  High-dimensional entanglement with spatial modes of light promises increased
security and information capacity over quantum channels. Unfortunately,
entanglement decays due to perturbations, corrupting quantum links which cannot
be repaired without a tomography of the channel. Paradoxically, the channel
tomography itself is not possible without a working link. Here we overcome this
problem with a robust approach to characterising quantum channels by means of
classical light. Using free-space communication in a turbulent atmosphere as an
example, we show that the state evolution of classically entangled degrees of
freedom is equivalent to that of quantum entangled pho- tons, thus providing
new physical insights into the notion of classical entanglement. The analysis
of quantum channels by means of classical light in real time unravels
stochastic dynamics in terms of pure state trajectories and thus enables
precise quantum error-correction in short and long haul optical communication,
in both free-space and fibre.
