Photon polarization entanglement induced by biexciton: experimental   evidence for violation of Bell's inequality
  We have investigated the polarization entanglement between photon pairs generated from a biexciton in a CuCl single crystal via resonant hyper parametric scattering. The pulses of a high repetition pump are seen to provide improved statistical accuracy and the ability to test Bell's inequality. Our results clearly violate the inequality and thus manifest the quantum entanglement and nonlocality of the photon pairs. We also analyzed the quantum state of our photon pairs using quantum state tomography. 
Goro Oohata, Ryosuke Shimizu, Keiichi Edamatsu