  Coherent information quantifies the achievable rate of the reliable quantum
information transmission through a communication channel. Use of the correlated
quantum states instead of the factorized ones may result in an increase in the
coherent information, a phenomenon known as superadditivity. However, even for
simple physical models of channels it is rather difficult to detect the
superadditivity and find the advantageous multipartite states. Here we consider
the case of polarization dependent losses and propose some physically motivated
multipartite entangled states which outperform all factorized states in a wide
range of the channel parameters. We show that in the asymptotic limit of the
infinite number of channel uses the superadditivity phenomenon takes place
whenever the channel is neither degradable nor antidegradable. Besides the
superadditivity identification, we also provide a method how to modify the
proposed states and get a higher quantum communication rate by doubling the
number of channel uses. The obtained results give a deeper understanding of
coherent information in the multishot scenario and may serve as a benchmark for
quantum capacity estimations and future approaches toward an optimal strategy
to transfer quantum information.
