  We cast diffraction-based interferometry in the framework of post-selected
unitary description towards enabling it as a platform for quantum information
processing. We express slit-diffraction as an infinite-dimensional
transformation and truncate it to a finite-dimensional transfer matrix by
post-selecting modes. Using such a framework with classical fields, a
customized double-slit setup is effectively a lossy beam splitter in a
post-selected sense. Diffraction optics provides a robust alternative to
conventional multi-beam interferometry with scope for miniaturization, and also
has applications in matter wave interferometry. In this work, the classical
treatment sets the stage for quantization of fields and implementing higher
dimensional quantum information processing like that done with other platforms
such as orbital angular momentum.
