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

TL;DR

This paper introduces a composable finite-size CV QKD system with discrete modulation, optimized for urban atmospheric channels, demonstrating a laboratory implementation with a novel security proof and key extraction process.

Contribution

It presents the first finite-size, composable CV QKD protocol with discrete modulation tailored for atmospheric channels, including a novel security proof and practical implementation.

Findings

Successful laboratory demonstration of the protocol

Implementation of a new security proof for finite-size keys

Analysis of frame error impact on key generation

Abstract

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.