Wobbling nanoparticles enable scientists to sense ultra-weak forces

Pioneering research at the University of Southampton has levitated nanoparticles using a Nobel Prize winning technique to an unprecedented level of sensing capability.

The experiment, which suspended glass nanoparticles using a method known as optical tweezers, observed and measured the material wobbling much like a spinning top rotating on a table. The findings could enable ultra-precise sensing with potential to detect the torque of a single nuclear spin, something yet to be reached by any experiment.

Scientists described the implementation and control of this precession motion in a paper for the Physical Review Letters that has been highlighted as an Editors' Suggestion.

Research Fellow Dr Muddassar Rashid says, "Different types of control of these trapped nanoparticles have been demonstrated experimentally in recent years, making such systems promising candidates to explore quantum mechanical features at an unprecedented large-mass scale and could yield precise sensors such as state-of-the-art gyroscopes.

"We have discovered that these levitated nanoparticles not only demonstrate translation and rotation motion but they also exhibit precession motion, a wobble of one of the axes about which the object is rotating, due to the optical force of light generating a torque on the particle.

"The research team here at Southampton worked tirelessly to characterise and understand the precession motion from both an experimental and theoretical standpoint. The analysis is complicated and requires great care, but the promotion of this work in Physical Review Letters highlights its importance and broad impact".

The optical tweezers technique was recognised in the Nobel Prize give to Arthur Ashkin in October 2018. The method uses highly focused light, producing a focal point, where particles are trapped and using properties of light can be controlled and their motion detected. This presents scientists with a powerful toolkit to study fundamental physics at the nanoscale.

"From a fundamental research point of view, we require new and novel tools to measure phenomena in the lab," Muddassar says. "Thus, this precession motion of a levitated particle becomes a tool, when coupled to other systems or forces, which can be used to detect extremely weak changes that we find in nature, for example forces generated by nuclear spins of the nucleus of the atoms. I hope our work will encourage others to utilise this approach for real-world applications and carry out further novel experiments in study of natural phenomena."

These experiments were part of a larger series of projects in Southampton's Quantum Nanophysics and Matter Wave Interferometry Group, led by Professor Hendrik Ulbricht, which are designed to detect weak forces in nature that may help address some of the most foundational questions in quantum physics.

"Our next steps will be to use this precession motion to carry out proof-of-principle experiments to detect electrical and magnetic forces, and maybe even Earth's gravity," Muddassar explains. "One of the critical requirements to this work is being able to control the shape of the nanoparticles, so we will be focusing on the ideal shapes that we can use for generating the desired precession motion."