Quantum theory of light interaction with a Lorenz-Mie particle: Optical detection and three-dimensional ground-state cooling

TL;DR

This paper develops a quantum theoretical framework for the interaction of light with levitated dielectric spheres, enabling ground-state cooling of their three-dimensional motion through optical detection and feedback, with implications for fundamental physics tests.

Contribution

It derives a comprehensive Hamiltonian including Stokes and anti-Stokes processes for arbitrary sphere sizes, and proposes feasible experimental setups for ground-state cooling beyond the point-dipole approximation.

Findings

Predicts configurations for three-dimensional ground-state cooling

Provides formulas for recoil heating and scattering patterns

Identifies regimes for effective optical detection and feedback

Abstract

We analyze theoretically the motional quantum dynamics of a levitated dielectric sphere interacting with the quantum electromagnetic field beyond the point-dipole approximation. To this end, we derive a Hamiltonian describing the fundamental coupling between photons and center-of-mass phonons, including Stokes and anti-Stokes processes, and the coupling rates for a dielectric sphere of arbitrary refractive index and size. We then derive the laser recoil heating rates and the information radiation patterns (the angular distribution of the scattered light that carries information about the center-of-mass motion) and show how to evaluate them efficiently in the presence of a focused laser beam, in either a running- or a standing-wave configuration. This information is crucial to implement active feedback cooling of optically levitated dielectric spheres beyond the point-dipole approximation. Our results predict several experimentally feasible configurations and parameter regimes where optical detection and active feedback can simultaneously cool to the ground state the three-dimensional center-of-mass motion of dielectric spheres in the micrometer regime. Scaling up the mass of the dielectric particles that can be cooled to the center-of-mass ground state is relevant not only for testing quantum mechanics at large scales but also for current experimental efforts that search for new physics (e.g., dark matter) using optically levitated sensors.