a quantum key distribution cascaded network with loop - back capability is disclosed . the qkd system network includes a plurality of cascaded qkd relays each having two qkd stations alice and bob . each qkd relay also includes an optical switch optically coupled to each qkd station in the relay , as well as to input ports of the relay . in a first position , the optical switch allows for communication between adjacent relays and in a second position allows for pass - through communication between the qkd relays that are adjacent the relay whose switch is in the first position .