  Quantum key distribution (QKD) which enables the secure distribution of
symmetric keys between two legitimate parties is of great importance in future
network security. Access network that connects multiple end-users with one
network backbone can be combined with QKD to build security for end-users in a
scalable and cost-effective way. Though previous QKD access networks are all
implemented in the upstream direction, in this paper, we prove that downstream
access network can also be constructed by using continuous-variable (CV) QKD.
The security of the CV-QKD downstream access network is analyzed in detail,
where we show the security analysis is secure against other parties in the
network. The security analysis we proved corresponds to the downstream access
network where only passive beamsplitter is sufficient to distribute the quantum
signals and no other active controls are demanded. Moreover, standard CV-QKD
systems can be directly fitted in the downstream access network, which makes it
more applicable for practical implementations. Numerous simulation results are
provided to demonstrate the performance of the CV-QKD downstream access
network, where up to 64 end-users are shown to be feasible to access the
network. Our work provides the security analysis framework for realizing QKD in
the downstream access network which will boost the diversity for constructing
practical QKD networks.
