Quantum optomechanics in the unresolved sideband regime

Abstract

A cavity optomechanical system is formed when the resonance frequency of an optical cavity is dependent upon the position of a mechanical oscillator. This is achieved in a plethora of physical systems, spanning the range from cold atom clouds trapped in miniaturised optical cavities through to the kilogram-scale test masses of the four kilometre long advanced Laser Interferometer Gravitationalwave Observatory. The dynamical coupling between light and vibration thus engendered modifies both the optical and mechanical behaviours, and is responsible for a number of intriguing phenomena.Indeed, with optomechanical tools it becomes possible to observe the fact that macroscopic mechanical oscillators are governed by the laws of quantum mechanics, rather than familiar classical equations of motion. The preparation of mechanical oscillators in truly quantum states, such as squeezed states and two-mode entangled states, has already been experimentally demonstrated. It is predicted that further advances along these lines will permit sensitive tests of fundamental physics, in addition to delivering significant improvements to sensor and information processing technologies.The majority of optomechanical protocols developed to date operate in the so-called resolved sideband limit, where the cavity bandwidth is small compared to the mechanical resonance frequency.This ensures that the optical cavity may be employed as a frequency-selective element for performing coherent control, but it also limits the quantity of circulating optical power and prevents one from reaching the standard quantum limit for position and force detection.In this thesis we report research that has expanded the optomechanical toolbox in the unresolved sideband regime, where the linewidth of the cavity is larger than the mechanical resonance frequency. This is a natural operating regime for large-mass and/or low-frequency mechanical oscillators; microcavity devices with small mode volumes and large optomechanical coupling rates; high-bandwidth, high-quantum-efficiency position and force detection; optical pulse manipulation; and hybrid devices couple to low-frequency quantum ancillae, such as the motional degrees of freedom of cold atoms.We begin this thesis by providing introductory material which covers advances in optomechanics and related fields; useful theoretical tools and their physical meanings; and some insights into technical details. These materials are included in the hope that they might be useful to other researchers in the field.Our first contribution to optomechanics in the unresolved sideband regime is a theoretical study of a remotely-coupled hybrid atom-optomechanical system. This uses a cloud of cold atoms as a 'quantum handle' with which to control the state of a mechanical oscillator. The two systems are coupled via light, which significantly relaxes experimental requirements on integration of vacuum and cryogenic apparatus. We derive the conditions under which laser cooling of the atomic ensemble permits one to sympathetica...

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“…In this description we consider the fields to be classical and describe the heterodyne scheme based on Ref. [92].…”

“…3.13. The experimental setup was built by James Bennett [92]. The details about the experimental setup used to measure the resonance frequency and quality factor can be found in James's thesis [92], however I will discuss those details that are relevant to the work I did.…”

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