Operator entanglement of two-qubit joint unitary operations revisited:   Schmidt number approach
  Operator entanglement of two-qubit joint unitary operations is revisited. Schmidt number is an important attribute of a two-qubit unitary operation, and may have connection with the entanglement measure of the unitary operator. We found the entanglement measure of two-qubit unitary operators is classified by the Schmidt number of the unitary operators. The exact relation between the operator entanglement and the parameters of the unitary operator is clarified too. 
Hui-Zhi Xia, Chao Li, Qing Yang, Ming Yang, and Zhuo-Liang Cao
quant-ph