On one-parameter families of hermiticity-preserving superoperators which are not positive

Abstract

A one-parameter family of hermiticity-preserving superoperators is a time-dependent family $\{\Phi_{t}\colon\mathbb{M}_{n}(\mathbb{C})\rightarrow\mathbb{M}_{n}(\mathbb{C})\}_{t\in\mathbb{R}}$ of hermiticity-preserving superoperators determined, in a certain sense, by real and complex polynomial functions in the variable $t\in\mathbb{R}$. The paper studies sufficient computable criteria for nonpositivity of superoperators in one-parameter families. More precisely, we give sufficient conditions for the following assertions to hold: $(1)$ every $\Phi_{t}$ is not positive, $(2)$ $\Phi_{t}$ is not positive for $t$ in some open interval $(u,v)\subseteq\mathbb{R}$ and $(3)$ there is some $\Phi_{t}$ which is not positive. We show that in some situations $(3)$ implies $(2)$. Our approach to the problem is based on the Descartes rule of signs and the Sturm-Tarski theorem. In order to apply these facts, we introduce the sign variation formulas. These formulas are first order logical formulas in one free variable $t$, generalising sign sequences of polynomials used in Descartes rule of signs.