  Performing complex cryptographic tasks will be an essential element in future
quantum communication networks. These tasks are based on a handful of
fundamental primitives, such as coin flipping, where two distrustful parties
wish to agree on a randomly generated bit. Although it is known that quantum
versions of these primitives can offer information-theoretic security
advantages with respect to classical protocols, a demonstration of such an
advantage in a practical communication scenario has remained elusive. Here, we
experimentally implement a quantum coin flipping protocol that performs
strictly better than classically possible over a distance suitable for
communication over metropolitan area optical networks. The implementation is
based on a practical plug&play system, designed for quantum key distribution.
We also show how to combine our protocol with coin flipping protocols that are
almost perfectly secure against bounded adversaries, hence enhancing them with
a level of information-theoretic security. Our results offer a powerful toolbox
for future secure quantum communications.
