Francesco Graffiti

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.