Composable free-space continuous-variable quantum key distribution using discrete modulation

• Main Authors: Jaksch, Kevin, Dirmeier, Thomas, Weiser, Yannick, Richter, Stefan, Bayraktar, Ömer, Hacker, Bastian, Rösler, Conrad, Khan, Imran, Petscharning, Stefan, Grafenauer, Thomas, Hentschel, Michael, Ömer, Bernhard, Pacher, Christoph, Kanitschar, Florian, Upadhyaya, Twesh, Lin, Jie, Lütkenhaus, Norbert, Leuchs, Gerd, Marquardt, Christoph
• Format: Journal Article
• Language: English
• Published: 16.10.2024
• Subjects: Physics - Quantum Physics
• DOI: 10.48550/arxiv.2410.12915

Continuous-variable (CV) quantum key distribution (QKD) allows for quantum secure communication with the benefit of being close to existing classical coherent communication. In recent years, CV QKD protocols using a discrete number of displaced coherent states have been studied intensively, as the modulation can be directly implemented with real devices with a finite digital resolution. However, the experimental demonstrations until now only calculated key rates in the asymptotic regime. To be used in cryptographic applications, a QKD system has to generate keys with composable security in the finite-size regime. In this paper, we present a CV QKD system using discrete modulation that is especially designed for urban atmospheric channels. For this, we use polarization encoding to cope with the turbulent but non-birefringent atmosphere. This will allow to expand CV QKD networks beyond the existing fiber backbone. In a first laboratory demonstration, we implemented a novel type of security proof allowing to calculate composable finite-size key rates against i.i.d. collective attacks without any Gaussian assumptions. We applied the full QKD protocol including a QRNG, error correction and privacy amplification to extract secret keys. In particular, we studied the impact of frame errors on the actual key generation.