  Realizing a spatial superposition with massive objects is one of the most
fundamental challenges, as it will test quantum theory in new regimes, probe
quantum-gravity, and enable to test exotic theories like gravitationally
induced collapse. A natural extension of the successful implementation of an
atomic Stern-Gerlach interferometer (SGI), is a SGI with a nano-diamond (ND) in
which a single spin is embedded in the form of a nitrogen-vacancy center (NV).
As the ND rotation, and with it the rotation of the NV spin direction, may
inhibit such a realization, both in terms of Newtonian trajectories and quantum
phases, we analyze here the role of rotations in the SGI. We take into account
fundamental limits, such as those imposed by the quantum angular uncertainty
relation and thermal fluctuations. We provide a detailed recipe for which a
superposition of massive objects is enabled. This may open the door not only to
fundamental tests, but also to new forms of quantum technology.
