New considerations on the separability of very noisy mixed states and   implications for NMR quantum computing
  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. 
J.D. Bulnes (CBPF/Rio de Janeiro, Brazil), R.S. Sarthour (CBPF/Rio de   Janeiro, Brazil), E.R. de Azevedo (IFSC-USP/S\~ao Pa...