  A quantum system consisting of two subsystems is separable if its density
matrix can be written as $\rho=\sum_A w_A\,\rho_A'\otimes\rho_A''$, where
$\rho_A'$ and $\rho_A''$ are density matrices for the two subsytems. In this
Letter, it is shown that a necessary condition for separability is that a
matrix, obtained by partial transposition of $\rho$, has only non-negative
eigenvalues. This criterion is stronger than Bell's inequality.
