Transportation cost and contraction coefficient for channels on von Neumann algebras

TL;DR

This paper develops a noncommutative optimal transport framework for quantum channels on von Neumann algebras, introducing Lipschitz cost and contraction measures to analyze quantum state transformations and their properties.

Contribution

It introduces the Lipschitz cost measure and contraction coefficient for quantum channels, establishing their properties and applications in quantum information theory.

Findings

Lipschitz cost measure and contraction coefficient are well-defined and continuous.

These measures can recover classical distances like group word length and Carnot-Carathéodory distance.

A contraction coefficient less than one implies entropy contraction and mixing time bounds.

Abstract

We present a noncommutative optimal transport framework for quantum channels acting on von Neumann algebras. Our central object is the Lipschitz cost measure, a transportation-inspired quantity that evaluates the minimal cost required to move between quantum states via a given channel. Accompanying this is the Lipschitz contraction coefficient, which captures how much the channel contracts the Wasserstein-type distance between states. We establish foundational properties of these quantities, including continuity, dual formulations, and behavior under composition and tensorization. Applications include recovery of several mathematical quantities including expected group word length and Carnot-Carath\'eodory distance, via transportation cost. Moreover, we show that if the Lipschitz contraction coefficient is strictly less than one, one can get entropy contraction and mixing time estimates for certain classes of non-symmetric channels.