A room temperature optomechanical squeezer

Abstract

Decades of advancement in technologies pertaining to interferometric measurements have made it possible for us to make the first ever direct observation of gravitational waves (GWs). These GW emitted from violent events in the distant universe bring us crucial information about the nature of matter and gravity. In order for us to be able to detect GWs from even farther or weaker sources, we must further reduce the noise in our detectors. One of the noise sources that currently limits GW detectors comes from the fundamental nature of measurement itself. When a certain measurement reaches very high precision, the Heisenberg uncertainty principle comes into play. In GW detectors, this uncertainty manifests itself in the quantum nature of the light. Due to its quantum nature, light (or electromagnetic field) has an uncertain amplitude and phase.

Since the interferometric measurement is directly measuring the phase of light, this uncertainty poses a limit on the precision of GW measurements. Additionally, this measurement is also subject to quantum back-action, which arises due to the radiation pressure force fluctuations caused by the amplitude uncertainty (QRPN). In order to lower this quantum noise, GW detectors plan to use squeezed light injection. Squeezed light is a special quantum state of light which has lower uncertainty in a certain quadrature, at the expense of higher uncertainty in the orthogonal quadrature. In this thesis, I focus on using radiation-pressure-mediated optomechanical (OM) interaction to generate squeezed light. Creating squeezed states by using optomechanical interaction opens up possibilities for engineering truly wavelength-independent squeezed light sources that may also be more compact and robust than traditionally used non-linear crystals.

Additionally, this project inherently involves studying the OM interaction, which is the mechanism for back-action noise in GW detectors. Our basic setup is a Fabry-Perot cavity with a movable mirror. We start by understanding the physics of this system in the presence of realistic imperfections like losses and classical noise. This study furthers the previous work done on OM squeezing in an ideal Fabry-Perot cavity. We use this understanding of the system to optimize the experimental parameters to obtain the most possible squeezing in a broad audio-frequency band at room temperature. This optimization involves choosing the optical properties of the cavity, and the mechanical properties of the oscillator. We then present the experimental implementation of this design, and subsequent observation of QRPN as well as OM squeezing from the optimized design.

These observations are the first ever direct observation of a room temperature oscillator's motion being overwhelmed by vacuum fluctuations. More so, this is also the first time it has been shown in the low frequency band, which is relevant to GW detectors, but poses its own technical challenges, and hence has not been done before. Being in the back-action dominated regime along with optimized optical properties has also enabled us to observe OM squeezing in this system. That is the first direct observation of quantum noise suppression in a room temperature OM system. It is also the first direct evidence of quantum correlations in a audio frequency band, in a broadband at non-resonant frequencies.