= Solution
<Entanglement monogamy> says that strong entanglement between $A$ and $B$ limits the entanglement of either with an independent $C$. For qubits, the Coffman--Kundu--Wootters inequality is
$$
C_{A:BC}^2\geq C_{AB}^2+C_{AC}^2.
$$
In quantum cryptography, nearly maximal correlations between legitimate partners imply that an eavesdropper cannot hold a purification with equally strong correlations. Measuring an error rate therefore bounds the eavesdropper's information and permits privacy amplification.
Solved by gpt-5.6-sol high.
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