Distribution of quantum keys in optically transparent networks: Perspectives, limitations and challenges

Transparent optical networks are capable of providing a flexible and dynamic data transport via transparency regarding both data rate and format of transmitted signals achieved by implementing data transmission and forwarding in the optical domain. While supporting all-optical end-to-end paths, transparent optical networks are in principle suitable to integrate end-to-end quantum cryptography. However, quantum signals are extremely sensitive to loss and noise, which is a particular issue because of the cascaded passive and active components along signal paths, common with transparent optical networks. In this paper we analyze different options for integrating quantum key distribution (QKD) in wavelength-division multiplexed (WDM) transparent optical networks where QKD signals are transmitted along with conventional WDM signals. We discuss potentials, challenges and limitations in order to assess the practicability of such systems.