Role of interference in quantum cloning
Résumé
We study the role of interference in the process of quantum cloning. We show that in order to achieve better than classical cloning of a qubit no interference is needed. In particular, a large class of symmetric universal 1$\to$ 2 qubit cloners exists which achieve the optimal average fidelity 5/6 for such machines, without using any interference. We also obtain optimal average fidelities for interference--free cloning in asymmetric situations, and discuss the relation of the quantum cloners found to the Bužek--Hillery quantum cloner.