  Atom and, more recently, molecule interferometers are used in fundamental
research and industrial applications. Most atom interferometers rely on
gratings made from laser beams, which can provide high precision but cannot
reach very short wavelengths and require complex laser systems to function.
Contrary to this, simple monolithic interferometers cut from single crystals
offer (sub) nano-meter wavelengths with an extreme level of stability and
robustness. Such devices have been conceived and demonstrated several decades
ago for neutrons and electrons. Here, we propose a monolithic design for a
thermal-beam molecule interferometer based on (quantum) reflection. We show, as
an example, how a reflective, monolithic interferometer (Mach-Zehnder type) can
be realised for a helium beam using Si(111)-H(1x1) surfaces, which have
previously been demonstrated to act as very robust and stable diffractive
mirrors for neutral helium atoms.
