  It is shown that any quantum operation that perfectly clones the entanglement
of all maximally-entangled qubit pairs cannot preserve separability. This
``entanglement no-cloning'' principle naturally suggests that some approximate
cloning of entanglement is nevertheless allowed by quantum mechanics. We
investigate a separability-preserving optimal cloning machine that duplicates
all maximally-entangled states of two qubits, resulting in 0.285 bits of
entanglement per clone, while a local cloning machine only yields 0.060 bits of
entanglement per clone.
