Physics – Quantum Physics
Scientific paper
2004-04-03
Physics
Quantum Physics
12 pages
Scientific paper
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.
Bonagamba T. J.
Bonk F. A.
Bulnes J. D.
de Azevedo E. R.
Freitas J. C. C.
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