  Multicore fibers are expected to be a game-changer in the coming decades
thanks to their intrinsic properties, allowing a larger transmission bandwidth
and a lower footprint in optical communications. In addition, multicore fibers
have recently been explored for quantum communication, attesting their
uniqueness in transporting high-dimensional quantum states. However,
investigations and experiments reported in literature have been carried out in
research laboratories, typically making use of short fiber links in controlled
environments. Thus, the possibility of using long distance multicore fibers for
quantum applications is still to be proven. We here characterize for the first
time, in terms of phase stability, multiple strands of a 4-core multicore fiber
installed underground in the city of L'Aquila, with an overall fiber length up
to about 25 km. In this preliminary study, we investigate the possibility of
using such an infrastructure to implement quantum-enhanced schemes, such as
high-dimensional quantum key distribution, quantum-based environmental sensors,
and more in general quantum communication protocols.
