$C^{\ast }$-extreme contractive completely positive maps

Abstract

In this paper we generalize a specific quantized convexity structure of the generalized state space of a $C^{\ast }$-algebra and examine the associated extreme points. We introduce the notion of $P$-$C^{\ast }$-convex subsets, where $P$ is any positive operator on a Hilbert space $\mathcal{H}$. These subsets are defined with in the set of all completely positive (CP) maps from a unital $C^{\ast }$-algebra $\mathcal{A}$ into the algebra $B(\mathcal{H})$ of bounded linear maps on $\mathcal{H}$. In particular, we focus on certain $P$-$C^{\ast }$-convex sets, denoted by ${\mathrm{CP}}^{(P)}(\mathcal{A},B(\mathcal{H}))$, and analyze their extreme points with respect to this new convexity structure. This generalizes the existing notions of $C^{\ast }$-convex subsets and $C^{\ast }$-extreme points of unital completely positive maps. We significantly extend many of the known results regarding the $C^{\ast }$-extreme points of unital completely positive maps into the context of $P$-$C^{\ast }$-convex sets we are considering. This includes abstract characterization and structure of $P$-$C^{\ast }$-extreme points. Further, using these studies, we completely characterize the $C^{\ast }$-extreme points of the $C^{\ast }$-convex set of all contractive completely positive maps from $\mathcal{A}$ into $B(\mathcal{H})$, where $\mathcal{H}$ is finite-dimensional. Additionally, we discuss the connection between $P$-$C^{\ast }$-extreme points and linear extreme points of these convex sets, as well as Krein-Milman type theorems.

To appear in Journal of Mathematical Analysis and Applications with the title "Generalized $C^{\ast }$-convexity in Completely Positive Maps". Removed the closed range assumption in Lemma 4.10 and subsequent results are improved accordingly