Experimental entanglement distillation and 'hidden' non-locality

Entangled states are central to quantum information processing, including q uantum teleportation(1), efficient quantum computation(2) and quantum crypt ography(3). In general, these applications work best with pure, maximally e ntangled quantum states. However, owing to dissipation and decoherence, pra ctically available states are likely to be non-maximally entangled, partial ly mixed (that is, not pure), or both. To counter this problem, various sch emes of entanglement distillation, state purification and concentration hav e been proposed(4-11). Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using pa rtial polarizers, we perform a filtering process to maximize the entangleme nt of pure polarization-entangled photon pairs generated by spontaneous par ametric down-conversion(12,13).(.) We have also applied our methods to init ial states that are partially mixed. After filtering, the distilled states demonstrate certain non-local correlations, as evidenced by their violation of a form of Bell's inequality(14,15). Because the initial states do not h ave this property, they can be said to possess 'hidden' non-locality(6,16).