Optically Levitated Nanodumbbell Torsion Balance and GHz Nanomechanical Rotor

TLDR

Levitated optomechanics offers powerful tools for precision measurement, quantum sensing, and fundamental physics, with linearly‑polarized optical tweezers providing a restoring torque analogous to a torsion wire and enabling studies of ultrafast rotation for material and vacuum friction investigations. The authors synthesize and optically levitate silica nanodumbbells in high vacuum to create a torsion balance that can probe Casimir torque and the quantum nature of gravity. They synthesize silica nanodumbbells, levitate them with a linearly polarized laser in high vacuum, and observe their torsional vibrations. Calculations predict that the levitated nanodumbbell torsion balance can detect torque several orders of magnitude more sensitively than existing balances, while a circularly‑polarized laser can spin a 170‑nm nanodumbbell above 1 GHz—currently the fastest nanomechanical rotor—and smaller beads could reach frequencies beyond 10 GHz.

Abstract

Levitated optomechanics has great potentials in precision measurements, thermodynamics, macroscopic quantum mechanics and quantum sensing. Here we synthesize and optically levitate silica nanodumbbells in high vacuum. With a linearly polarized laser, we observe the torsional vibration of an optically levitated nanodumbbell in vacuum. The linearly-polarized optical tweezer provides a restoring torque to confine the orientation of the nanodumbbell, in analog to the torsion wire which provides restoring torque for suspended lead spheres in the Cavendish torsion balance. Our calculation shows its torque detection sensitivity can exceed that of the current state-of-the-art torsion balance by several orders. The levitated nanodumbbell torsion balance provides rare opportunities to observe the Casimir torque and probe the quantum nature of gravity as proposed recently. With a circularly-polarized laser, we drive a 170-nm-diameter nanodumbbell to rotate beyond 1~GHz, which is the fastest nanomechanical rotor realized to date. Our calculations show that smaller silica nanodumbbells can sustain rotation frequency beyond 10 GHz. Such ultrafast rotation may be used to study material properties and probe vacuum friction.