Summary: Levitated cavity optomechanics in high vacuum
U Delić, D Grass, M Reisenbauer, T Damm, M Weitz, N Kiesel and M Aspelmeyer
TL;DR Summary
The authors experiment on dispersive coupling with an optically trapped nanoparticle in a setup formed by placing two parallel, highly reflective mirrors facing each other (a Fabry-Përot cavity) in high vacuum. They show nanometre-level control for positioning the nanoparticle with respect to the cavity field. Coupling rates of the system are determined and they expect that by decreasing the mode volume of the cavity, strong cooperativity can be reached.
Introduction
The setup of cavity optomechanics allows optical quantum control of mechanical motion. When the motion of a levitated object is coupled to an optical cavity, this can enable considerable isolation of the mechanical motion from the environment at ultra-high vacuum at room temperatures. In a Fabry-Përot cavity, the particle shifts the cavity resonance, coupling the particle motion dispersively to the cavity field, when resembles the fundamental cavity optomechanical interaction.
Method
The experiment uses a free space Fabry-Përot resonator with an optical tweezer. To cool the particle’s centre of mass motion, parametric feedback cooling is used to remove energy from the particle’s motion whereby a velocity dependent spring constant is used.
The optomechanical interface is made with a high-finesse, near-confocal Fabry-Përot cavity. Two laser beams drive the cavity, a locking laser for homodyne detection and a second control laser which is modulated with an electro-optical modulator. Silica nanoparticles are loaded into the optical tweezer using a nebulizer.
Levitated Cavity Optomechanics Setup
Toggle the controls below to activate the different laser fields interacting with the levitated silica nanoparticle in high vacuum.
Results
The coherent optomechanical interaction competes with noise acting on the nanoparticles’ centre of mass motion. The noise comes from two sources: from collisions with surrounding gas molecules giving a heating rate is , where is the pressure dependent mechanical line width and the thermal occupation of the bath which is . The nanoparticle also scatters photons off the optical trap creating a white and Gaussian but anisotropic force noise.
The recoil heating is dominated by the tweezer trapping field, but it also has a contribution from the control and locking field of the cavity.
The pressure dependent heating rate in the optical tweezer without cavity fields present was measured first. Then, the assumption that the cavity field has a negligible impact on heating rate was tested. This causes no significant increase in heating rate compared to the case without cavity fields which was expected.
The cooperativity was characterised by measuring the coupling between optics and mechanics via optomechanically induced transparency. Cooperativity is a regime where the coherent interaction between light and matter overwhelms the system’s tendency to lose energy to its environment.
Conclusion
Dispersive coupling of a levitated silica nanoparticle in high vacuum to a high-finesse optical cavity was demonstrated. This experiment provides a foundation for protocols to move towards strong quantum cooperativity.