Intrinsic Non-Locality and Device-Independent Conference Key Agreement.

In this work, we introduce the multipartite intrinsic non-locality as a resource quantifier for the multipartite scenario of device-independent (DI) conference key agreement. We prove that this quantity is additive, convex, and monotone under a class of free operations called local operations and common randomness. As one of our technical contributions, we establish a chain rule for multipartite mutual information, which we then use to prove that the multipartite intrinsic non-locality is additive. This chain rule may be of independent interest in other contexts. All of these properties of multipartite intrinsic non-locality are helpful in establishing the main result of our paper: multipartite intrinsic non-locality is an upper bound on secret key rate in the general multipartite scenario of DI conference key agreement. We discuss various examples of DI conference key protocols and compare our upper bounds for these protocols with known lower bounds. Finally, we calculate upper bounds on recent experimental realizations of DI quantum key distribution.