Optimal quantum cloning via stimulated emission

We show that optimal universal quantum cloning can be realized via stimulated emission. Universality of the cloning procedure is achieved by chousing systems that have appropriate symmetries. We first discuss a scheme based on stimulated emission in certain three-level systems, e.g., atoms in a cavity. Then we present a way of realizing optimal universal cloning based on stimulated parametric down-conversion. This scheme also implements the optimal universal NOT operation.