Robust Anonymous Conference Key Agreement enhanced by Multipartite Entanglement.

Users of quantum networks can securely communicate via so-called quantum conference key agreement -- making their identities publicly known. In certain cases, however, users may seek anonymity. Here, exploiting multipartite Greenberger-Horne-Zeilinger (GHZ) states, we design efficient and noise-tolerant protocols for anonymous conference key agreement. Inspired by the composable security of quantum key distribution, we introduce security definitions encompassing various levels of anonymity and prove the security of our protocols. We analyze the performance of our protocols in noisy and lossy quantum networks and compare with protocols that only use bipartite entanglement to achieve the same functionalities. Our simulations show that GHZ-based protocols can outperform protocols based on bipartite entanglement in the finite-key regime and that the advantage increases for protocols with higher anonymity requirements. Our results strongly advocate the use of multipartite entanglement for cryptographic tasks involving several users.