  A simple argument is presented that explicitly shows how to construct an
arbitrary quantum gate acting on orbital angular momentum (OAM) of single
photons. The scheme can be applied to implement subspace multiplexing, where a
single high-dimensional OAM qudit represents effectively a stack of multiple
independent lower-dimensional qudits. A special subclass of unitaries composed
of single-photon controlled gates is studied in detail and notable examples of
the general approach are discussed. The generalization of the simple argument
leads to the parallelization scheme, which results in the savings of resources.
The presented schemes utilize only conventional optical elements and apply not
only to single photons but also to classical light.
