Thermal Noise Measurement Below the Standard Quantum Limit

TL;DR

This paper introduces a method to measure thermal noise below the quantum noise limit in an optical cavity, enabling detailed analysis of quantum noise suppression effects at cryogenic temperatures.

Contribution

The paper presents a novel technique for characterizing thermal noise independently from quantum noise, even when thermal noise is below the quantum limit.

Findings

Thermal noise was measured up to 5 dB below the SQL.

Quantum noise suppression from the optical spring reached 10 dB below the SQL.

The method enables precise thermal noise characterization at cryogenic temperatures.

Abstract

We present a method characterizing thermal noise in an optical cavity independent from quantum noise despite the thermal noise falling below the quantum noise limit. Using this method, we measured the thermal noise contribution from a GaAs AlGaAs micro-mirror suspended on a GaAs cantilever microresonator when brought to a cryogenic temperature (approximately 25 K) and incorporated into a Fabry Perot cavity. An optical spring is formed in this optical cavity. Previously, this setup exploited an optical spring to produce a displacement sensitivity falling 2.8 dB below the free-mass standard quantum limit (SQL), as reported by Cullen et. al. Here we use a similar setup to measure thermal noise which fell a maximum of 5 dB below the SQL. This measurement, in turn, allowed for an investigation of quantum noise suppression resulting from the optical spring effect, falling a maximum of 10 dB below the SQL.