  Spatial transformations of light are ubiquitous in optics, with examples
ranging from simple imaging with a lens to quantum and classical information
processing in waveguide meshes. Multi-plane light converter (MPLC) systems have
emerged as a platform that promises completely general spatial transformations,
i.e., a universal unitary. However until now, MPLC systems have demonstrated
transformations that are far from general, e.g., converting from a Gaussian to
Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability
to impose an arbitrary unitary transformation that can be reconfigured
dynamically. Specifically, we consider transformations on superpositions of
parallel free-space beams arranged in an array, which is a common information
encoding in photonics. We experimentally test the full gamut of unitary
transformations for a system of two parallel beams and make a map of their
fidelity. We obtain an average transformation fidelity of $0.85 \pm 0.03$. This
high-fidelity suggests MPLCs are a useful tool implementing the unitary
transformations that comprise quantum and classical information processing.
