# Detecting Casimir torque with an optically levitated nanorod

## TL;DR

This paper proposes an experimental method to detect the elusive Casimir torque using an optically levitated nanorod near a birefringent crystal, accounting for thermal and quantum noise effects.

## Contribution

It introduces a novel approach to measure the Casimir torque with a levitated nanorod, which has not been experimentally observed before.

## Key findings

- Calculated the Casimir torque and force on the nanorod
- Demonstrated the feasibility of detection under realistic conditions
- Analyzed effects of thermal noise and photon recoils

## Abstract

The linear momentum and angular momentum of virtual photons of quantum vacuum fluctuations can induce the Casimir force and the Casimir torque, respectively. While the Casimir force has been measured extensively, the Casimir torque has not been observed experimentally though it was predicted over forty years ago. Here we propose to detect the Casimir torque with an optically levitated nanorod near a birefringent plate in vacuum. The axis of the nanorod tends to align with the polarization direction of the linearly polarized optical tweezer. When its axis is not parallel or perpendicular to the optical axis of the birefringent crystal, it will experience a Casimir torque that shifts its orientation slightly. We calculate the Casimir torque and Casimir force acting on a levitated nanorod near a birefringent crystal. We also investigate the effects of thermal noise and photon recoils on the torque and force detection. We prove that a levitated nanorod in vacuum will be capable of detecting the Casimir torque under realistic conditions.