A new method for cooling levitated nanoparticles using optical tweezers, discovered by researchers at the University of Vienna, the Austrian Academy of Sciences, and the Massachusetts Institute of Technology (MIT), could advance scientists’ ability to observe the quantum effects on nanoparticles. Previous attempts to obtain full control over particle motion — a necessary prerequisite to observing quantum effects — have also used optical tweezers, but these methods have been limited by laser noise and large required laser intensities. “Our new cooling scheme is directly borrowed from the atomic physics community, where similar challenges for quantum control exist,” said researcher Uros Delic. The researchers drew from early works by Innsbruck physicist Helmut Ritsch and U.S. physicists Vladan Vuletic and Steve Chu, who realized that it is sufficient to use the light that is scattered directly from the optical tweezer itself if the particle is kept inside an initially empty optical cavity. A tightly focused laser field traps a nanoparticle between two highly reflecting mirrors, that is, an optical cavity. Preferential scattering along this optical resonator induces cooling of the nanoparticle motion in all three directions. Courtesy of Aspelmeyer Group/University of Vienna. A nanoparticle in an optical tweezer scatters a tiny part of the tweezer light in nearly all directions. If the particle is positioned inside an optical cavity, a portion of the scattered light can be stored between its mirrors. As a result, photons are preferentially scattered into the optical cavity. However, this is only possible for light of specific colors, or specific photon energies. To obtain full control over particle motion, the researchers used tweezer light of a color that corresponded to a slightly smaller photon energy than required, causing the nanoparticles to give up some of their kinetic energy to allow photon scattering into the optical cavity. This loss of kinetic energy effectively cooled the particle’s motion. This method has been demonstrated for atoms before by Vladan Vuletic, a co-author of the current research. However, the current work is the first time this approach has been applied to nanoparticles and used to cool particles in all three directions of motion. “Our cooling method is much more powerful than all the previously demonstrated schemes,” Delic said. “Without the constraints imposed by laser noise and laser power, quantum behavior of levitated nanoparticles should be around the corner.” Such isolated particles could be used to study fundamental processes of nanoscopic heat engines, or quantum phenomena involving large masses. The research was published in Physical Review Letters (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.123602), and a Viewpoint on the work was published in Physics (https://physics.aps.org/articles/v12/34).