Quantum control of a nanoparticle optically levitated in cryogenic free space (2103.03853v2)

Abstract: Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence. Quantum control of mechanical motion has been achieved by engineering the radiation-pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states. In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects, since their trapping potential is fully controllable. In this work, we optically levitate a femto-gram dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its center-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 43%. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales.

• The paper reports achieving near ground-state cooling of a cryogenically levitated nanoparticle with an occupancy of 0.65 quanta and 43% state purity.
• The paper details a measurement-based feedback control methodology using combined homodyne and heterodyne detection to mitigate decoherence through cold damping.
• The paper demonstrates a significant measurement efficiency of 24% and offers a scalable platform for probing macroscopic quantum effects.

Quantum Control of Nanoparticles in Cryogenic Environments

The paper titled "Quantum control of a nanoparticle optically levitated in cryogenic free space" presents a significant advancement in the quest for testing quantum mechanics at macroscopic scales. The research team, based at the Photonics Laboratory, ETH Zürich, reports on their successful experimentation with a novel setup in which a dielectric nanoparticle is optically levitated in a cryogenic environment. This paper addresses challenges in controlling mechanical motion at the quantum level, specifically by mitigating decoherence mechanisms that usually hinder such experiments.

In the context of quantum optomechanics, the authors leverage a cryogenic environment to optically trap a particle, achieving a significant suppression of thermal effects. This reduction allows the measurement back-action to become the predominant factor of decoherence. The nanoparticle's center-of-mass motion is cooled to nearly its quantum ground state, with an average occupancy of 0.65 motional quanta. Notably, this achievement corresponds to a state purity of 43%.

The authors provide a detailed analysis of their experimental system, where the levitated particle is subject to measurement-based feedback control, significantly distinct from mechanically-clamped systems that often rely on cavity-enhanced detection methods. The nanoparticle, situated in cryogenic free space, is monitored and manipulated using a combined homodyne and heterodyne detection system, enabling high-efficiency quantum measurement.

The paper employs a technique known as cold damping, utilizing feedback forces derived from the homodyne signal to increase dissipation while minimizing fluctuations. This approach has allowed the achievement of a substantial measurement efficiency of 24%.

Strong experimental results were demonstrated by the team's ability to extract the phonon occupancy through multiple methodologies, including sideband asymmetry and cross-correlation measurements, which confirmed the ground-state cooling of the nanoparticle. The implications of achieving such cooling without the constraints of an optical cavity suggests potential for greater scalability and simplification in experimental setups for future explorations of quantum mechanics on macroscopic objects.

Aside from its immediate results, this research contributes to the theoretical and practical understanding of quantum systems focusing on measurement back-action and decoherence management. The findings open avenues for further refining quantum control mechanisms, potentially reaching even higher measurement efficiencies and less decoherence. Future directions may involve extending these methods to larger particles or exploring novel feedback mechanisms, presenting a significant opportunity for innovation in the field of quantum optomechanics.

The paper not only enhances the fundamental understanding of quantum measurement and control, but it also lays down a robust platform for testing quantum effects in macroscopic objects under unprecedented conditions, stimulating ongoing research into the boundaries of quantum mechanics.