Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 343 / 2 / a

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 isIn 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.