1. Physics Division, P-23, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
2. Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland
3. Present addresses: University of Illinois, Urbana-Champaign, Department of Physics, 1110 West Green Street, Urbana, Illinois 61801-3080, USA (P.G.K.); Escuela Superior de Física y Matemáticas, IPN, Mexico D.F., 07738, Mexico (S.B.-L.).

Correspondence to: Paul G. Kwiat^1,2 Correspondence and requests for materials should be addressed to P.G.K. (e-mail: Email: Kwiat@uiuc.edu).

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 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}.