Experimental entanglement distillation and ‘hidden’ non-locality

Entangled states are central to quantum information processing, including quantum teleportation1, efficient quantum computation2 and quantum cryptography3. 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 proposed4,5,6,7,8,9,10,11. 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-conversion12,13. 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 inequality14,15. Because the initial states do not have this property, they can be said to possess ‘hidden’ non-locality6,16.