Dense coding with multipartite quantum states
  We consider generalisations of the dense coding protocol with an arbitrary number of senders and either one or two receivers, sharing a multiparty quantum state, and using a noiseless channel. For the case of a single receiver, the capacity of such information transfer is found exactly. It is shown that the capacity is not enhanced by allowing the senders to perform joint operations. We provide a nontrivial upper bound on the capacity in the case of two receivers. We also give a classification of the set of all multiparty states in terms of their usefulness for dense coding. We provide examples for each of these classes, and discuss some of their properties. 
Dagmar Bruss, Giacomo Mauro D'Ariano, Maciej Lewenstein, Chiara   Macchiavello, Aditi Sen De, Ujjwal Sen