By Josh Perry, Editor
[email protected]

Physicists from the University of Vienna (Austria), the Austrian Academy of Sciences and the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. demonstrated a new method for cooling isolated and levitated nanoparticles.

A tightly focused laser field traps a nanoparticle between two highly reflecting mirrors, i.e. an optical cavity. (Aspelmeyer group/University of Vienna)

According to a report from the University of Vienna, the researchers used a technique in which lasers act as optical tweezers to trap nanoparticles (which earned Arthur Askin the Nobel prize in physics in 2018), but they performed this in an ultra-high vacuum, which limited the laser noise and mitigated the need for large laser intensities. 

The particle is trapped by the lasers inside optical cavities so that the scattered light from the particle can be stored between the mirrors.

“As a result, photons are preferentially scattered into the optical cavity,” the article explained. “However, this is only possible for light of specific colors, or said differently, specific photon energies.”

Researchers used a tweezer of a color that is at a slightly lower photon energy than required to force the nanoparticle to use some of its kinetic energy to scatter photons within the cavity. This loss of energy effectively cools the motion of the particle.

While previous efforts have demonstrated the viability of cooling particles in this manner, this research is the first to demonstrate cooling particles in all three directions of motion.

The research was recently published in Physical Review Letters. The abstract stated:

“We report three-dimensional (3D) cooling of a levitated nanoparticle inside an optical cavity. The cooling mechanism is provided by cavity-enhanced coherent scattering off an optical tweezer. The observed 3D dynamics and cooling rates are as theoretically expected from the presence of both linear and quadratic terms in the interaction between the particle motion and the cavity field.

“By achieving nanometer-level control over the particle location we optimize the position-dependent coupling and demonstrate axial cooling by two orders of magnitude at background pressures of 6×10−2  mbar. We also estimate a significant (>40  dB) suppression of laser phase noise heating, which is a specific feature of the coherent scattering scheme.

“The observed performance implies that quantum ground state cavity cooling of levitated nanoparticles can be achieved for background pressures below 1×10−7  mbar.”