Entanglement witness measurement for two-qubit states by optical interference Zhi-Wei Wang, Jian Li, Yun-Feng Huang, Yong-Sheng Zhang, Xi-Feng Ren, Pei Zhang and Guang-Can Guo EPL, 82 6 (2008) 60003 Published online: 30 May 2008 DOI: 10.1209/0295-5075/82/60003