  Quantum key distribution (QKD) networks constitute promising solutions for
secure communication. Beyond conventional point-to-point QKD, we developed 1xN
QKD network systems with a sub-ns resolution optical path length compensation
scheme. With a practical plug-and-play QKD architecture and compact timing
control modules based on a field programmable gate array (FPGA), we achieved
long term stable operation of a 1x64 QKD network system. Using this
architecture, 64 users can simultaneously share secret keys with one server,
without using complex software algorithms and expensive hardware. We
demonstrated the working of a 1x4 QKD network system using the fiber network of
a metropolitan area.
