Experimental asymmetric phase-covariant quantum cloning of polarization   qubits
  We report on two optical realizations of the $1 \to 2$ asymmetric phase-covariant cloning machines for polarization states of single photons. The experimental setups combine two-photon interference and tunable polarization filtering that enables us to control the asymmetry of the cloners. The first scheme involves a special unbalanced bulk beam splitter exhibiting different splitting ratios for vertical and horizontal polarizations, respectively. The second implemented scheme consists of a balanced fiber coupler where photon bunching occurs, followed by a free-space part with polarization filters. With this later approach we were able to demonstrate very high cloning fidelities which are above the universal cloning limit. 
Jan Soubusta, Lucie Bartuskova, Antonin Cernoch, Miloslav Dusek,   Jaromir Fiurasek