Stefan Baeuml

Continuous variable quantum key distribution (CVQKD) with discrete modulation combines advantages of CVQKD, such as the implementability using readily available technologies, with advantages of discrete variable quantum key distribution, such as easier error correction procedures. In this work we consider a phase-shift keying protocol using four coherent states (4-PSK protocol) and coarse-grained heterodyne measurements. We provide a security proof against coherent attacks and compute the achievable key rate in a finite size setting, i.e. with a finite number of rounds. To this end, we employ the generalized entropy accumulation theorem, as well conic optimisation, providing us with improved key rates compared to previous works. At metropolitan distances our method can provide positive key rates for the order of $10^9$ rounds. We also provide a theoretical method to overcome the assumption of a finite photon number cutoff made in previous works.