  The quantum network makes use of the quantum states to transmit data, which
will revolutionize classical communication and allow for some breakthrough
applications. The quantum key distribution (QKD) is one prominent application
of quantum networks, and can protect the data transmission through quantum
mechanics. In this work, we propose an expandable and cost-effective quantum
access network, in which the round-trip structure makes quantum states travel
in a circle to carry the information, and the multi-band technique is proposed
to support multi-user access. Based on the round-trip multi-band quantum access
network, we realize multi-user secure key sharing through the
continuous-variable QKD (CV-QKD) protocol. Due to the encoding characteristics
of CV-QKD, the quadrature components in different frequency bands can be used
to transmit key information for different users. The feasibility of this scheme
is confirmed by comprehensive noise analysis, and is verified by a
proof-of-principle experiment. The results show that each user can achieve
excess noise suppression and 600 bps level secure key generation under 30 km
standard fiber transmission. Such networks have the ability of multi-user
access theoretically and could be expanded by plugging in simple modules.
Therefore, it paves the way for near-term large-scale quantum secure networks.
