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Any physical process is supposed to take physical states into physical states (Schrödinger picture). If density matrices are used to describe quantum states in quantum mechanics, then it must be some operation that sends density matrices to density matrices. So for finite-dimensional state spaces then an process it should be a linear map of vector spaces of matrices
(so far this is a general “superoperator”) that preserves the subset of density matrices, in that
it preserves the trace of matrices;
takes hermitian matrices with non-negative eigenvalues to hermitian matrices with non-negative eigenvalues.
Such a map is then called a quantum operation. The notion of a quantum operation is built from the Stinespring factorization theorem.
We first give the traditional definition in terms of linear algebra and matrices in
Then we consider the general abstract formulation
Let $k,n \in \mathbb{N}$.
A matrix $A \in Mat(n \times n, \mathbb{C})$ is called positive if it is hermitian – if $A^\dagger = A$ – and if all its eigenvalues (which then are necessarily real) are non-negative.
A linear map (morphism of vector spaces of matrices)
is called positive if it takes positive matrices to positive matrices.
The map $\Phi$ is called completely positive if for all $p \in \mathbb{N}$ the tensor product
is positive.
… due to (Selinger 05) … see for instance (Coecke-Heunen 11, section 2) for a quick summary
A map $\Phi$ as above is completely positive precisely if there exists a set $I$ and an $I$-family $\{E_i \in Mat(k \times n, \mathbb{C}| i \in I)\}$ of matrices, such that for all $A \in Mat(n \times n, \mathbb{C})$ we have
Moreover, such $\Phi$ preserves the trace of matrices precisely if
This is originally due to (Stinespring 55). The decomposition in the theorem is called Kraus decomposition after (Kraus 71). See also (Choi 76, theorem 1). A brief review is for instance in (Kuperberg 05, theorem 1.5.1). A general abstract proof in terms of †-categories is given in (Selinger 05). A characterization of completely positive maps entirely in terms of $\dagger$-categories is given in (Coecke 07).
The matrices $\{E_i\}$ that are associated to a completely positive and trace-preserving map by the above theorem are called Kraus operators.
In the physics literature the above theorem is then phrased as: Every quantum channel can be represented using Kraus operators .
Notice that the identity map is clearly completely positive and trace preserving, and that the composite of two maps that preserve positivity and trace clearly still preserves positivity and trace. Therefore we obtain a category $QChan \subset Vect$ – a subcategory of Vect${}_{\mathbb{C}}$ – whose
objects are the vector spaces $Mat(n \times n, \mathbb{C})$ for all $n \in \mathbb{N}$;
morphism are completely positive and trace-preserving linear maps $\Phi : Mat(n\times n , \mathbb{C}) \to Mat(m \times m, \mathbb{C})$;
composition of morphisms is, of course, the composition in Vect, i.e. the ordinary composition of linear maps.
See also extremal quantum channels and graphical quantum channels.
A quantum measurement is formally represented by a quantum operation that is induced by a positive-operator valued probability measure (POVM).
A very common example of this formalism comes from its use in open quantum systems, that is systems that are coupled to an environment. Let $\rho$ be the state of some quantum system and $\rho_{env}$ be the state of the environment. The action of a unitary transformation, $U$, on the system is
The Kraus-decomposition characterization of completely positive maps is due to
Reviews and surveys include
See also
Caleb J. O’Loan (2009), Topics in Estimation of Quantum Channels , PhD thesis, University of St. Andrews, (arXiv)
Christian B. Mendl, Michael M. Wolf, Unital Quantum Channels - Convex Structure and Revivals of Birkhoff’s Theorem , Commun. Math. Phys. 289, 1057-1096 (2009) (arXiv:0806.2820)
Smolin, John A., Verstraete, Frank, and Winter, Andreas Entanglement of assistance and multipartite state distillation , Phys. Rev. A, vol. 72, 052317, 2005 (arXiv:quant-ph/0505038)
John Watrous, Mixing doubly stochastic quantum channels with the completely depolarizing channel (2008) (arXiv)
The description of completely positive maps in terms of dagger-categories (see at finite quantum mechanics in terms of dagger-compact categories) goes back to
This is further explored in
Bob Coecke, Eric Paquette, Dusko Pavlovic, Classical and quantum structures (pdf)
Bob Coecke, Chris Heunen, Pictures of complete positivity in arbitrary dimension, EPTCS 95, 2012, pp. 27-35 (arXiv:1110.3055)