  The capacity of information delivered by single photons is boosted by
encoding high-dimensional quantum dits in their transverse shape. Transporting
such high-dimensional quantum dits in optical networks may be accomplished
using multimode optical fibers, which support the low-loss transmission of
multiple spatial modes over existing infrastructure. However, when photons
propagate through a multimode fiber their transverse shape gets scrambled
because of mode mixing and modal interference. This is usually corrected using
free-space spatial light modulators, inhibiting a robust all-fiber operation.
In this work, we demonstrate an all-fiber approach for controlling the shape of
single photons and the spatial correlations between entangled photon pairs,
using carefully controlled mechanical perturbations of the fiber. We optimize
these perturbations to localize the spatial distribution of a single photon or
the spatial correlations of photon pairs in a single spot, enhancing the signal
in the optimized spot by over an order of magnitude. Using the same approach we
show a similar enhancement for coupling light from a multimode fiber into a
single-mode fiber.
