Fixed-ray escort representations of sandwiched and $α$--$z$ Rényi divergences on von Neumann algebras

Abstract

We represent sandwiched and $α$-$z$ Rényi divergences as averages of ordinary relative entropy. The $α$-$z$ Rényi divergence is shown to be an integral over the relative entropy of a canonical family of fixed-ray escort states along the ray $z=cα$. We prove this representation for normal states on an arbitrary von Neumann algebra, using Haagerup non-commutative $L^p$ spaces and interpolation. The formula holds for every $z>0$: for $0<α<1$ it holds when the support of the first state is contained in that of the reference state, and for $α>1$ it holds whenever the divergence is finite. When the lower-order support condition fails, we identify the exact fixed-ray support-boundary term. The representation yields a monotone escort profile and a convex order potential. We use these to reformulate one-shot testing converses, exact sandwiched strong-converse exponents, and work-extraction reliability as signed-area or level-crossing statements, and discuss a restricted two-parameter pair-conversion rate.