# Ultrasensitive torque detection with an optically levitated nanorotor

## TL;DR

This paper presents an ultrasensitive torque sensor using an optically levitated nanorotor in vacuum, achieving record sensitivity at room temperature and enabling new studies in nanoscale physics without complex fabrication or cryogenics.

## Contribution

Developed a highly sensitive optically levitated nanorotor torque sensor that surpasses previous nanofabricated devices in sensitivity and operational simplicity.

## Key findings

- Measured torque as small as 1.2e-27 Nm in 100 seconds at room temperature
- Achieved nanoparticle rotation speeds beyond 5 GHz (300 billion rpm)
- System can potentially detect vacuum friction and study nanoscale magnetism

## Abstract

Torque sensors such as the torsion balance enabled the first determination of the gravitational constant by Cavendish and the discovery of Coulomb's law. Torque sensors are also widely used in studying small-scale magnetism, the Casimir effect, and other applications. Great effort has been made to improve the torque detection sensitivity by nanofabrication and cryogenic cooling. The most sensitive nanofabricated torque sensor has achieved a remarkable sensitivity of $10^{-24} \rm{Nm}/\sqrt{\rm{Hz}}$ at millikelvin temperatures in a dilution refrigerator. Here we dramatically improve the torque detection sensitivity by developing an ultrasensitive torque sensor with an optically levitated nanorotor in vacuum. We measure a torque as small as $(1.2 \pm 0.5) \times 10^{-27} \rm{Nm}$ in 100 seconds at room temperature. Our system does not require complex nanofabrication or cryogenic cooling. Moreover, we drive a nanoparticle to rotate at a record high speed beyond 5 GHz (300 billion rpm). Our calculations show that this system will be able to detect the long-sought vacuum friction near a surface under realistic conditions. The optically levitated nanorotor will also have applications in studying nanoscale magnetism and quantum geometric phase.