Mechanical oscillators are extensively used in applications ranging from chronometry using quartz oscillators to ultra fast sensors and actuators as in atomic force microscopy and spin/mass/force sensing. The continuous push towards miniaturized high precision sensors has motivated the development of increasingly high quality mechanical oscillators at length scales as small as nanometers, with integration into cryogenic environments used to avoid thermal noise. This raises the prospect of observing mechanical oscillators in a new regime where their dynamics are dictated by quantum, rather than classical, mechanics. Utilizing the quantum behaviour of mechanical oscillators promises to enhance applications in sensing and metrology. In this thesis experimental techniques are reported that enhance optomechanical sensors and enable fundamental experiments in quantum optomechanics. In particular, there is a strong emphasis towards experimentally developing detection based feedback control techniques, for example to enable feedback cooling towards the mechanical ground state or to stabilize parasitic instabilities. In addition to these experimental advances, I detail a real-time estimation strategy that invalidates the use of linear feedback to enhanced the performance of linear optomechanical sensors. Finally, a unique and promising optomechanical systems is developed and characterized based on surface waves of a thin film of superfluid helium-4.
The first topic considered in this thesis is feedback cooling of a generalized optomechanical system. In that chapter a detailed mathematical treatment is derived that highlights the importance of using a dual probe position measurement to accurately characterize a feedback cooled oscillator. The theoretical results are then experimentally verified using a microtoroidal resonator controlled via electrostatic actuation. This chapter provides a brief introduction into feedback cooling and serves to introduce the fundamental optomechanical system that underscores the majority of the work presented in this thesis. In the following chapter, the problem of estimating an unknown force driving a linear oscillator is considered. In this context it is well known that linear feedback control can improve the performance of nonlinear mechanical sensors. However, for completely linear systems, feedback is often cited as a mechanism to enhance bandwidth, sensitivity or resolution. For such systems it is shown that as long as the oscillator dynamics are known, there exists a real-time estimation strategy that reproduces the same measurement record as any arbitrary feedback protocol. Consequently, some form of nonlinearity is required in the controller, plant, or sensor, to gain any advantage beyond estimation alone. This result holds true in both quantum and classical systems, with non-stationary forces and feedback, and in the general case of non-Gaussian and correlated noise. As a proof-of-principle, a specific case of feedback enhanced incoherent force resolution is experimentally reproduced using straightforward filtering on the position measurement record from an optomechanical sensor.
The fundamental limits to optomechanical force sensing are defined by the well-known standard quantum limit (SQL) which is characterized by an equal contribution from two fundamental noise sources, imprecision noise in the form of optical shot noise and quantum back-action noise due to the stochastic arrival of photons. However, well before the SQL is reached, dynamical back-action may become sufficiently strong to severely alter the dynamics of the sensor and hence degrade the sensitivity. In this thesis a technique based on opto-electromechanical feedback control is proposed and experimentally demonstrated. The parametric-instability-induced degradation in mechanical transduction sensitivity is experimentally characterized, then it is shown that application of the proposed feedback technique suppresses the instability, enabling higher optical powers to be used, resulting in enhanced transduction sensitivity.
In addition to dynamical backaction effects, the optical power incident on a mechanical system is typically bounded from above, for example from the thermal damage threshold characterized by intrinsic absorption. If this bound prevents the SQL from being reached, one may enhance the imprecision by surpassing the shot noise limit using quadrature squeezed light. In such a proposal, the reduced noise on one quadrature permits enhanced measurement sensitivity, for example, in a phase sensitive interferometer. While implementation of squeezed light in enhanced sensing has been demonstrated in spectroscopy, and atomic, polarization and gravitational wave interferometry its use for position measurement of mesoscopic mechanical oscillators is still largely unexplored. Here, phase squeezed light is experimentally shown to enhance the transduction sensitivity of a room temperature microtoroidal resonator; extending the applicability of non-classical light to the regime of micro-mechanical oscillators and potentially leading to quantum enhanced feedback.
Undoubtedly, the most exciting prospects of mesoscopic mechanical systems lie in fundamental quantum research where such systems could enable new quantum information technologies and experimental tests of quantum nonlinear mechanics. Among the most successful optomechanical systems are the collective modes of ultra cold atoms where ponderomotive squeezing and ground state cooling have been observed. Therefore the collective motion of thin films of superfluid helium-4 also appears to be a promising candidate for quantum optomechanics given its zero viscosity flow and low effective mass. To this end, for the first time, precise optical readout and control of superfluid helium-4 surface waves is demonstrated. Readout is achieved by evanescent coupling of the film to an optical whispering gallery mode resonator, allowing the intrinsic Brownian motion to be directly observed. Laser control is achieved by detuning from the optical cavity, resulting in a modified spring constant and dynamical heating and cooling of the superfluid mode. By weakly modulating the injected optical field the mechanical mode is also shown to exhibit a strong Duffing nonlinearity. This system, with the prospect of extremely low dissipation rates and strong mechanical nonlinearities, is a promising candidate for fundamental research into highly non-classical mechanical systems.