Quantum CDMA-based Continuous Variable Quantum Key Distribution using Chaotic Phase Shifters

We present a novel quantum code-division multiple-access (q-CDMA)-based continuous-variable quantum key distribution (CV-QKD) system that enables secure multiuser QKD over a shared quantum channel. In the proposed system, quantum states generated by multiple transmitters are chaotically encoded using chaotic phase shifters and subsequently multiplexed through a beam splitter (BS) network before transmission over a shared quantum channel. At the receiver, a corresponding set of synchronized chaotic phase shifters is employed for decoding, followed by demultiplexing through the inverse BS network to recover the individual user signals. For an arbitrary number of users, we characterize the input–output quadrature relations and analyze the achievable secret key rate (SKR) under collective Gaussian attacks with reverse–reconciliation (RR). The effects of key system parameters, including the correction factor, multiuser interference noise, and channel transmittance, are systematically investigated to evaluate their impact on the achievable asymptotic SKR. Furthermore, the finite-size effects performance is compared with the asymptotic limit to provide insights into the associated performance trade-offs. Numerical results demonstrate that the proposed q-CDMA-based CV-QKD system achieves superior SKR performance when compared with existing quantum multiple-access schemes, namely frequency-division multiple access (FDMA), orthogonal frequency-division multiple access (OFDMA), time-division multiple access (TDMA), and wavelength division multiple access (WDMA). The developed analytical framework provides a comprehensive foundation for evaluating the security and performance of a multi-user q-CDMA-based CV-QKD system and offers valuable design insights for scalable and secure quantum communication networks.