Unidimensional-modulation Continuous-variable Quantum Key Distribution under Light-injection Attacks

Unidimensional-modulation continuous-variable quantum key distribution has received considerable attention due to its simplified transmitter architecture, reduced random-number consumption, and low deployment cost. However, its security against source-side light-injection attacks remains unexplored. In this paper, the effects of such attacks on unidimensional-modulation continuous-variable quantum key distribution systems are systematically investigated. A theoretical model of quantum-state preparation at the transmitter under light-injection attacks is established, parameter estimation bias formulas are derived, the resulting degradation in the secret key rate is quantified, and the theoretical results are validated through numerical simulations. In addition, the security implications of light-injection attacks in the finite-size regime are examined. The results demonstrate that light-injection attacks cause overestimation of the channel transmittance and underestimation of excess noise. Under finite-size conditions, these attacks reduce the minimum data-block length required to achieve a positive secret key rate, thereby creating a false indication of security even when no secure key can actually be generated. These findings demonstrate that light-injection attacks constitute a serious threat to unidimensional-modulation continuous-variable quantum key distribution systems and provide an important theoretical foundation for future secure practical deployment.