Introduction

High-quality optical cavities – and hence optical coatings – have emerged as an indispensable tool in physics. One of their key uses is in precision sensing and spectroscopy, where high-performance coatings bridge a broad spectrum of applications from large-scale gravitational wave interferometers (see Chapter 14) to atomic optical clocks (see Chapter 15). Radiation pressure forces inside of such cavities, i.e. the momentum transfer of photons onto the cavity boundaries, impose a fundamental limit on their performance, the so-called standard quantum limit (see Section 1.4), when combined with unavoidable quantum fluctuations in a laser beam. Another limitation arises from random phase fluctuations that are induced by the thermal noise of the coating's mechanical motion (see Chapter 3). With the improved performance of optical coatings, optical cavities are gradually becoming sensitive enough to observe these optomechanical effects. Curiously, what is being considered a nuisance by some fields of research has led within only a few years to the development of a completely new field of optical physics – cavity optomechanics – that exploits optomechanical interactions in high-finesse optical cavities as a new interface between light and matter. A common goal in this field is to exploit the quantum regime of mechanical devices through quantum optical control. This work has immediate implications with respect to fundamental questions, such as the quantum-classical transition of massive objects, and new applications including quantum limited mechanical sensors or mechanical interfaces for quantum information processing.