  Quantum key distribution (QKD), the distribution of quantum secured keys
useful for data encryption, is expected to have a crucial impact in the next
decades. However, although the notable achievements accomplished in the last
twenty years, many practical and serious challenges are limiting the full
deployment of this novel quantum technology in the current telecommunication
infrastructures. In particular, the co-propagation of quantum signals and
high-speed data traffic within the same optical fiber, is not completely
resolved, due to the intrinsic noise caused by the high intensity of the
classical signals. As a consequence, current co-propagation schemes limit the
amount of classical optical power in order to reduce the overall link noise.
However, this ad-hoc solution restrains the overall range of possibilities for
a large-scale QKD deployment. Here, we propose and demonstrate a new method,
based on up-conversion assisted receiver, for co-propagating classical light
and QKD signals. In addition, we compare the performances of this scheme with
an off-the-shelf quantum receiver, equipped with a standard InGaAs detector,
over different lengths of an installed fiber link. Our proposal exhibits higher
tolerance for noise in comparison to the standard receiver, thus enabling the
distribution of secret keys in the condition of 4 dB-higher classical power.
