Continuous optimal ensembles I: A geometrical characterization of robustly separable quantum states
arXiv:quant-ph/0503173
Abstract
A geometrical characterization of robustly separable (that is, remaining separable under sufficiently small variiations) mixed states of a bipartite quantum system is given. It is shown that the density matrix of any such state can be represented as a normal vector to a hypersurface in the Euclidean space of all self-adjoint operators in the state space of the whole system. The expression for this hypersurface is provided.