Sufficient conditions for the Kadison--Schwarz property of unital positive maps on $M_3$

Abstract

Kadison--Schwarz (KS) maps form a natural class of positive linear maps lying strictly between positivity and complete positivity. Despite their relevance in operator algebras and quantum dynamics, explicit analytic sufficient conditions for the KS property remain scarce beyond low-dimensional or highly symmetric settings. In this work we analyze unital positive linear maps on $M_3$ within the Bloch--Gell--Mann representation and derive explicit analytic sufficient conditions ensuring the Kadison--Schwarz property. The approach exploits unitary equivalence together with structural properties of the Lie algebra $\mathfrak{su}(3)$ and does not rely on numerical optimization or semidefinite-programming methods. A key mechanism is the cancellation of contributions associated with antisymmetric structure constants, which reduces the problem to estimates governed solely by the symmetric tensor $d_{ijk}$. The results clarify how the Kadison--Schwarz property can hold under assumptions substantially weaker than complete positivity and yield a structural criterion for the KS property on $M_3$ in terms of Bloch parameters.

1. The title has been reformulated. 3. The proof of the main theorem has been expanded and clarified. 4. Addition of an illustrative two-parameter diagonal family $T(t,s)$, highlighting the relation between positivity, the Kadison--Schwarz property, and complete positivity. 5. Reorganization and structural clarification of the Appendix