Christopher L. Morrison

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.

Quantum key distribution (QKD) with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. We report a bright quantum dot based source of telecom photons by frequency converting a near- infrared InGaAs quantum dot to the telecom C-band (1). We implement polarisation encoded BB84 quantum key distribution (QKD), achieving a positive asymptotic key rate over 175 km of optical fibre. We also present finite key analysis optimised for typically non-ideal single- photon sources, achieving 8 orders of magnitude improvement with finite key rates of 40 kbps over 50 km in practical acquisition times of one hour (2). To extend the distances further, we take inspiration from decoy state QKD protocols – typically used to overcome photon- splitting attacks when using weak coherent states – and demonstrate a QD excitation scheme for implementing modulation of the photon number distribution. We show experimentally that the decoy state protocol enables the distribution of a secret key over more than 200 km of optical fibre.

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.