Proposal on Detecting Casimir Torque

Many efforts were made to improve the precision in the Casimir force measurement and to harness the Casimir effect in various devices. Similar to the Casimir force that comes from the linear momentum of virtual photons, virtual photons also carry angular momentum, and this induces the Casimir torque between anisotropic materials. In spite of significant interests and theoretical proposals toward detecting Casimir torque, there is only one reported experimental observation of Casimir torque in 2018. The measurement was conducted between a liquid crystal and a solid birefringent crystal. To ensure the parallelism of two surfaces, the vacuum gap is replaced by an isotropic material. To our knowledge, Casimir torque between two objects with a vacuum gap has never been detected yet. In this chapter, we propose a scheme to detect the Casimir torque by our ultrasensitive optical levitation system. We will present the calculation of the Casimir torque between a levitated nanorod and a birefringent surface at a sub-wavelength distance. Compared to the torque sensitivity of the levitation system, we will demonstrate that it is promising to detect the Casimir torque across a vacuum gap in the near future, and our levitation setup is expected to provide a better accuracy for the Casimir torque measurement. Parts of the contents in this chapter have been published in Xu and Li (Phys Rev A 96:033843, 2017).