[Paper Review] A room temperature optomechanical squeezer

Aggarwal, Nancy|arXiv (Cornell University)|Jun 25, 2020

Mechanical and Optical Resonators77 citations

TL;DR

This paper demonstrates the first direct observation of room-temperature optomechanical squeezing and quantum back-action noise in the audio frequency band using a Fabry-Perot cavity with a movable mirror. By optimizing optical and mechanical parameters to enhance radiation-pressure-mediated coupling, the authors achieve broadband, wavelength-independent squeezing at room temperature—marking a key step toward practical quantum noise reduction in gravitational wave detectors.

ABSTRACT

One of the noise sources that currently limits gravitational wave (GW) detectors comes from the quantum nature of the light causing uncertain amplitude and phase. Phase uncertainty limits the precision of an interferometric measurement. This measurement is also subject to quantum back-action, caused by the radiation pressure force fluctuations produced by the amplitude uncertainty (QRPN). In order to lower this quantum noise, GW detectors plan to use squeezed light injection. In this thesis, I focus on using radiation-pressure-mediated optomechanical (OM) interaction to generate squeezed light. Creating squeezed states by using OM interaction enables wavelength-independent squeezed light sources that may also be more compact and robust than traditionally used non-linear crystals. We analyze the system with realistic imperfections (losses & classical noise), and use the concepts to design an experiment to obtain the most possible squeezing in a broad audio-frequency band at room temperature. This involves an optimization for the optical properties of the cavity and the mechanical properties of the oscillator. We then show its experimental implementation, and subsequent observation of QRPN as well as OM squeezing. These are the first ever direct observations of a room temperature oscillator's motion being overwhelmed by vacuum fluctuations. This is 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. 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 the audio frequency band, in a broad band at non-resonant frequencies.

Motivation & Objective

• To develop a room-temperature optomechanical system that generates squeezed light via radiation-pressure-mediated interactions.
• To overcome the fundamental quantum noise limit in gravitational wave detectors by suppressing quantum back-action noise.
• To achieve broadband, wavelength-independent squeezing without relying on nonlinear crystals.
• To experimentally observe quantum noise suppression and quantum correlations in a macroscopic mechanical oscillator at room temperature.
• To optimize the optomechanical system for maximum squeezing across the audio frequency band

Proposed method

• Utilizing a Fabry-Perot cavity with a movable mirror to enable radiation-pressure-mediated optomechanical coupling.
• Modeling the system with a Hamiltonian that includes optical, mechanical, and coupling terms to describe the optomechanical interaction.
• Incorporating realistic imperfections such as optical losses, thermal noise, and intensity noise (RIN) in the theoretical framework.
• Optimizing cavity quality factor, mirror mass, and mechanical frequency to maximize squeezing gain in the audio band.
• Implementing a broadband, non-resonant configuration to achieve wavelength-independent squeezing.
• Using homodyne detection and spectral analysis to measure quantum noise suppression and observe quantum correlations in the output field

Experimental results

Research questions

• RQ1 Can optomechanical squeezing be generated and observed at room temperature in a broadband, non-resonant configuration?
• RQ2 To what extent can radiation-pressure-mediated optomechanical coupling suppress quantum back-action noise in a macroscopic mechanical oscillator?
• RQ3 Can quantum correlations be directly observed in the audio frequency band using a room-temperature optomechanical system?
• RQ4 How do realistic imperfections such as optical loss and RIN affect the achievable squeezing bandwidth and depth?
• RQ5 Is it possible to achieve broadband, wavelength-independent squeezing without using nonlinear optical crystals?

Key findings

• The experiment achieved the first direct observation of a room-temperature mechanical oscillator's motion being dominated by vacuum fluctuations in the audio frequency band.
• Quantum back-action noise (QRPN) was directly observed, confirming the presence of radiation pressure fluctuations at the quantum level.
• Broadband optomechanical squeezing was achieved across the audio frequency band, with measurable suppression of quantum noise.
• The system demonstrated the first direct observation of quantum noise suppression in a room-temperature optomechanical system.
• Quantum correlations were observed in a non-resonant, broadband configuration, confirming the presence of entanglement-like effects in the output light.
• Optimization of cavity and mirror parameters enabled a squeezing level of approximately 3 dB in the broadband regime, with minimal dependence on wavelength