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 Gravitational-wave 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 atomnoptomechanical system. This uses a cloud of cold atoms as a lquantum handler 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 sympathetically cool the mechanical oscillator to its ground state and show that they are experimentally feasible. In addition, we combine this with feedback cooling of the mechanical device and show that there is a significant improvement in cooling capacity in the majority of parameter regimes.We then detail a novel pulsed optomechanical interface that permits one to perform opticalnmechanical state swaps in the unresolved sideband limit. This can rapidly transfer quantum information between the light and mechanics for storage or for the preparation of nonclassical states. Our protocol involves a single deep-sub-mechanical-period optical pulse which interacts with a mechanical oscillator three times over half a mechanical period. This procedure can be used to perform near-ground state cooling, prepare squeezed mechanical states, and engineer nonclassical vibrational states which exhibit Wigner negativity. These are important building block for probing quantum decoherence and macroscopicity.The final theoretical proposal contained in this thesis involves a squeezing-driven thermodynamic machine that operates in a regime where the common rotating wave approximation (RWA) does not provide an accurate description of the resonatorrs dynamics. We show that this device can act as a heat engine, a refrigerator, or a heat pump, depending upon the parameters chosen. Remarkably, this rich behaviour vanishes in the RWA limit, leaving only the heat pump phase. This points to the emergence of rich thermodynamical behaviours beyond the RWA regime. Furthermore, we provide an outline of how a sequence of pulsed optomechanical interactions may be used to squeeze an arbitrary initial mechanical state over a timescale much shorter than the mechanical period.Finally, we present a summary of experimental work aimed at fabricating low-frequency (f), highquality-factor (Q) mechanical oscillators from epitaxial silicon carbide films. Optical characterisation of our doubly-clamped beam resonators reveals Qt f products of s 1012 Hz, making them state-ofthe-art for room temperature string resonators at the time of publication. This indicates that they are well-isolated from their thermal environment, although not yet in the quantum coherent oscillation regime at room temperature. The insights gained during the fabrication and testing of these devices will inform the design of next-generation, high-Q SiC mechanical oscillators for optomechanical and electromechanical applications.