Optimal cloning of coherent states with a linear amplifier and beam   splitters
  A transformation achieving the optimal symmetric N-to-M cloning of coherent states is presented. Its implementation only requires a phase-insensitive linear amplifier and a network of beam splitters. An experimental demonstration of this continuous-variable cloner should therefore be in the scope of current technology. The link between optimal quantum cloning and optimal amplification of quantum states is also pointed out. 
Samuel L. Braunstein, Nicolas J. Cerf, Sofyan Iblisdir, Peter van   Loock, and Serge Massar