New considerations on the separability of very noisy mixed states and implications for NMR quantum computing
Abstract
We revise the problem first addressed by Braunstein and coworkers (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 nonphysical 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.
 Publication:

arXiv eprints
 Pub Date:
 April 2004
 arXiv:
 arXiv:quantph/0404020
 Bibcode:
 2004quant.ph..4020B
 Keywords:

 Quantum Physics
 EPrint:
 12 pages