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Abstract

We discuss the case where a set of states {|psi_i>} cannot be cloned by itself, but is clonable with the assistance of another system prepared in a state rho_i. When S is pair-wise nonorthogonal, Jozsa has shown that one can generate the copy |psi_i> from rho_i alone, with no interaction with the original system. |Here we show that a set containing orthogonal pairs exhibits a property forming a striking contrast; For any such set, there is a choice of rho_i that enables cloning only when the two systems are interacted in a purely quantum way that is not achievable by classical communication.