Dagger compact category explained

In category theory, a branch of mathematics, dagger compact categories (or dagger compact closed categories) first appeared in 1989 in the work of Sergio Doplicher and John E. Roberts on the reconstruction of compact topological groups from their category of finite-dimensional continuous unitary representations (that is, Tannakian categories).[1] They also appeared in the work of John Baez and James Dolan as an instance of semistrict k-tuply monoidal n-categories, which describe general topological quantum field theories,[2] for n = 1 and k = 3. They are a fundamental structure in Samson Abramsky and Bob Coecke's categorical quantum mechanics.[3] [4] [5]

Overview

Dagger compact categories can be used to express and verify some fundamental quantum information protocols, namely: teleportation, logic gate teleportation and entanglement swapping, and standard notions such as unitarity, inner-product, trace, Choi–Jamiolkowsky duality, complete positivity, Bell states and many other notions are captured by the language of dagger compact categories.[3] All this follows from the completeness theorem, below. Categorical quantum mechanics takes dagger compact categories as a background structure relative to which other quantum mechanical notions like quantum observables and complementarity thereof can be abstractly defined. This forms the basis for a high-level approach to quantum information processing.

Formal definition

C

which is also compact closed, together with a relation to tie together the dagger structure to the compact structure. Specifically, the dagger is used to connect the unit to the counit, so that, for all

A

in

C

, the following diagram commutes:

To summarize all of these points:

CC\toC

that is associative, natural and has left and right identities obeying certain coherence conditions.

\sigmaA,:AB\simeqBA

that is natural in both A and B, and, again, obeys certain coherence conditions (see symmetric monoidal category for details).

A\inC

has a dual object

A*

. Categories with dual objects are equipped with two morphisms, the unit

ηA:I\toA*A

and the counit

\varepsilonA:AA*\toI

, which satisfy certain coherence or yanking conditions.

\dagger\colonCopC

that is the identity on objects, but maps morphisms to their adjoints.

A dagger compact category is then a category that is each of the above, and, in addition, has a condition to relate the dagger structure to the compact structure. This is done by relating the unit to the counit via the dagger:

\sigma
A,A*
\dagger
\circ\varepsilon
A

=ηA

shown in the commuting diagram above. In the category FdHilb of finite-dimensional Hilbert spaces, this last condition can be understood as defining the dagger (the Hermitian conjugate) as the transpose of the complex conjugate.

Examples

The following categories are dagger compact.

Infinite-dimensional Hilbert spaces are not dagger compact, and are described by dagger symmetric monoidal categories.

Structural theorems

Selinger showed that dagger compact categories admit a Joyal-Street style diagrammatic language[7] and proved that dagger compact categories are complete with respect to finite dimensional Hilbert spaces[8] [9] i.e. an equational statement in the language of dagger compact categories holds if and only if it can be derived in the concrete category of finite dimensional Hilbert spaces and linear maps. There is no analogous completeness for Rel or nCob.

This completeness result implies that various theorems from Hilbert spaces extend to this category. For example, the no-cloning theorem implies that there is no universal cloning morphism.[10] Completeness also implies far more mundane features as well: dagger compact categories can be given a basis in the same way that a Hilbert space can have a basis. Operators can be decomposed in the basis; operators can have eigenvectors, etc.. This is reviewed in the next section.

Basis

(A,\delta,\varepsilon)

. The two operations are a copying or comultiplication δ: AAA morphism that is cocommutative and coassociative, and a deleting operation or counit morphism ε: AI . Together, these obey five axioms:

Comultiplicativity:

(1A\varepsilon)\circ\delta=1A=(\varepsilon1A)\circ\delta

Coassociativity:

(1A\delta)\circ\delta=(\delta1A)\circ\delta

Cocommutativity:

\sigmaA,A\circ\delta=\delta

Isometry:

\delta\dagger\circ\delta=1A

Frobenius law

(\delta\dagger1A)\circ(1A\delta)=\delta\circ\delta\dagger

To see that these relations define a basis of a vector space in the traditional sense, write the comultiplication and counit using bra–ket notation, and understanding that these are now linear operators acting on vectors

|j\rangle

in a Hilbert space H:

\begin{align} \delta:H&\toHH\\ |j\rangle&\mapsto|j\rangle|j\rangle=|jj\rangle\\ \end{align}

and

\begin{align} \varepsilon:H&\toC\\ |j\rangle&\mapsto1\\ \end{align}

The only vectors

|j\rangle

that can satisfy the above five axioms must be orthogonal to one-another; the counit then uniquely specifies the basis. The suggestive names copying and deleting for the comultiplication and counit operators come from the idea that the no-cloning theorem and no-deleting theorem state that the only vectors that it is possible to copy or delete are orthogonal basis vectors.

General results

Given the above definition of a basis, a number of results for Hilbert spaces can be stated for compact dagger categories. We list some of these below, taken from[11] unless otherwise noted.

(A,\delta,\varepsilon)

.

(A,\delta,\varepsilon)

is any object

\psi

for which

\delta\circ\psi=\psi\psi

Eigenstates are orthogonal to one another.

\psi

is complementary to the observable

(A,\delta,\varepsilon)

if

\delta\dagger\circ(\overline\psi\psi)=\varepsilon\dagger

(In quantum mechanics, a state vector

\psi

is said to be complementary to an observable if any measurement result is equiprobable. viz. an spin eigenstate of Sx is equiprobable when measured in the basis Sz, or momentum eigenstates are equiprobable when measured in the position basis.)

(A,\deltaX,\varepsilonX)

and

(A,\deltaZ,\varepsilonZ)

are complementary if
\dagger
\delta
Z

\circ\deltaX=\varepsilonZ\circ

\dagger
\varepsilon
X

\delta\dagger\circ(\psi1A)

is unitary if and only if

\psi

is complementary to the observable

(A,\delta,\varepsilon)

Notes and References

  1. S. . Doplicher . J. . Roberts . A new duality theory for compact groups . Invent. Math. . 98 . 157–218 . 1989 . 10.1007/BF01388849 . 1989InMat..98..157D . 120280418 .
  2. J.C.. Baez . J.. Dolan . Higher-dimensional Algebra and Topological Quantum Field Theory . J. Math. Phys.. 36 . 11. 6073–6105. 1995. 10.1063/1.531236 . 10.1.1.269.4681 . q-alg/9503002 . 1995JMP....36.6073B. 14908618 .
  3. Book: Samson Abramsky . S.. Abramsky . Bob Coecke . B. . Coecke . A categorical semantics of quantum protocols. Proceedings of the 19th IEEE conference on Logic in Computer Science (LiCS'04) . IEEE. 2004 . 0-7695-2192-4 . 415–425 . 10.1109/LICS.2004.1319636. quant-ph/0402130 . 10.1.1.330.7289. 1980118.
  4. Book: S.. Abramsky . B. . Coecke. Categorical quantum mechanics . . K.. Engesser. D.M.. Gabbay . D. . Lehmann . Handbook of Quantum Logic and Quantum Structures. Elsevier . 2009 . 978-0-08-093166-1. 261–323. 0808.1023.
  5. Abramsky and Coecke used the term strongly compact closed categories, since a dagger compact category is a compact closed category augmented with a covariant involutive monoidal endofunctor.
  6. M.. Atiyah . Topological quantum field theories. Inst. Hautes Études Sci. Publ. Math. . 68 . 175–186 . 1989 . 10.1007/BF02698547 . 121647908 .
  7. Peter . Selinger . Dagger compact closed categories and completely positive maps: (Extended Abstract) . Electronic Notes in Theoretical Computer Science . 170 . Proceedings of the 3rd International Workshop on Quantum Programming Languages (QPL 2005) . 139–163 . 2007 . 10.1016/j.entcs.2006.12.018 . 10.1.1.84.8476 .
  8. P. . Selinger . Finite dimensional Hilbert spaces are complete for dagger compact closed categories . Electronic Notes in Theoretical Computer Science . 270 . Proceedings of the Joint 5th International Workshop on Quantum Physics and Logic and 4th Workshop on Developments in Computational Models (QPL/DCM 2008) . 113–9 . 2011 . 10.1016/j.entcs.2011.01.010 . 1207.6972 . 10.1.1.749.4436 .
  9. Book: Hasegawa . M. . Hofmann . M. . Plotkin . G. . Finite Dimensional Vector Spaces Are Complete for Traced Symmetric Monoidal Categories . Avron . A. . Dershowitz . N. . Rabinovich . A. . Pillars of Computer Science . Springer . Lecture Notes in Computer Science . 4800 . 2008 . 978-3-540-78127-1 . 367–385 . 10.1007/978-3-540-78127-1_20 . 10.1.1.443.3495. 15045491 .
  10. Book: Abramsky, S. . No-Cloning in categorical quantum mechanics . . I. . Mackie . S. . Gay . Semantic Techniques for Quantum Computation . Cambridge University Press . 2010 . 978-0-521-51374-6 . 1–28 .
  11. Bob . Coecke . Quantum Picturalism . Contemporary Physics . 51 . 59–83 . 2009 . 10.1080/00107510903257624 . 0908.1787. 752173 .