Universal and phase covariant superbroadcasting for mixed qubit states

Abstract

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.

19 pages, 8 figures, strictly related to quant-ph/0506251 and quant-ph/0510155