We revise the problem first addressed by Braunstein and co-workers (Phys. Rev. Lett. { \bf 83} (5) (1999) 1054) concerning the separability of very noisy mixed states represented by general density matrices with the form $\rho_\epsilon = (1-\epsilon)M_d+\epsilon\rho_1$. From a detailed numerical analysis, it is shown that: (1) there exist infinite values in the interval taken for the density matrix expansion coefficients, $-1\le c_{ \alpha_1,...,\alpha_N}\le 1$, which give rise to { \em non-physical density matrices}, with trace equal to 1, but at least one { \em negative} eigenvalue; (2) there exist entangled matrices outside the predicted entanglement region, and (3) there exist separable matrices inside the same region. It is also shown that the lower and upper bounds of $\epsilon$ depend on the coefficients of the expansion of $\rho_1$ in the Pauli basis. If $\rho_{ 1}$ is hermitian with trace equal to 1, but is allowed to have { \em negative} eigenvalues, it is shown that $\rho_\epsilon$ can be entangled, even for two qubits.
Categories: quant-ph