 
Summary: Positivity for matrix systems: a case study
from quantum mechanics
Claudio Altafini
SISSAISAS, International School for Advanced Studies
via Beirut 24, 34014 Trieste, Italy
altafini@sissa.it
Abstract. We discuss an example from quantum physics of "positive system" in
which the state (a density operator) is a square matrix constrained to be positive
semidefinite (plus Hermitian and of unit trace). The positivity constraint is captured
by the notion of complete positivity of the corresponding flow. The infinitesimal
generators of all possible admissible ODEs can be characterized explicitly in terms
of cones of matrices. Correspondingly, it is possible to determine all linear time
varying systems and bilinear control systems that preserve positivity of the state
space.
1 Introduction
For a matrix system, i.e., a system of ODEs having as state space a set of
square matrices, the idea of state that can assume only positive values nat
urally generalizes to operator positivity i.e., to positive (semi)definiteness of
the square matrix representing the state variables. Such a concept is not new
in applied mathematics and for example it has long been used in model
