properties that

• it is a field of characteristic 0 with involution $\dagger$;

• the subfield $\mathbb{R}_C$ fixed under $\dagger$ is orderable.

If furthermore every bounded sequence of measurements in $C$ with values in $\mathbb{R}_C$ has a least upper bound, then it follows that this field coincides with the complex numbers

$\mathbb{C}_C = \mathbb{C}$

and moreover

$\mathbb{R}_C = \mathbb{R} \,.$

### Completely positive maps

The behaviour of quantum channels and completely positive maps has an elegant categorical description in terms of $\dagger$-compact categories. See (Selinger and Coecke).

## Quantum logic

Symmetric monoidal categories such as †-compact categories have as internal logic a fragment of linear logic and as type theory a flavor of linear type theory. In this fashion everything that can be formally said about quantum mechanics in terms of †-compact categories has an equivalent expression in formal logic/type theory. It has been argued (Abramsky-Duncan 05, Duncan 06) that this linear logic/linear type theory of quantum mechanics is the correct formalization of “quantum logic”. An exposition of this point of view is in (Baez-Stay 09).

## References

### Quantum information theory via String diagrams

The observation that a natural language for quantum information theory and quantum computation, specifically for quantum circuit diagrams, is that of string diagrams in †-compact categories (see quantum information theory via dagger-compact categories):

On the relation to quantum logic/linear logic:

Early exposition with introduction to monoidal category theory:

• Bob Coecke, Kindergarten quantum mechanics $[$arXiv:quant-ph/0510032$]$

• Bob Coecke, Introducing categories to the practicing physicist $[$arXiv:0808.1032$]$

• John Baez, Mike Stay, Physics, topology, logic and computation: a rosetta stone in: New Structures for Physics, Bob Coecke (ed.), Lecture Notes in Physics 813, Springer (2011) 95-174 $[$arxiv/0903.0340$]$

• Bob Coecke, Eric Oliver Paquette, Categories for the practising physicist $[$arXiv:0905.3010$]$

• Bob Coecke, Quantum Picturalism $[$arXiv:0908.1787$]$

With further emphasis on quantum computation:

• Jamie Vicary, The Topology of Quantum Algorithms, (LICS 2013) Proceedings of 28th Annual ACM/IEEE Symposium on Logic in Computer Science (2013) 93-102 $[$arXiv:1209.3917, doi:10.1109/LICS.2013.14$]$

Generalization to quantum operations on mixed states (completely positive maps of density matrices):

Formalization of quantum measurement via Frobenius algebra-structures:

Textbook accounts (with background on relevant monoidal category theory):

Refinement to the ZX-calculus for specific control of quantum circuit-diagrams:

Relating the ZX-calculus to braided fusion categories for anyon braiding:

The role of complex numbers in general $\dagger$-compact categories:

Last revised on November 24, 2022 at 16:48:15. See the history of this page for a list of all contributions to it.