Hermitian Tensor Product Approximation of Complex Matrices and   Separability
  The approximation of matrices to the sum of tensor products of Hermitian matrices is studied. A minimum decomposition of matrices on tensor space $H_1\otimes H_2$ in terms of the sum of tensor products of Hermitian matrices on $H_1$ and $H_2$ is presented. From this construction the separability of quantum states is discussed. 
Shao-Ming Fei, Naihuan Jing, Bao-Zhi Sun