Repeater device and transmission control techniques in quantum cryptographic communication system

A network management device in a communication network in which a plurality of nodes including a transmitter, a receiver and a plurality of repeater devices are connected by spatial links, wherein the transmitter generates signal light and reference light from coherent light and transmits the signal light and the reference light to the receiver through at least one repeater device, wherein the signal light is light of weak intensity having quantum states and the reference light is light of normal intensity having no quantum states, the network management device comprising:

• a storage that stores a network configuration of the nodes and the spatial links;
• a link state collector that collects link states of spatial links accommodated by each node from the plurality of nodes;
• and
• a network controller configured to:
• select an optimal route in the communication network based on the link states of the spatial links;
• and
• control each of repeater devices on the route so that each repeater device selects spatial links constituting the route.

A transmission control method in a repeater device that receives signal light and reference light and transmits them in a communication network, wherein the signal light and the reference light are generated from coherent light, wherein the signal light is light of weak intensity having quantum states and the reference light is light of normal intensity having no quantum states, the method comprising:

• by an optical route selector, selecting an input-side spatial link connected to an upstream adjacent node and an output-side spatial link connected to a downstream adjacent node;
• by a link state detector, detecting a link state of at least one of the input-side spatial link and the output-side spatial link, wherein the link state is an attenuation rate or a transmission rate;
• and
• by a controller, controlling the optical route selector to select an optimal pair of output-side spatial link and input-side spatial link according to the link state.