Degree

Doctor of Philosophy (PhD)

Department

Physics

Document Type

Dissertation

Abstract

Quantum mechanics places sensitivity restrictions on physical measurements. These limitations manifest themselves in interferometric force and displacement measurements as uncertainty both in the photon number measured at a photodetector (shot noise), and in the quantum radiation pressure applied to the interferometer (quantum back action noise manifested as quantum radiation pressure noise). The balance between shot noise and quantum radiation pressure noise imposes the Standard Quantum Limit (SQL). A large body of work has been dedicated to measuring these quantum effects and mitigating them. Owing to the inherently small scale at which quantum effects are visible, classical effects must be subverted before these quantum effects are studied. One formidable source of noise, present in any system, is thermal noise.

The central goal of the work presented in this thesis is to measure the thermal noise contribution from a GaAs/AlGaAs micro-mirror suspended on a GaAs cantilever microresonator when brought to a cryogenic temperature ∼ 25 K. These materials exhibit an intrinsically low mechanical loss which results in a small contribution of thermal noise and high reflectivity allowing for a high finesse when used to form one end of an op tomechanical Fabry–Pérot cavity. In this configuration, the cantilever mirror enabled the observation of optomechanically generated squeezed light at room temperature [1], the observation of quantum back-action (QBA) in the audio band [2], a demonstration that this QBA noise is reduced via squeezed light injection [3] and that it can be suppressed when operating a detuned optomechanical (OM) cavity [4]. Finally, the low thermal noise allowed for a measurement of sensitivities falling below the free mass SQL [5]. All these demonstrations were aimed towards improving ground based interferometric gravitational wave detectors (LIGO, Virgo, KAGRA). Our motivation to study the thermal noise produced by the GaAs/AlGaAs cantilever mirror is also, in part, to improve gravitational wave detectors. Because of the low mechanical loss, and corresponding low thermal noise contribution of the coating, combined with its high reflectivity, an effort to implement GaAs/AlGaAs crystalline mirror coatings into gravitational wave detectors is currently underway [6].

While reducing thermal noise to a level that allows these measurements is a noteworthy accomplishment, it produces an interesting quandary: how to characterize the thermal noise at this level while removing the contribution from quantum noise. This thesis explores a quantum correlation measurement which removes shot noise, and a backaction evasion technique which utilizes the optical spring effect to mitigate quantum radiation pressure noise, comparing this technique to more conventional approaches to limiting quantum back-action noise. The result of this investigation is twofold: a measurement of thermal noise falling 5 dB below the SQL, and in turn a measurement of quantum noise, free from thermal noise, which falls 10 dB below the SQL.

Date

1-10-2025

Committee Chair

Thomas Corbitt

Included in

Physics Commons

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