Optical Cavities for Optical Atomic Clocks, Atom Interferometry and Gravitational-Wave Detection

Abstract

A mis hermanas Ángela e Isabel y a mis abuelas Pepita y Consuelo. v Abstract I t is an extremely exciting time for physics. In the last 100 years we have moved from the formulation of Einstein's general relativity to the first direct observation of gravitational waves in late 2015 by the Laser Interferometer Gravitational-wave Observatory (LIGO). Within that time science and technology have come a long way: we have learned to use light to cool atoms to nearly absolute zero temperature, and to use atomic transitions in the microwave and optical regimes to devise the most accurate time and frequency references. We have observed the wave-like behaviour of cold atoms in diffraction experiments using both micro-fabricated structures and the periodic structure of light beams. Exploiting this wave-like behaviour, we have constructed atom interferometers which allow us to test and measure gravity in a new scale. All of these amazing experiments have one thing in common, from LIGO's giant 4 km arms to the transportable atomic clocks sent to space or the atom interferometers that will someday replace current navigation systems, they all make use of a device that has become essential in many areas of science and technology: the Fabry-Perot optical cavity.This thesis delves deeply into the application of optical cavities at the forefront of experimental physics, and it is divided into three parts, each pertaining to a different field where optical cavities are a key technology. Part I of the thesis follows the development of a next-generation, thermal-noise-limited, ultra-stable optical cavity for use as the reference oscillator in optical atomic clocks. This is part of the effort being carried out at the National Physical Laboratory in the UK in the field of precision metrology of time and frequency towards redefining the base SI unit of time, the second, in terms of an optical frequency standard. Part II presents work in the application of optical cavities for enhancing the sensitivity of atom interferometers, with an analysis of multi-photon Bragg diffraction inside an optical cavity. This led to an exploration of the true potential and limitations of the technique. The findings were used to aid the design of the MIGA experiment in France, and design a multi-mirror cavity for enhanced atom interferometry that overcomes some of the limitations of two-mirror cavities. Lastly, Part III presents work towards enhancing current and future laser interferometer gravitational-wave detectors by using near-unstable optical cavities in order to reduce the mirror coating thermal-noise floor, and by modelling parametric oscillatory instabilities that arise in the interferometers from the coupling between the optical field and the mechanical resonances of the test masses. The work carried out during the extent of this PhD, and the variety of contexts, accounts for the relevance and versatility of such an elegant setup as the Fabry-Perot optical cavity is. vii AcknowledgementsThis work was realised with the financial support of the Defen...

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