Optimal estimation of two-qubit pure-state entanglement - art. no. 062307

We present optimal measuring strategies for an estimation of the entangleme nt of unknown two-qubit pure states and of the degree of mixing of unknown single-qubit mixed states, of which N identical copies are available. The m ost general measuring strategies are considered in both situations, to conc lude in the first case that a local, although collective, measurement suffi ces to estimate entanglement, a nonlocal property, optimally.