Quantum nonlocality obtained from local states by entanglement   purification
  We have applied an entanglement purification protocol to produce a single entangled pair of photons capable of violating a CHSH Bell inequality from two pairs that individually could not. The initial poorly-entangled photons were created by a controllable decoherence that introduced complex errors. All of the states were reconstructed using quantum state tomography which allowed for a quantitative description of the improvement of the state after purification. 
P. Walther, K.J. Resch, C. Brukner, A.M. Steinberg, J.-W. Pan, A.   Zeilinger