  Optically levitated nonspherical particles in vacuum are excellent candidates
for torque sensing, rotational quantum mechanics, high-frequency gravitational
wave detection, and multiple other applications. Many potential applications,
such as detecting the Casimir torque near a birefringent surface, require
simultaneous cooling of both the center-of-mass motion and the torsional
vibration (or rotation) of a nonspherical nanoparticle. Here we report the
first 5D cooling of a levitated nanoparticle. We cool the 3 center-of-mass
motion modes and 2 torsional vibration modes of a levitated nanodumbbell in a
linearly-polarized laser simultaneously. The only uncooled rigid-body degree of
freedom is the rotation of the nanodumbbell around its long axis. This free
rotation mode does not couple to the optical tweezers directly. Surprisingly,
we observe that it strongly affects the torsional vibrations of the
nanodumbbell. This work deepens our understanding of the nonlinear dynamics and
rotation coupling of a levitated nanoparticle and paves the way towards full
quantum control of its motion.
