$\alpha$-$z$-R\'enyi divergences in von Neumann algebras: data-processing inequality, reversibility, and monotonicity properties in $\alpha,z$

Abstract

We study the $\alpha$-$z$-R\'enyi divergences $D_{\alpha,z}(\psi\|\varphi)$ where $\alpha,z>0$ ($\alpha\ne1$) for normal positive functionals $\psi,\varphi$ on general von Neumann algebras, introduced in [S.~Kato and Y.~Ueda, arXiv:2307.01790] and [S.~Kato, arXiv:2311.01748]. We prove the variational expressions and the data processing inequality (DPI) for the $\alpha$-$z$-R\'enyi divergences. We establish the sufficiency theorem for $D_{\alpha,z}(\psi\|\varphi)$, saying that for $(\alpha,z)$ inside the DPI bounds, the equality $D_{\alpha,z}(\psi\circ\gamma\|\varphi\circ\gamma)=D_{\alpha,z}(\psi\|\varphi)<\infty$ in the DPI under a quantum channel (or a normal $2$-positive unital map) $\gamma$ implies the reversibility of $\gamma$ with respect to $\psi,\varphi$. Moreover, we show the monotonicity properties of $D_{\alpha,z}(\psi\|\varphi)$ in the parameters $\alpha,z$ and their limits to the normalized relative entropy as $\alpha\nearrow1$ and $\alpha\searrow1$.