Francesco Graffitti

Quantum conference key agreement (CKA) is a cryptographic task for sharing a secret common key between multiple users. CKA has been established as a network protocol that can leverage multipartite entanglement (NQKD) to gain an advantage over contemporary two-party communication primitives (2QKD). Specifically, when performing QCKA in constrained quantum networks, e.g., with limited channel capacities, NQKD schemes can produce the conference key between N users with up to an N-1 rate advantage over 2QKD. QCKA has previously been implemented by direct transmission of a 4-photon GHZ state, however did not show the advantage comparison. Here we show this advantage using a universal network resource represented by a 6-qubit photonic graph state.

We demonstrate fibre-based quantum key distribution over 175 km using a bright frequency converted quantum dot single-photon source. The source is capable of producing count rates approaching 2 MHz at 1550 nm with second order correlations on the order of 3%. This allows for a measured key rate of 130 bps (100 kbps) at 175 km (50 km) in the asymptotic regime using static encoding and predicted positive key rate out to 188 km. This can be extended to 240 km using ultra-low loss fibre based on the measured source parameters.