Experimental Measurement of Lower and Upper Bounds of Concurrence for   Mixed Quantum States
  We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method to provide two faithful copies of a two-photon mixed state required for parity measurements: Two photon pairs generated by two neighboring pump laser pulses through optical parametric down conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multi-photon mixed states. 
Xiao-Ling Niu, Yan-Xiao Gong, Jian Li, Liang Peng, Cheng-Jie Zhang,   Yong-Sheng Zhang, Yun-Feng Huang, and Guang-Can Guo