Experimental preparation of Werner state via spontaneous parametric   down-conversion
  We present an experiment of preparing Werner state via spontaneous parametric down-conversion and controlled decoherence of photons in this paper. In this experiment two independent BBO (beta-barium borate) crystals are used to produce down-conversion light beams, which are mixed to prepare Werner state. 
Yong-Sheng Zhang, Yun-Feng Huang, Chuan-Feng Li, and Guang-Can Guo