  Matter-wave interferometry has become an essential tool in studies on the
foundations of quantum physics and for precision measurements. Mechanical
gratings have played an important role as coherent beamsplitters for atoms,
molecules and clusters since the basic diffraction mechanism is the same for
all particles. However, polarizable objects may experience van der Waals shifts
when they pass the grating walls and the undesired dephasing may prevent
interferometry with massive objects. Here we explore how to minimize this
perturbation by reducing the thickness of the diffraction mask to its ultimate
physical limit, i.e. the thickness of a single atom. We have fabricated
diffraction masks in single-layer and bilayer graphene as well as in 1 nm thin
carbonaceous biphenyl membrane. We identify conditions to transform an array of
single layer graphene nanoribbons into a grating of carbon nanoscrolls. We show
that all these ultra-thin nanomasks can be used for high-contrast quantum
diffraction of massive molecules. They can be seen as a nanomechanical answer
to the question debated by Bohr and Einstein whether a softly suspended double
slit would destroy quantum interference. In agreement with Bohr's reasoning we
show that quantum coherence prevails even in the limit of atomically thin
gratings.
