A clock signal of a master clock of a sender is transmitted to a receiver through a classical channel and is returned from the receiver. The clock signal is transmitted with strong light from a sender-side quantum unit to a receiver-side quantum unit through a quantum channel. A sender-side synchronization section establishes phase synchronization between the clock signal returned from the receiver and the clock signal detected by the sender-side quantum unit, and generates a calibration clock signal. At the receiver as well, a receiver-side synchronization section establishes phase synchronization between the clock signal detected from the classical channel and the clock signal detected by the receiver-side quantum unit, and generates a calibration clock signal.