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

We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around $10^{-5}\,{\rm mbar}$, minimal temperatures along the cavity axis in the mK regime are observed. Simultaneously, the center-of-mass (COM) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred $\rm mK$. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behaviour of the COM temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined

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

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

Motional Sideband Asymmetry of a Nanoparticle Optically Levitated in Free Space

ISSN:0031-9007ISSN:1079-711

GHz Rotation of an Optically Trapped Nanoparticle in Vacuum

ISSN:0031-9007ISSN:1079-711

Optically levitated rotor at its thermal limit of frequency stability

Optically levitated rotors are prime candidates for torque sensors whose precision is limited by the fluctuations of the rotation frequency. In this work we investigate an optically levitated rotor at its fundamental thermal limit of frequency stability, where rotation-frequency fluctuations arise solely due to coupling to the thermal bath.ISSN:1094-1622ISSN:0556-2791ISSN:1050-294

Scalable all-optical cold damping of levitated nanoparticles

Motional control of levitated nanoparticles relies on either autonomous feedback via a cavity or measurement-based feedback via external forces. Recent demonstrations of the measurement-based ground-state cooling of a single nanoparticle employ linear velocity feedback, also called cold damping, and require the use of electrostatic forces on charged particles via external electrodes. Here we introduce an all-optical cold damping scheme based on the spatial modulation of trap position, which has the advantage of being scalable to multiple particles. The scheme relies on programmable optical tweezers to provide full independent control over the trap frequency and position of each tweezer. We show that the technique cools the centre-of-mass motion of particles along one axis down to 17 mK at a pressure of 2 × 10−6 mbar and demonstrate its scalability by simultaneously cooling the motion of two particles. Our work paves the way towards studying quantum interactions between particles; achieving three-dimensional quantum control of particle motion without cavity-based cooling, electrodes or charged particles; and probing multipartite entanglement in levitated optomechanical systems.ISSN:1748-3387ISSN:1748-339

Simultaneous ground-state cooling of two mechanical modes of a levitated nanoparticle

The quantum ground state of a massive mechanical system is a stepping stone for investigating macroscopic quantum states and building high fidelity sensors. With the recent achievement of ground-state cooling of a single motional mode, levitated nanoparticles have entered the quantum domain. To overcome detrimental cross-coupling and decoherence effects, quantum control needs to be expanded to more system dimensions, but the effect of a decoupled dark mode has so far hindered cavity-based ground-state cooling of multiple mechanical modes. Here, we demonstrate two-dimensional ground-state cooling of an optically levitated nanoparticle. Utilizing coherent scattering into an optical cavity mode, we reduce the occupation numbers of two separate centre-of-mass modes to 0.83 and 0.81, respectively. By controlling the frequency separation and the cavity coupling strengths of the nanoparticle’s mechanical modes, we show the transition from 1D to 2D ground-state cooling. This 2D control lays the foundations for quantum-limited orbital angular momentum states for rotation sensing and, combined with ground-state cooling along the third motional axis shown previously, may allow full 3D ground-state cooling of a massive object.ISSN:1745-2473ISSN:1745-248