Gravitational-waves, predicted by Einstein's general theory of relativity, will open up a new window into the universe. Directly detecting them and eventually extracting information about astronomical phenomena requires new instruments with extremely high sensitivity, which only became feasible recently. The current paradigm of gravitational-wave detectors uses kilometre-scale laser interferometers with suspended mirror-endowed test masses; the so-called advanced gravitational-wave detectors currently under construction are expected to achieve the first direct detection of gravitational waves. However, in order to establish gravitational wave detectors as efficient sources for astrophysical information, the signal to noise ratio of these instruments needs to be improved further. Advanced detectors are expected to be limited by quantum noise around their most sensitive band, which arises from fundamental quantum fluctuations in the optical field. On the one hand, this implies that we need to use quantum mechanics to describe them, and that we must manipulate the quantum coherence to enhance their sensitivities. On the other hand, they provide us, for the first time, with platforms for probing the quantum behaviour of macroscopic objects --- kilogram-scale test masses. In this project, we aim (i) to explore different approaches for reducing quantum noise and (ii) to study tests of quantum mechanics via precision measurements of quantum dynamics of the macroscopic test masses. In particular, we will (i) develop numerical tools for optimizing the quantum noise of complex interferometer configurations; (ii) use quantum measurement theory to better understand the fundamental quantum limit of gravitational wave detectors; and (iii) make a systematic study of how quantum dynamics of macroscopic test masses encode the information of possible modifications to quantum mechanics.