  We address the experimental determination of entanglement for systems made of
a pair of polarization qubits. We exploit quantum estimation theory to derive
optimal estimators, which are then implemented to achieve ultimate bound to
precision. In particular, we present a set of experiments aimed at measuring
the amount of entanglement for states belonging to different families of pure
and mixed two-qubit two-photon states. Our scheme is based on visibility
measurements of quantum correlations and achieves the ultimate precision
allowed by quantum mechanics in the limit of Poissonian distribution of
coincidence counts. Although optimal estimation of entanglement does not
require the full tomography of the states we have also performed state
reconstruction using two different sets of tomographic projectors and
explicitly shown that they provide a less precise determination of
entanglement. The use of optimal estimators also allows us to compare and
statistically assess the different noise models used to describe decoherence
effects occuring in the generation of entanglement.
