Quantum no-stretching: A geometrical interpretation of the no-cloning theorem

We consider the ideal situation in which a space rotation is transferred from a quantum spin j   to a quantum spin l≠jl≠j. Quantum-information theoretical considerations lead to the conclusion that such operation is possible only for l⩽jl⩽j. For l>jl>j the optimal stretching transformation is derived. We show that for qubits the present no-stretching theorem is equivalent to the usual no-cloning theorem.