Vol 241, No 2 (2019)

Kraus representation of quantum information transfer channels is widely used in practice. We present examples of Kraus decompositions for channels that possess the covariance property with respect to the maximal commutative group of unitary operators. We show that in some problems (for example, the problem on the estimate of the minimal output entropy of the channel), the choice of a Kraus representation with nonminimal number of Kraus operators is relevant. We also present certain algebraic properties of noncommutative operator graphs generated by Kraus operators for the case of quantum channels that demonstrate the superactivation phenomenon.

We apply methods of the representation theory, combinatorial algebra, and noncommutative geometry to various problems of quantum tomography. We introduce the algebra of projectors that satisfy a certain commutation relation, examine this relation by combinatorial methods, and develop the representation theory of this algebra. We also present a geometrical interpretation of our problem and apply the results obtained to the description of the Petrescu family of mutually unbiased bases in dimension 7.

We propose a quantum branch-and-bound algorithm based on the general scheme of the branch-and-bound method and the quantum nested searching algorithm and examine its computational efficiency. We also compare this algorithm with a similar classical algorithm on the example of the travelling salesman problem. We show that in the vast majority of problems, the classical algorithm is quicker than the quantum algorithm due to greater adaptability. However, the operation time of the quantum algorithm is constant for all problem, whereas the classical algorithm runs very slowly for certain problems. In the worst case, the quantum branch-and-bound algorithm is proved to be several times more efficient than the classical algorithm.

We discuss the question of the general definition of the production of entropy per unit time for a quantum system governed by the Lindblad equation. The difficulty is as follows: in order to determine the total production of entropy, one must know the entropy flow from the system into the environment. This requires additional information on the environment and on its interaction with the system. The Lindblad equation for the reduced density matrix of the system does not contain such information. Therefore, the following question arises: What minimum additional information about the environment must be added to the Lindblad equation in order to find the flow of entropy into the environment and the total production of entropy? To answer this question, we use the concept of a complementary quantum channel known from the the quantum information theory. We also prove a theorem on the nonnegativity of production of entropy, and, under certain assumptions, the adiabatic and nonadiabatic contribution to it.

By using estimates for the variation of quantum mutual information and the relative entropy of entanglement, we obtain ε-exact lower estimates for distances from a given quantum channels to sets of degradable, antidegradable, and entanglement-breaking channels. As an auxiliary result, we obtain ε-exact lower estimates for the distance from a given two-particle state to the set of all separable states.