  A levitated non-spherical nanoparticle in a vacuum is ideal for studying
quantum rotations and is an extremely sensitive torque and force detector. It
has been proposed to probe fundamental particle-surface interactions such as
the Casimir torque and the rotational quantum vacuum friction, which require it
to be driven to rotate near a surface at sub-micrometer separations. Here, we
optically levitate a silica nanodumbbell in a vacuum at about 430 nm away from
a sapphire surface and drive it to rotate at GHz frequencies. The relative
linear speed between the tip of the nanodumbbell and the surface reaches 1.4
km/s at a sub-micrometer separation. The rotating nanodumbbell near the surface
demonstrates a torque sensitivity of $(5.0 \pm 1.1) \times 10^{-26} {\rm
NmHz}^{-1/2}$ at room temperature. Moreover, we levitate a nanodumbbell near a
gold nanograting and use it to probe the near-field intensity distribution
beyond the optical diffraction limit. Our numerical simulation shows it is
promising to detect the Casimir torque between a nanodumbbell and a
nanograting.

Quantum Physics;Optics