A Kernel Approach to the Stinespring and Kraus Representations

Abstract

We give a self-contained derivation of the Stinespring theorem for completely positive maps and the Kraus representation for normal completely positive maps using two scalar positive definite kernels. The first kernel realizes the minimal Stinespring space directly as a reproducing kernel Hilbert space, without passing through a quotient construction, while the second separates the multiplicity space and produces the Kraus operators as its coordinate operators. For holomorphic self-maps $f$ of the disk, this identifies the de Branges-Rovnyak space $\mathcal{H}\left(f\right)$ as the canonical Kraus multiplicity space of the associated map ${\Phi }_f$ on $B\left(H^2\right)$. Composition of symbols is reflected by canonical isometries between these multiplicity spaces, and the same kernel formulas describe the iterates of ${\Phi }_f$. We show in particular that every ${\Phi }_f$ is extreme and that, when the iterates of $f$ converge to a point of the disk, the preadjoint iterates converge in trace norm to the corresponding Szegő kernel state.