Estimating the output entropy of a tensor product of two quantum channels

For a class of bipartite quantum states, we find a nontrivial lower bound on the entropy gain resulting from the action of a tensor product of the identity channel with an arbitrary channel. We use the obtained result to bound the output entropy of the tensor product of a dephasing channel with an arbitrary channel from below. We characterize phase-damping channels that are particular cases of dephasing channels.