Quantum back-action evasion on a kg-scale mirror

The Laser Interferometric Gravitational-Wave Observatory (LIGO) provides a quantum-limited platform to study measurement back-action in macroscopic optomechanical systems. We feedback-trap the differential motion of its 40 kg mirrors to form a harmonic oscillator at \(\sim\)151 Hz with quality factor \(\approx\)100 and use it to probe correlations between the quantum noises that form the imprecision and back-action components. Injecting phase-squeezed light reduces imprecision but amplifies measurement back-action, while frequency-dependent squeezing (FDS) introduces correlations that partially cancel the latter. We observe a \(\approx 45\,\%\) reduction in total displacement-noise power within \(\pm 3\) coherence rates of the mechanical resonance. These measurements witness quantum back-action on a kilogram-scale oscillator without any classical noise subtraction, and its evasion through correlations from frequency-dependent squeezed-light. Extending this approach in future detector configurations promises a path toward ground-state operation and experimental probes of gravity-induced decoherence.