Kubo-Ando Means and Rigidity of Quantum Positivity Cones
Abstract
We investigate the stability of quantum positivity cones under nonlinear operator means. Specifically, we examine how Kubo--Ando means interact with the separable, positive partial transpose (PPT), and Schmidt-number cones. By analyzing the curvature of operator monotone functions at the identity, we give a strict rigidity phenomenon: weighted arithmetic means are the only Kubo--Ando means that preserve the separable cone in all dimensions. We show that the strictly positive curvature of any non-arithmetic mean explicitly forces a violation of the PPT condition, even in the foundational two-qubit setting, and can strictly increase the Schmidt number of the resulting operator. Finally, using the Choi--Jamio{\l}kowski correspondence, we translate these geometric obstructions to the map-theoretic setting, concluding that convex mixing is the uniquely permissible Kubo--Ando operation for preserving entanglement-breaking quantum channels.
Keywords
Cite
@article{arxiv.2605.26272,
title = {Kubo-Ando Means and Rigidity of Quantum Positivity Cones},
author = {Mohsen Kian},
journal= {arXiv preprint arXiv:2605.26272},
year = {2026}
}