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

摘要

We consider the ideal situation in which a space rotation is transferred from a quantum spin j to a quantum spin l not equal j. Quantum-information theoretical considerations lead to the conclusion that such operation is possible only for l <= j. For l > 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.