A quantum cryptographic key distribution system, including: an optical source, which generates a plurality of optical pulses; an optical beam splitter, which generates, starting from each optical pulse, a first and a second optical sub-pulse; a first and a second peripheral device; and an optical path having a first and a second end connected to the optical beam splitter, the optical path extending through the first and second peripheral devices and being traversed in opposite directions by the first and second optical sub-pulses. The peripheral device randomly phase shifts the second optical sub-pulse by a first phase, and the second peripheral device randomly phase shifts the first optical sub-pulse by a second phase. Furthermore, the optical path is such as to cause interference in the first optical beam splitter between the first and second optical sub-pulses, as a function of first and second phases.