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 micromirror suspended on a GaAs cantilever microresonator when brought to a cryogenic temperature ($\sim 25\mathrm{K}$) and incorporated into a Fabry-Pérot 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. [Phys. Rev. Lett. 133, 113602 (2024)]. 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, a new benchmark for sub-SQL optomechanical measurements.

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• Quantum optics
• Quantum states of light
• Gravitational wave detectors