  We present an experiment demonstrating entanglement-enhanced classical
communication capacity of a quantum channel with correlated noise. The channel
is modelled by a fiber optic link exhibiting random birefringence that
fluctuates on a time scale much longer than the temporal separation between
consecutive uses of the channel. In this setting, introducing entanglement
between two photons travelling down the fiber allows one to encode reliably up
to one bit of information into their joint polarization degree of freedom. When
no quantum correlations between two separate uses of the channel are allowed,
this capacity is reduced by a factor of more than three. We demonstrated this
effect using a fiber-coupled source of entagled photon pairs based on
spontaneous parametric down-conversion, and a linear-optics Bell state
measurement.
