An additive and operational entanglement measure: conditional   entanglement of mutual information
  Based on the monogamy of entanglement, we develop the technique of quantum conditioning to build an {\it additive} entanglement measure: the conditional entanglement of mutual information. Its {\it operational} meaning is elaborated to be the minimal net "flow of qubits" in the process of partial state merging. The result and conclusion can also be generalized to multipartite entanglement cases. 
Dong Yang, Michal Horodecki, and Z. D. Wang