  We combine matter-wave interferometry and cavity optomechanics to propose a
coherent matter--light interface based on mechanical motion at the quantum
level. We demonstrate a mechanism that is able to transfer non-classical
features imprinted on the state of a matter-wave system to an optomechanical
device, transducing them into distinctive interference fringes. This provides a
reliable tool for the inference of quantum coherence in the particle beam.
Moreover, we discuss how our system allows for intriguing perspectives, paving
the way to the construction of a device for the encoding of quantum information
in matter-wave systems. Our proposal, which highlights previously unforeseen
possibilities for the synergetic exploitation of these two experimental
platforms, is explicitly based on existing technology, available and widely
used in current cutting-edge experiments.
