Separable approximations of density matrices of composite quantum systems

We investigate optimal separable approximations (decompositions) of states rho of bipartite quantum systems A and B of arbitrary dimensions M x N following the lines of Lewenstein and Sanpera. Such approximations allow to represent in an optimal way any density operator as a sum of a separable state and an entangled state of a certain form. For two-qubit systems (M = N = 2) the best separable approximation has the form of a mixture of a separable state and a projector onto a pure entangled state. We formulate a necessary condition that the pure state in the best separable approximation is not maximally entangled. We demonstrate that the weight of the entangled state in the best separable approximation in arbitrary dimensions provides a good entanglement measure. We prove for arbitrary M and N that the best separable approximation corresponds to a mixture of separable and entangled states, both of which are unique. We develop also a theory of optimal separable approximations for states with positive partial transpose (PPT states). Such approximations allow to decompose any density operator with positive partial transpose as a sum of a separable state and an entangled PPT state. We discuss procedures for constructing such decompositions.