  High-dimensional entanglement is a valuable resource for quantum
communication, and photon pairs entangled in orbital angular momentum are
commonly used for encoding high-dimensional quantum states. However, methods
for preparation of maximally entangled states of arbitrary dimensionality are
still lacking, and currently used approaches essentially rely on filtering and
entanglement concentration. Here we experimentally realize a method for
generation of high-dimensional maximally entangled OAM states of photon pairs
which does not require any of these procedures. Moreover, the prepared state is
restricted to the subspace of the specified dimensionality, thus requiring
minimal postselection.
