Free Stein kernels, free moment maps, and higher order derivatives

Abstract: In this work, we describe new constructions of free Stein kernels. Firstly, in dimension one, we propose a free analog to the construction of Stein kernels using moment maps as the one proposed by Fathi. This will be possible for a class of measures called the free moment measures via the notion of free moment map (convex functions), introduced in the free case by Bahr and Boschert. In a second time, we introduce the notion of higher-order free Stein kernels relative to a potential, which can be thought as the free counterpart of a recent and powerful idea introduced in the classical case by Fathi, and which generalize the notion of free Stein kernels by introducing higher-order derivatives of test functions (in our context noncommutative polynomials). We then focus our attention to the case of homothetic semicircular potentials. We prove as in the classical case, that their existence implies moments constraints. Finally, we relate these discrepancies to various metrics: the free (quadratic) Wasserstein distance, the relative free Fisher information along the Ornstein-Uhlenbeck flow or the relative non-microstates free entropy. Finally, as an important application, we provide new rates of convergences in the entropic free CLT under higher moments constraints.