  Levitated nanoparticles are a promising platform for sensing applications and
for macroscopic quantum experiments. While the nanoparticles' motional
temperatures can be reduced to near absolute zero, their uncontrolled internal
degrees of freedom remain much hotter, inevitably leading to the emission of
heat radiation. The decoherence and motional heating caused by this thermal
emission process is still poorly understood beyond the case of the
center-of-mass motion of point particles. Here, we present the master equation
describing the impact of heat radiation on the motional quantum state of
arbitrarily sized and shaped dielectric rigid rotors. It predicts the
localization of spatio-orientational superpositions only based on the bulk
material properties and the particle geometry. A counter-intuitive and
experimentally relevant implication of the presented theory is that
orientational superpositions of optically isotropic bodies are not protected by
their symmetry, even in the small-particle limit.
