  Precise synchronization between transmitter and receiver is crucial for
quantum communication protocols, such as Quantum Key Distribution (QKD), to
efficiently correlate the transmitted and received signals and increase the
signal-to-noise ratio. In this work, we introduce a synchronization technique
that exploits a co-propagating classical optical communication link and test
its performance in a free-space QKD system. Previously, existing techniques
required additional laser beams or relied on the capability of retrieving the
synchronization from the quantum signal itself, though this is not applicable
in high channel loss scenarios. On the contrary, our method exploits classical
and quantum signals locked to the same master clock, allowing the receiver to
synchronize both the classical and quantum communication links by performing a
clock-data-recovery routine on the classical signal. In this way, by exploiting
the same classical communication already required for post-processing and key
generation, no additional hardware is required, and the synchronization can be
reconstructed from a high-power signal. Our approach is suitable for both
satellite and fiber infrastructures, where a classical and quantum channel can
be transmitted through the same link.
