No Cloning of Orthogonal States in Composite Systems

The no-cloning principle tells us that nonorthogonal quantum states cannot be cloned, but it d not tell us that orthogonal states can always be cloned. We suggest a situation where the clo transformations are restricted, leading to a novel type of no-cloning principle. In the case of composite system made of two subsystems: if the subsystems are only available one after the o then there are various cases when orthogonal statescannot be cloned. Surprising examples are given, which give a radically better insight regarding the basic concepts of quantum cryptograp [S0031-9007(98)05708-1]