  The field of levitodynamics has made substantial advancements in manipulating
the translational and rotational degrees of freedom of levitated nanoparticles.
Notably, rotational degrees of freedom can now be cooled to millikelvin
temperatures and driven into GHz rotational speeds. However, in the case of
cylindrically symmetric nanorotors, only the rotations around their short axes
have been effectively manipulated, while the possibility to control rotation
around the longer axis has remained a notable gap in the field. Here, we extend
the rotational control toolbox by engineering an optically levitated
nanodumbbell in vacuum into controlled spinning around its long axis with
spinning rates exceeding 1 GHz. This fast spinning introduces deep-strong
coupling between the nanodumbell's libration modes, such that the coupling rate
$g$ exceeds the bare libration frequencies $\Omega_0$ by two orders of
magnitude with $g/\Omega_0=724\pm 33$. Our control over the long-axis rotation
opens the door to study the physics of deep-strong coupled mechanical
oscillators and to observe macroscopic rotational quantum interference effects,
thus laying a solid foundation for future applications in quantum technologies.
Additionally, we find that our system offers great potential as a nanoscopic
gyroscope with competitive sensitivity.
