Brillouin cavity optomechanics with whispering-gallery microresonators

Abstract

Cavity quantum optomechanics is a field of investigation which studies the interaction between optical and mechanical degrees of freedom applying the powerful methods of quantum optics. These theoretical methods and experimental techniques have been developed through the second half of the 20th century and gained significant momentum following the invention of the laser. Upon the realisation that coherent manipulation of the motion of mesoscopic, or even macroscopic, mechanical systems was feasible using laser light, interest in cavity quantum optomechanics grew and groups across the globe began working towards the preparation of non-classical mechanical states. The field is now well established, but the preparation of non-classical states of the motion via interaction with optical fields remains challenging to explore, which is believed will open the door to studies of fundamental physics regarding decoherence, the quantum-to-classical transition or may even shed light on the interface between quantum mechanics and gravity. This thesis explores Brillouin scattering in whispering-gallery-mode microresonators. The parametric coupling between high frequency (11 GHz) elastic waves and infrared (1550 nm) light via electrostriction is used to demonstrate several interesting cavity optomechanical phenomena. This thesis contributes to developing this new approach to optomechanics in four key ways. By use of a pair of optical resonances of different transverse structure spaced by the material's Brillouin shift, reaching the cavity optomechanical strong coupling regime was experimentally demonstrated with a fused silica microrod resonator. Operation in this regime is crucial for many optomechanical protocols, importantly optomechanical state-swap between the optical and mechanical modes. A similar resonator was employed to demonstrate measurement-enhanced optomechanical sideband cooling. Here the measurement record of the continuously monitored heterodyne measurement of Brillouin anti-Stokes scattered light is used to reduce the phase-space uncertainty of the mechanical state, effectively cooling the mode beyond the sideband-cooling limit. For this purpose the conservation of Gaussianity of the state (initially a thermal state) under linear measurement (heterodyne detection) is used within a stochastic master equation approach. In another experiment, single-phonon addition and subtraction to a mechanical thermal state was demonstrated, showing a characteristic doubling of the mean phonon number of the mechanical mode. For this experiment a crystalline microresonator of barium fluoride was used, a material which allows reaching superb optical quality factors combined with highest elastic isotropy for a crystalline substance. The non-Gaussianity of the single-quantum subtracted state is demonstrated. Finally, the platform of Brillouin cavity optomechanics with high-frequency phonons in crystalline (barium fluoride) whispering-gallery resonators at cryogenic temperatures is discussed.