  Quantum key distribution (QKD) provides an attractive means for securing
communications in optical fibre networks. However, deployment of the technology
has been hampered by the frequent need for dedicated dark fibres to segregate
the very weak quantum signals from conventional traffic. Up until now the
coexistence of QKD with data has been limited to bandwidths that are orders of
magnitude below those commonly employed in fibre optic communication networks.
Using an optimised wavelength divisional multiplexing scheme, we transport QKD
and the prevalent 100 Gb/s data format in the forward direction over the same
fibre for the first time. We show a full quantum encryption system operating
with a bandwidth of 200 Gb/s over a 100 km fibre. Exploring the ultimate limits
of the technology by experimental measurements of the Raman noise, we
demonstrate it is feasible to combine QKD with 10 Tb/s of data over a 50 km
link. These results suggest it will be possible to integrate QKD and other
quantum photonic technologies into high bandwidth data communication
infrastructures, thereby allowing their widespread deployment.

Quantum Physics