Optimal photon cloning - art. no. 032302

We show that optimal universal cloning of the polarization state of photons can be achieved via stimulated emission in three-level systems, both of th e Lambda and the V type. We establish the equivalence of our systems with c oupled harmonic oscillators, which permits us to analyze the structure of t he cloning transformations realized. These transformations are shown to be equivalent to the optimal cloning transformations for qubits discovered by Buzek and Hillery and Gisin and Massar. The down-conversion cloner discover ed previously by some of the authors is obtained as a limiting case. We dem onstrate an interesting equivalence between systems of Lambda atoms and sys tems of pairwise entangled V atoms. Finally we discuss the physical differe nces between our photon cloners and the qubit cloners considered previously and prove that the bounds on the fidelity of the clones derived for qubits also apply in our situation.