Bipartite entanglement is sufficient for standard device-independent conference key agreement

Abstract: Conference key agreement (CKA) aims to establish shared, private randomness among many separated parties in a network. Device-independent (DI) CKA is a variant in which no assumptions are placed on the nature of the source, or the measurements performed by each party. So far, DICKA protocols largely fall into two categories: those that rely on violating a joint Bell inequality using genuinely multi-partite entangled states, and those that concatenate many bipartite protocols. The question of whether a hybrid protocol exists, where a multi-partite Bell inequality can be violated using only bipartite entanglement, was asked by Grasselli et al. in [Quantum 7, 980, (2023)]. We answer this question affirmatively, by constructing an asymptotically secure DICKA protocol achieving the same rate as the concatenation of bipartite DIQKD, yet relying on a single joint Bell violation. Our results prompt further discussion on the benefits of multi-partite entanglement for DICKA over its bipartite alternative, and we give an overview of different arguments for near-term devices.