Cooling of Thermal Motion of Levitated Optically Coupled Nanoparticles

Individual optically levitated nanoparticles have been employed in experimental testing of quantum physics and statistical mechanics in previously inaccessible parameter regimes. Such experiments typically require cooling of the effective kinetic temperature of the particles well below the ambient temperature. We report on cooling of a system of two optically levitated charged nanoparticles using a cold-damping force provided by an external electric field controlled by an electronic feedback loop. The particles interacting through optical binding act as linearly coupled harmonic oscillators displaying normal modes of oscillatory motion. We systematically study the cooling of the normal modes and succeed in reaching sub-Kelvin temperatures for both modes at pressures of 5 × 10
⁻³
mbar. Experimental observations are captured by a theoretical model that is used to survey the parameter space more widely and to quantify the limits imposed by measurement noise and loop delays. Our work paves the way for the study of quantum interactions between meso-scale particles and the exploration of multiparticle entanglement in levitated optomechanical systems.