Approximate quantum cloning and the impossibility of superluminal   information transfer
  We show that nonlocality of quantum mechanics cannot lead to superluminal transmission of information, even if most general local operations are allowed, as long as they are linear and trace preserving. In particular, any quantum mechanical approximate cloning transformation does not allow signalling. On the other hand, the no-signalling constraint on its own is not sufficient to prevent a transformation from surpassing the known cloning bounds. We illustrate these concepts on the basis of some examples. 
D. Bruss, G. M. D'Ariano, C. Macchiavello, M. F. Sacchi