  The development of prototype metropolitan-scale quantum networks is underway
and entails transmitting quantum information via single photons through
deployed optical fibers spanning several tens of kilometers. The major
challenges in building metropolitan-scale quantum networks are compensation of
polarization mode dispersion, high-precision clock synchronization, and
compensation for cumulative transmission time fluctuations. One approach
addressing these challenges is to co-propagate classical probe signals in the
same fiber as the quantum signal. Thus, both signals experience the same
conditions, and the changes of the fiber can therefore be monitored and
compensated. Here, we demonstrate the distribution of polarization entangled
quantum signals co-propagating with the White Rabbit Precision Time Protocol
(WR-PTP) classical signals in the same single-core fiber strand at
metropolitan-scale distances. Our results demonstrate the feasibility of this
quantum-classical coexistence by achieving high-fidelity entanglement
distribution between nodes separated by 100 km of optical fiber. This
advancement is a significant step towards the practical implementation of
robust and efficient metropolitan-scale quantum networks.
