Levitating nanoparticle improves 'torque sensing'

A bar balancing two lead spheres at either end was suspended on a thin metal wire. Gravity acting on the two weights caused the wire and bar to twist, and this twisting - or torsion - was measured to calculate the gravitational force.

In the new experiment, an oblong-shaped nanodiamond levitated by a laser beam in a vacuum chamber served the same role as the bar, and the laser beam served the same role as the wire in Cavendish's experiment.

"A change of the orientation of the nanodiamond caused the polarisation of the laser beam to twist," said Tongcang Li, an assistant professor of physics and astronomy and electrical and computer engineering at Purdue University.

"Torsion balances have played historic roles in the development of modern physics. Now, an optically levitated ellipsoidal nanodiamond in a vacuum provides a new nanoscale torsion balance that will be many times more sensitive." Findings are detailed in a paper that appeared in the journal Physical Review Letters.

"This is the first experimental observation of torsional motion of a nanoparticle levitated in a vacuum and represents a very sensitive torque detector," Li said. "In principle, we could detect the torque on a single electron or a single proton."

The paper was authored by Purdue postdoctoral research associate Thai M. Hoang; student Yue Ma from Tsinghua University in China; Purdue graduate students Jonghoon Ahn and Jaehoon Bang; Francis Robicheaux, a Purdue professor of physics and astronomy; Zhang-Qi Yin, an assistant research fellow at Tsinghua University; and Li.

The paper details the detection of torsional vibration, a proposal to use the technique for torque sensing and also to achieve torsional "ground state cooling," which could aid efforts to study quantum theory and realise potential applications in quantum information processing and high-precision measurement for sensors.

This cooling reduces "noise" caused by vibrating molecules and atoms, making it possible to precisely measure torque and probe the relationships between motion and electron "spin." Electrons can be thought of as having two distinct spin states, "up" or "down," and this phenomenon might be used in future quantum simulations.

The paper includes experimental and theoretical portions. "Experimentally, we observed torsional motion, and the theoretical part is a proposal of how to cool down the motion to achieve quantum ground state," Li said.

The nanodiamonds are about 100 nm in diameter, or roughly the size of a virus. Future research will include efforts to achieve ground state cooling. The research was supported by the National Science Foundation and by the National Natural Science Foundation of China.