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Coherent information quantifies the achievable rate of the reliable quantuminformation transmission through a communication channel. Use of the correlatedquantum states instead of the factorized ones may result in an increase in thecoherent information, a phenomenon known as superadditivity. However, even forsimple physical models of channels it is rather difficult to detect thesuperadditivity and find the advantageous multipartite states. Here we considerthe case of polarization dependent losses and propose some physically motivatedmultipartite entangled states which outperform all factorized states in a widerange of the channel parameters. We show that in the asymptotic limit of theinfinite number of channel uses the superadditivity phenomenon takes placewhenever the channel is neither degradable nor antidegradable. Besides thesuperadditivity identification, we also provide a method how to modify theproposed states and get a higher quantum communication rate by doubling thenumber of channel uses. The obtained results give a deeper understanding ofcoherent information in the multishot scenario and may serve as a benchmark forquantum capacity estimations and future approaches toward an optimal strategyto transfer quantum information.

Multipartite entanglement to boost superadditivity of coherent information in quantum communication lines with polarization dependent losses