The quantum strategy (or quantum combs) framework is a useful tool for reasoning about interactions among entities that process and exchange quantum information over the course of multiple turns. We prove a time-reversal property for a class of linear functions, defined on quantum strategy representations within this framework, that corresponds to the set of rank-one positive semidefinite operators on a certain space. This time-reversal property states that the maximum value obtained by such a function over all valid quantum strategies is also obtained when the direction of time for the function is reversed, despite the fact that the strategies themselves are generally not time reversible. An application of this fact is an alternative proof of a known relationship between the conditional minand max-entropy of bipartite quantum states, along with generalizations of this relationship.
The quantum strategy frameworkThe quantum strategy framework [9], which is also known as the quantum combs framework [2, 4], provides a useful framework for reasoning about networks of quantum channels. It may be used to model scenarios in which two or more entities, which we will call players, process and exchange quantum information over the course of multiple rounds of communication; and it is particularly useful when one wishes to consider an optimization over all possible behaviors of one player, for any given specification of the other player or players. Various developments, applications, and variants of the quantum strategy framework can be found in [1,3,5,8,10], for instance, and in a number of other sources.In the discussion of the quantum strategy framework that follows, as well as in the subsequent sections of this paper, we assume that the reader is familiar with quantum information theory and semidefinite programming. References on this material include [11,13,15,16] as well as [14], which we follow closely with respect to notation and terminology. In particular,