Cavity-Based 3D Cooling of a Levitated Nanoparticle via Coherent Scattering

Abstract

After pioneering work on optically levitated particles in the 1970s by Ashkin, so called optical tweezers have not only become a popular tool in biology and medicine but also experienced a renaissance in vacuum trapping over the last ten years. While the broader field of optomechanics already gained popularity previously and investigated the classical-to-quantum transition in cryogenic systems at the beginning of the last decade, levitated particle optomechanics opened the door to room temperature quantum experiments. The versatile levitated particle system has been utilized in various applications, such as rotation experiments at world record speeds exceeding 6 GHz, highly sensitive force sensors or to study stochastic effects of thermodynamics. Most intriguing, however, was the transition into the quantum regime by ground-state cooling of the particle's center-of-mass motion in one dimension, setting the first milestone of genuine quantum experiments. Ten years after the first cooling attempt, a technique adopted from the atom and ion cooling community finally led to ground-state cooling of a levitated particle. This technique, called cooling by coherent scattering, is the central scheme of this thesis. In the first part of this thesis we describe the construction of a double vacuum chamber system to efficiently trap and transfer a levitated particle into an optical cavity. In contrast to previous systems, no particle transfer to a second tweezer is necessary, minimizing the risk of particle loss and enabling experiments within minutes after particle loading. In the second part, we present the working principle of our coherent scattering setup. It was the first pure optical trapping configuration to transition a levitated particle into high vacuum, while being stabilized exclusively through cavity cooling. At the time, we achieved record low cavity cooling center-of-mass energies reflected by an effective temperature of a few millikelvin and demonstrated genuine 3D cooling of the motional degrees of freedom. The central feat of the coherent scattering setup is the ability to cool the particle motion in a cavity field node, reducing the impact of laser phase noise compared to the dispersive regime. In the last part of the thesis, we find that the mechanical instability of the particle position limits the suppression of phase noise, leading to particle cooling to about 10 phonons.