Separable approximation for mixed states of composite quantum systems
Thomas Wellens, Marek Kuś · Physical Review A · 2001
We describe a purely algebraic method for finding the best separable approximation to a mixed state of a composite $2\ifmmode\times\else\texttimes\fi{}2$ quantum system, consisting of a decomposition of the state into a linear combination of a mixed separable part and a pure entangled one. We prove that, in a generic case, the weight of the pure part in the decomposition equals the concurrence of the state.