Methods and systems are provided for distributing key information by securely transmitting light information in a network along a path comprised of a plurality of network devices and by using a key distribution center (KDC) connected with the network. Three or more user devices connected with the network are specified as constituting a secure communication group. Each user device sets up an end-to-end path to the KDC by configuring at least one of the network devices to direct light information along the path. Thereafter, a user key is established between each user device and the KDC, by sending light information using randomly selected quantum bases through the path. The KDC determines a shared secret key, and notifies each user device of the shared secret key by sending the result of a calculation based on the shared secret key and each user key through the network.