  We show that optimal universal quantum cloning can be realized via stimulated
emission. Universality of the cloning procedure is achieved by choosing 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.
