Contributed Talks 2a: QKD (Chair: Andreas Poppe)

Abstract: Continuous-variable quantum key distribution offers a practical way for doing secure key exchange by means of broadband modulators and coherent detectors operating in the telecom band. Recent advances in theory and practice have improved the security and eased the system implementation. These include composable security with a finite number of distributed Gaussian-modulated coherent states and the use of pilot/reference signals and a real local oscillator for sharing the phase reference across the communicating parties. Here we report the first prepare-and-measure continuous-variable quantum key distribution experiment that can produce composable keys in the finite-size regime with security against collective attacks. Through novel improvements in the existing security proofs and a fast, yet low-noise and highly stable system operation, we obtain a secret key rate $>$5 Mbps over a 20 km long fiber channel. Our demonstration verifies the security of practical continuous-variable quantum key distribution when used for encryption or other cryptographic tasks.

Abstract: We have shown how to conduct QKD vulnerability assessment in practice, based on a sound methodology inherited from Common Criteria. Taking a running CV-QKD system as a reference platform, we have experimentally tested and rated two different attack paths exploiting a common threat: detector saturation. Our results illustrate the importance of rating attacks in order to prioritize the implementation of countermeasures and to steer the design and engineering of practical QKD systems towards the highest possible security standards, paving the way to their security certification.

Abstract: We establish an analytical lower bound on the asymptotic secret key rate of continuous-variable quantum key distribution with an arbitrary modulation of coherent states. Previously, such bounds were only available for protocols with a Gaussian modulation, and numerical bounds existed in the case of simple phase-shift-keying modulations. The latter bounds were obtained as a solution of a convex optimization problem and our new analytical bound matches them, up to numerical precision. The more relevant case of quadrature amplitude modulation (QAM) could not be analyzed with the previous techniques,due to their large number of coherent states. Our bound shows that relatively small constellation sizes, with say 64 states, are essentially sufficient to obtain a performance close to a true Gaussian modulation and are therefore an attractive solution for large-scale deployment of continuous-variable quantum key distribution. We also derive similar bounds when the modulation consists of arbitrary states, not necessarily pure.