Separability criterion for bipartite states and its generalization to   multipartite systems
  A group of symmetric operators are introduced to carry out the separability criterion for bipartite and multipartite quantum states. Every symmetric operator, represented by a symmetric matrix with only two nonzero elements, and their arbitrary linear combinations are found to be entanglement witnesses. By using these symmetric operators, Wootters' separability criterion for two-qubit states can be generalized to bipartite and multipartite systems in arbitrary dimensions. 
Jie-Hui Huang, Li-Yun Hu, Lei Wang, and Shi-Yao Zhu
quant-ph