Qubit- and entanglement-assisted optimal entanglement concentration

We present two methods for optimal entanglement concentration from pure entangled states by local actions only. However, a prior knowledge of the Schmidt coefficients is required. The first method is optimally efficient only when a finite ensemble of pure entangled states are available, whereas the second method realizes the single pair optimal concentration probability. We also propose an entanglement-assisted method that is again optimally efficient even for a single pair. We also discuss concentrating entanglement from N-partite catlike states.