A system and a method for quantum key distribution over a multi-user wavelength division multiplexing (WDM) network are disclosed. The system comprises a tunable or multi-wavelength transmitter; a plurality of receivers, each assigned a receiving-wavelength; and a multi-user WDM network linking the transmitter to the receivers. The transmitter can select a receiver among the receivers to be communicated therewith and transmit quantum signals to the selected receiver over the WDM network. The quantum signals are at a wavelength equal to a receiving-wavelength of the receiver. Therefore the WDM network allows quantum signals to be communicated between the transmitter and the receivers by wavelength routing.