LIGO Document T2500248-v1

2025 LIGO SURF: Mapping and Correcting the Surface of the GQuEST End Mirrors

Document #:
LIGO-T2500248-v1
Document type:
T - Technical notes
Other Versions:
Abstract:
The GQuEST (Gravity from the Quantum Entanglement of Space-Time) end mirrors are susceptible to surface deformations which can cause incomplete interference at the differential output of the interferometer. I used an optical profiler to image the surface of these end mirrors and calculated the coupling coefficients to higher order Hermite-Gauss (HG) modes. A custom adjustable mirror mount was used to actuate on the end mirrors to decrease quadratic deformations in the mirror surface, and mitigate coupling to the \( \text{HG}_{02} \), \( \text{HG}_{11} \), and \( \text{HG}_{20} \) modes. The mount corrected \( 3.0\times10^{-2} \) diopters of focusing power with a precision of \( 2.6 \times 10^{-4} \) diopters, reducing the total mirror loss from \( 43,461 \text{ ppm} \) to \( 312 \text{ ppm} \). This enables GQuEST to lower the classical noise floor by minimizing the mirror thickness while minimally increasing the contrast defect. The benefit consists in a sufficiently low end mirror loss to confidently distinguish and measure (or bound) the quantum gravity fluctuations.
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