A path computation engine for an optical communications network, the optical communications network comprising a plurality of nodes and a plurality of links, wherein the path computation engine comprises a processor and memory, the memory comprising instructions executable by the processor whereby the path computation engine is operative to: receive a request to configure a quantum key, Qkey, path from a first node to a second node in the optical communications network for a quantum key distribution, QKD, signal for a quantum key for a secure data transmission signal; calculate a feasible Qkey path from the first node to the second node that is logically different to a traffic path from the first node to the second node for the secure data transmission signal, wherein the Qkey path is feasible if an optical signal power originating from the secure data transmission signal within the Qkey path, caused by optical interference of the secure data transmission signal with the QKD signal, is below a predetermined threshold value, wherein to calculate a feasible Qkey path the path computation engine is operative to: - calculate a plurality of feasible Qkey paths from the first node to the second node, each calculated Qkey path being logically different to the traffic path and randomly select one of said plurality of feasible Qkey paths; or - assign a respective randomly calculated weight value to each link of the optical communications network and to calculate a feasible Qkey path depending on said weight values; and generate a control signal comprising instructions arranged to configure said feasible Qkey path. receive a request to configure a quantum key, Qkey, path from a first node to a second node in the optical communications network for a quantum key distribution, QKD, signal for a quantum key for a secure data transmission signal; calculate a feasible Qkey path from the first node to the second node that is logically different to a traffic path from the first node to the second node for the secure data transmission signal, wherein the Qkey path is feasible if an optical signal power originating from the secure data transmission signal within the Qkey path, caused by optical interference of the secure data transmission signal with the QKD signal, is below a predetermined threshold value, wherein to calculate a feasible Qkey path the path computation engine is operative to: - calculate a plurality of feasible Qkey paths from the first node to the second node, each calculated Qkey path being logically different to the traffic path and randomly select one of said plurality of feasible Qkey paths; or - assign a respective randomly calculated weight value to each link of the optical communications network and to calculate a feasible Qkey path depending on said weight values; and - calculate a plurality of feasible Qkey paths from the first node to the second node, each calculated Qkey path being logically different to the traffic path and randomly select one of said plurality of feasible Qkey paths; or - assign a respective randomly calculated weight value to each link of the optical communications network and to calculate a feasible Qkey path depending on said weight values; generate a control signal comprising instructions arranged to configure said feasible Qkey path.