Ultrasensitive torque detection with an optically levitated nanorotor

Torque sensors such as the torsion balance enabled the first determination of the gravitational constant by Henri Cavendish¹ and the discovery of Coulomb's law. Torque sensors are also widely used in studying small-scale magnetism^2,3, the Casimir effect⁴ and other applications⁵. Great effort has been made to improve the torque detection sensitivity by nanofabrication and cryogenic cooling. Until now, the most sensitive torque sensor has achieved a remarkable sensitivity of 2.9 × 10^-24 N m Hz^-1/2 at millikelvin temperatures in a dilution refrigerator⁶. Here, we show a torque sensor reaching sensitivity of (4.2 ± 1.2) × 10^-27 N m Hz^-1/2 at room temperature. It is created by an optically levitated nanoparticle in vacuum. Our system does not require complex nanofabrication. Moreover, we drive a nanoparticle to rotate at a record high speed beyond 5 GHz (300 billion r.p.m.). Our calculations show that this system will be able to detect the long sought after vacuum friction^7-10 near a surface under realistic conditions. The optically levitated nanorotor will also have applications in studying nanoscale magnetism^2,3 and the quantum geometric phase¹¹.