Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. Here, we present a four-user QCKA protocol using photonic GHZ states distributed over fibre with combined lengths up to 50 km. Furthermore, we investigate a constrained network consisting of a 6-qubit photonic graph state which we apply network coding routines to distil suitable resource states.

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Joseph Ho, Massimiliano Proietti, Federico Grasselli, Alexander Pickston, Peter Barrow, Andres Ulibarrena, Mehul Malik, and Alessandro Fedrizzi, "Quantum conference key agreement using photonic graph states," Proc. SPIE 11881, Quantum Technology: Driving Commercialisation of an Enabling Science II, 118810B (Presented at SPIE Photonex: September 28, 2021; Published: 6 October 2021); https://doi.org/10.1117/12.2599036.6275736426001.