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An analysis of completely-positive trace-preserving maps on ${\cal M}_{2}$. (English) Zbl 1032.47046
Completely positive, trace-preserving linear maps (so-called stochastic maps) are well-known to play a fundamental role in quantum information theory. Therefore, it is an important problem to find useful, easily checkable characterizations of the complete positivity of linear transformations on the space $B(H)$ of all bounded linear operators acting on the complex Hilbert space $H$. In this paper, the authors present such a characterization in the case when $H$ is two-dimensional. Although this case may seem rather special, it is of considerable interest because of the role it plays in quantum computation and quantum communication. It is known that if a linear transformation $\Phi$ on the space $M_2$ of $2\times 2$ complex matrices is positivity-preserving and trace-preserving, then the $4\times 4$ matrix representation of $\Phi$ can be reduced, via a change of basis in $\bbfC^2$, to a rather special form: the entry in the left upper corner is 1 and it has nonzero entries only in the first column and in the diagonal. The main result of the paper gives a rather computable criterion for a linear transformation on $M_2$ with a matrix representation mentioned above to be completely positive. The criterion is the existence of a contractive solution of a matrix equation of the form $A=BXC$ where $A,B,C$ are given $2\times 2$ matrices with entries which are algebraic expressions of the entries of the mentioned matrix representation of $\Phi$. Next, the authors determine explicitly all extreme points of the set of all stochastic maps on $M_2$. This allows a detailed examination of an important class of non-unital extreme points that can be characterized as having exactly two images on the Bloch sphere. The authors also discuss a number of related issues about the images and the geometry of the set of stochastic maps, and show that any stochastic map on $M_2$ can be written as a convex combination of two “generalized” extreme points. Several interesting examples are presented.

47L07Convex sets and cones of operators
81P68Quantum computation
15A99Miscellaneous topics in linear algebra
46L30States of $C^*$-algebras
46L60Applications of selfadjoint operator algebras to physics
94A40Channel models (including quantum)
Full Text: DOI arXiv
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