Universal and phase covariant superbroadcasting for mixed qubit states
  We describe a general framework to study covariant symmetric broadcasting maps for mixed qubit states. We explicitly derive the optimal N to M superbroadcasting maps, achieving optimal purification of the single-site output copy, in both the universal and the phase covariant cases. We also study the bipartite entanglement properties of the superbroadcast states. 
Francesco Buscemi, Giacomo Mauro D'Ariano, Chiara Macchiavello, and   Paolo Perinotti