Experimental entanglement distillation and ‘hidden’ non-locality

Entangled states are central to quantum information processing, including quantum teleportation 1 , efficient quantum computation 2 and quantum cryptography 3 . In general, these applications work best with pure, maximally entangled quantum states. However, owing to dissipation and decoherence, practically available states are likely to be non-maximally entangled, partially mixed (that is, not pure), or both. To counter this problem, various schemes of entanglement distillation, state purification and concentration have been proposed 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarization-entangled photon pairs generated by spontaneous parametric down-conversion 12 , 13 . We have also applied our methods to initial 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 have this property, they can be said to possess ‘hidden’ non-locality 6 , 16 .