  Whether quantum physics is universally valid is an open question with
far-reaching implications. Intense research is therefore invested into testing
the quantum superposition principle with ever heavier and more complex objects.
Here we propose a radically new, experimentally viable route towards studies at
the quantum-to-classical borderline by probing the orientational quantum
revivals of a nanoscale rigid rotor. The proposed interference experiment
testifies a macroscopic superposition of all possible orientations. It requires
no diffraction grating, uses only a single levitated particle, and works with
moderate motional temperatures under realistic environmental conditions. The
first exploitation of quantum rotations of a massive object opens the door to
new tests of quantum physics with submicron particles and to quantum gyroscopic
torque sensors, holding the potential to improve state-of-the art devices by
many orders of magnitude.
