Past exam of the mathematics course of the University of Cambridge / 2009 / iii / Paper 53 / 4 / Solution

The relevant security model gives Eve the purifying system and access to the public transcript. A useful preliminary fact is that purity decouples a subsystem from its purification. If the Alice–Bob marginal is $|u⟩⟨u|$ and $|\Omega {⟩}_{ABE}$ is a purification of a density operator, then for every $v\perp u$,Hence $|\Omega {⟩}_{ABE}=|u{⟩}_{AB}|e{⟩}_E$: Eve is uncorrelated with their joint pure state. A Bell pair then supplies a uniform shared secret bit when both parties measure in the computational basis. Pure product states have no shared random correlations, while a mixed state must be assessed with its purifying information included. The classifications below use the usual quantum key distribution setting with an authenticated public classical channel; no full security proof is required.