Entanglement is the central resource of quantum information processing and the precise characterization of entangled states is a crucial issue for the development of quantum technologies. This leads to the necessity of a precise, experimental feasible measure of entanglement. Nevertheless, such measurements are limited both from experimental uncertainties and intrinsic quantum bounds. Here we present an experiment where the amount of entanglement of a family of two-qubit mixed photon states is estimated with the ultimate precision allowed by quantum mechanics.

 

"Experimental Estimation of Entanglement at the Quantum Limit"
Giorgio Brida, Ivo Pietro Degiovanni, Angela Florio, Marco Genovese, Paolo Giorda, Alice Meda, Matteo G. A. Paris and Alexander Shurupov
Phys. Rev. Lett. 104, 100501 (2010)