Optically levitated nanoscale torsion balance for detecting Casimir torque and beyond.

The virtual photons of quantum vacuum fluctuations will not only have linear momentums that lead to the well-known Casimir force, but also have angular momentums which can induce the Casimir torque for anisotropic materials and structures. We propose to optically levitate a nonspherical nanoparticle in vacuum to detect the Casimir torque due to the angular momentum of vacuum fluctuations. We have experimentally levitated nonspherical nanoparticles in vacuum, observed their torsional vibration, and driven them to rotate at several hundred MHz. A nonspherical nanoparticle levitated in high vacuum will have a remarkable torque detection sensitivity on the order of 10$^{\mathrm{-28}}$ Nm/sqrt(Hz), which will be sufficient to detect the Casimir torque.