University of Vienna Researchers Achieve First Two-Dimensional Quantum Ground State Cooling of Rotating Nanoparticles

University of Vienna researchers reach the quantum limit of rotation, cooling a 100-million-atom nanorotor to 20 microkelvin for precision torque sensing.

By: AXL Media

Published: Apr 6, 2026, 8:53 AM EDT

Source: Information for this report was sourced from EurekAlert!

Breaking the Rotational Thermal Barrier

In a significant advancement for quantum mechanics, researchers from the University of Vienna, TU Wien, and Ulm University have demonstrated the ability to freeze the rotational jiggling of a microscopic object. By trapping a silica nanorotor in an ultra-high vacuum using focused laser light, the team successfully suppressed thermal oscillations to a point where the particle’s energy transitions become quantized. While previous experiments had achieved ground-state cooling for linear motion, this study represents the first time rotational motion, or libration, has been restricted to its lowest possible energy state in two dimensions simultaneously.

Precision Beyond Atomic Dimensions

The experiment utilized a nano-dumbbell structure, comprised of two 150-nanometer silica spheres, which were oriented by the electric field of a laser like an invisible mechanical spring. Through a process known as coherent scattering cooling, the team reduced the rotor's temperature to just a few ten microkelvin above absolute zero. At this extreme threshold, the alignment of the rotor reached a quantum-limited state of approximately 20 microradians. Lead author Stephan Troyer noted that the tip of the rotor now moves less than one-hundredth of the diameter of a single atom, a level of precision comparable to a compass needle oriented more accurately than the width of a bacterium.

Harnessing Coherent Scattering for Optical Cooling

To reach these unprecedented temperatures, the researchers subjected the nanoparticles to a staggering light intensity of 100 megawatts per square centimeter. Trapped within an optical resonator, the particles scatter photons that carry away individual quanta of mechanical energy. This interaction allows the resonator field to effectively "wick away" the kinetic energy of the rotation, leaving the particle in a state of zero-point fluctuation. This method of coherent scattering ensures that the only remaining motion is the unavoidable disorientation imposed by the laws of quantum physics, rather than external thermal interference.