Composable Continuous-Variable Multi-User QKD with Discrete Modulation: Theory and Implementation

Abstract

Establishing scalable, secure quantum networks requires advancing beyond conventional point-to-point quantum key distribution (QKD) protocols toward point-to-multipoint QKD protocols. Here, we generalize a well-established discrete-modulated continuous-variable (CV) QKD protocol from the point-to-point to the point-to-multipoint setting. We present a comprehensive security analysis across four trust scenarios and derive secret key rates for both loss-only and noisy channels, in the asymptotic and composable finite-size regimes. Experimentally, we validate the protocol in a passive optical network with 10 km access links, achieving a composable secure key rate of $2.185 \times 10^{-3}$ bits per symbol (0.273 Mbit/s) against independent and identically distributed collective attacks. Our results demonstrate that discrete-modulated CV-QKD can support access networks with multiple users while relying solely on cost-efficient, off-the-shelf telecommunication components, paving the way toward practical, scalable, and secure quantum networks.