Schemes of transmission of classical information via quantum channels with many senders: Discrete- and continuous-variable cases

Superadditivity effects in the classical capacity of discrete multiaccess channels and continuous variable (CV) Gaussian MACs are analyzed. Several examples of the manifestation of superadditivity in the discrete case are provided, including, in particular, a channel which is fully symmetric with respect to all senders. Furthermore, we consider a class of channels for which input entanglement across more than two copies of the channels is necessary to saturate the asymptotic rate of transmission from one of the senders to the receiver. The five-input entanglement of Shor error correction codewords surpass the capacity attainable by using arbitrary two-input entanglement for these channels. In the CV case, we consider the properties of the two channels (a beam-splitter channel and a nondemolition quadratures coupling sum (XP) gate channel) analyzed in Czekaj et al. [Phys. Rev. A 82, 020302(R) (2010)] in greater detail and also consider the sensitivity of capacity superadditivity effects to thermal noise. We observe that the estimates of the amount of two-mode squeezing required to achieve capacity superadditivity are more optimistic than previously reported.