Y-00 quantum stream cipher overlay in a coherent 256-Gbit/s polarization multiplexed 16-QAM WDM system

We present the operation of a 1.5-Gb/s real-time Y-00 quantum stream cipher as an overlay in a modern coherent wavelength-division multiplexed (WDM) transmission system. We investigate transmission performance in two different wavelength allocation scenarios. The first scenario places the Y-00 cipher signal in a vacant 50-GHz channel slot between two 50-GHz spaced real-time processed 32-Gbaud PDM-16-QAM (256-Gbit/s) channels. The second scenario puts the Y-00 cipher signal in the small gap between two adjacent 50-GHz spaced WDM channels, hence implementing a secure channel overlay in a fully loaded WDM system. In both scenarios, the Y-00 cipher signal and the 256-Gbit/s signals are transmitted over 320 km. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement