Fig. 2: Correlations between different parties in broadcasting protocols.
a Correlations after signal broadcasting. b Alice1 and Bob1 do not trust other users and decorrelate their joint data from everyone else, resulting in the unique decoupled identical secret key at Alice\({\prime}\) and Bob\({\prime}\). In the untrusted broadcasting protocol, all users perform only this step. c Alice2 and Bob2 now are decorrelated from Alice\({\prime}\) and Bob\({\prime}\), and hence, consider them trusted. d AliceN and BobN trust previous pairs to decorrelate their data from them, thus can pay the lowest privacy amplification cost
