Separable approximation for mixed states of composite quantum systems - art. no. 052302

We describe a purely algebraic method for finding the best separable approx imation to a mixed state of a composite 2 x 2 quantum system, consisting of a decomposition of the state into a linear combination of a mixed separabl e part and a pure entangled one. We prove that, in a generic case, the weig ht of the pure part in the decomposition equals the concurrence of the stat e.