  Covert communication offers a method to transmit messages in such a way that
it is not possible to detect that the communication is happening at all. In
this work, we report an experimental demonstration of covert communication that
is provably secure against unbounded quantum adversaries. The covert
communication is carried out over 10 km of optical fiber, addressing the
challenges associated with transmission over metropolitan distances. We deploy
the protocol in a dense wavelength-division multiplexing infrastructure, where
our system has to coexist with a co-propagating C-band classical channel. The
noise from the classical channel allows us to perform covert communication in a
neighbouring channel. We perform an optimization of all protocol parameters and
report the transmission of three different messages with varying levels of
security. Our results showcase the feasibility of secure covert communication
in a practical setting, with several possible future improvements from both
theory and experiment.
