  We report co-propagation experiments of the quantum channel (at 1310 nm) of a
Quantum Key Distribution (QKD) system with Dense Wavelength Division
Multiplexing (DWDM) data channels in the 1550 nm range. Two configurations are
assessed. The first one is a single span configuration where various lengths of
Standard Single Mode Fiber (SSMF) (from 20 to 70 km) are used and the total WDM
channels power is varied. The Secure Key Rate (SKR) and the Quantum Bit Error
Ratio (QBER) are recorded showing that up to ~17 dBm total power of 30 or 60
channels at 100 Gb/s can coexist with the quantum channel. A metric to evaluate
the co-propagation efficiency is also proposed to better evaluate the ability
of a QKD system to provide secure keys in a co-propagation regime. The second
experiment is a three spans link with a cascade of three QKD systems and two
trusted nodes in a 184 km total link length. We report the transmission of a
coherent 400 Gb/s Dual Polarization DP-16QAM (Quadrature Amplitude Modulation)
channel that transports a QKD secured 100 GbE data stream, with other
fifty-four 100 Gb/s WDM channels. Encryption is demonstrated at the same time
as co-propagation.

Quantum Physics