Phase-covariant quantum cloning - art. no. 012302

We consider an N-->M quantum cloning transformation acting on pure two-leve l states lying on the equator of the Bloch sphere. An upper bound for its f idelity is presented, by establishing a connection between optimal phase-co variant cloning and phase estimation. We give the explicit form of a clonin g transformation that achieves the bound for the case N = 1, M = 2, and fmd a link between this case and optimal eavesdropping in the quantum cryptogr aphic scheme BB84.