A method of key distribution, a key distribution system, a single photon source system and a method of generating single photons. The method of key distribution comprises the steps of: providing a free space optics, FSO, link between a transmitter and a receiver; detecting whether an eavesdropper is present along the FSO link; transmitting individual photons or weak coherent pulses, as an approximation of individual photons, each encoding a basic unit of the key according to a binary or higher number base system from the transmitter to the receiver; and comparing timing information associated with the transmission and reception of the individual photons for determining the key when it is detected that no eavesdropper is present along the FSO link.