Detecting the concurrence of an unknown state with a single observable
  While the detection of entanglement has been proved already to be quite a difficult task, experimental quantification of entanglement is even more challenging. In this work, we derive an analytical lower bound for the concurrence of a bipartite mix quantum state in arbitrary dimension. The lower bound is experimentally implementable in a feasible way, which enabling quantification of entanglement in a broad variety of cases 
Zhi-Hao Ma, Zhi-Hua Chen, Jing-Ling Chen
quant-ph