Equality cases in monotonicity of quasi-entropies, Lieb's concavity and Ando's convexity

Abstract

We revisit and improve joint concavity/convexity and monotonicity properties of quasi-entropies due to Petz in a new fashion. Then we characterize equality cases in the monotonicity inequalities (the data-processing inequalities) of quasi-entropies in several ways as follows: Let $\Phi:\mathcal{B}(\mathcal{H})\to\mathcal{B}(\mathcal{K})$ be a trace-preserving map such that $\Phi^*$ is a Schwarz map. When $f$ is an operator monotone or operator convex function on $[0,\infty)$, we present several equivalent conditions for the equality $S_f^K(\Phi(\rho)\|\Phi(\sigma))=S_f^{\Phi^*(K)}(\rho\|\sigma)$ to hold for given positive operators $\rho,\sigma$ on $\mathcal{H}$ and $K\in\mathcal{B}(\mathcal{K})$. The conditions include equality cases in the monotonicity versions of Lieb's concavity and Ando's convexity theorems. Specializing the map $\Phi$ we have equivalent conditions for equality cases in Lieb's concavity and Ando's convexity. Similar equality conditions are discussed also for monotone metrics and $\chi^2$-divergences. We further consider some types of linear preserver problems for those quantum information quantities.