  It is known that the additivity conjecture of Holevo capacity, output minimum
entoropy, and the entanglement of formation (EoF), are equivalent with each
other. Among them, the output minimum entropy is simplest, and hence many
researchers are focusing on this quantity.
  Here, we suggest yet another entanglement measure, whose strong
superadditivity and additivity are equivalent to the additivity of the
quantities mentioned above. This quantity is as simple as the output minimum
entropy, and in existing proofs of additivity conjecture of the output minimum
entropy for the specific examples, they are essentially proving the strong
superadditivity of this quantity.
