Abstract: This talk will start with gravitational wave detectors, which can be viewed as measuring weak gravitational forces. I will discuss the application of Quantum Fisher information of waveform estimation and how the injection of squeezed vacuum, the application of quantum parametric amplification, and the detection of frequency-dependent field quadratures allow us to saturate the QFI, increase the level of QFI, and eventually improve sensitivity. As we turn to estimate the spectral density of a noisy signal, I will show that injecting non-Gaussian optical states and measurement via photon counting (instead of homodyne detection) can be more advantageous. I will finally discuss estimating the quantum state of test masses and how that can allow us to test quantum mechanics in the macroscopic regime.