Linear feedback cooling of a levitated nanoparticle in free space

Abstract

About a decade ago, optically levitated nanoparticles have been proposed for macroscopic tests of quantum mechanics. For such tests, the thermal motion of the particle’s center of mass is required to be close to its ground state of energy. Ever since these proposals, research groups around the world try to achieve ground-state cooling of optically levitated glass particles. In this dissertation, we cool the center-of-mass motion of a nanoparticle in an optical trap. Based on the position measurement of the particle, we apply a damping force in proportion to the particle’s speed, which leads to a cooling effect. We find that the cooling performance of our cold damping scheme is limited by the measurement imprecision. We analyze our detection principle theoretically and find an ideal detection scheme whose imprecision is at the fundamental noise level dictated by quantum mechanics. Such a Heisenberg-limited detection would, in principle, allow for ground-state feedback cooling. With these insights applied to our experiment, we cool the motion of our particle to an average of four quanta. Moreover, we resolve an asymmetry between the Stokes and anti-Stokes scattered light from the particle. This quantum effect allows us to calibrate the system to the ground state energy. Our work advances the research field of levitated optomechanics toward quantum control and therefore toward macroscopic tests of quantum mechanics.