  Quantum protocols require access to large-scale entangled quantum states, due
to the requirement of channel capacity. As a promising candidate, the
high-dimensional orbital angular momentum (OAM) entangled states have been
implemented, but only one of four OAM Bell states in each individual subspace
can be distinguished. Here we demonstrate the first realization of complete OAM
Bell-state measurement (OAM-BSM) in an individual subspace, by seeking the
suitable unitary matrix performable using only linear optics and breaking the
degeneracy of four OAM Bell states in ancillary polarization dimension. We
further realize the superdense coding via our complete OAMBSM with the average
success probability of ~82% and the channel capacity of ~1.1(4) bits. This work
opens the window for increasing the channel capacity and extending the
applications of OAM quantum states in quantum information in future.
