The ability to directly detect gravitational waves will open a completely new branch of astronomy to view the Universe, one that is inaccessible to electromagnetic-based astronomy. First generation ground-based interferometric gravitational-wave detectors have achieved strain sensitivities of order one part in ten to the twenty-one, at 100 Hz detection frequency. A new generation of detectors are under construction, designed to improve on the sensitivity of the first-generation detectors by a factor of 10. The quantum nature of light will broadly limit the sensitivity of these new instruments. This quantum noise will originate from the quantum vacuum fluctuations that enter the unused port of the interferometer. One of the most promising options for reducing the quantum noise impact and further increasing the sensitivity is applying quantum squeezed vacuum states. These squeezed states have lower noise in one quadrature than the vacuum state. By replacing the quantum vacuum fluctuations entering the interferometer with squeezed vacuum states, the quantum noise impact is reduced. This thesis firstly details the development of a squeezed light source that produces squeezed states applicable for enhancing interferometric gravitational-wave detectors. A doubly-resonant, travelling-wave bow-tie cavity squeezed light source is presented. For the first time, greater than 10 dB of quantum noise suppression across the gravitational-wave detection band is directly observed, and 11.6 dB of quantum noise suppression is observed above 200 Hz. This squeezing cavity design also has benefits with intrinsic isolation to backscattered light. Experiments that quantify this isolation are reported. The properties affecting squeezing magnitude and low-frequency squeezing measurement are discussed. In addition, a modified squeezing-ellipse-phase control technique for squeezed vacuum states is presented. This thesis secondly presents results from the LIGO Squeezed Light Injection Experiment, undertaken to test squeezed light injection into a 4 km interferometric gravitational-wave detector. The results of the experiment show the first measurement of squeezing-enhancement in a 4 km gravitational-wave detector, with 2.15 dB measured above 250 Hz. This represents the best sensitivity to gravitational waves yet achieved at these frequencies by any single gravitational-wave detector to date. An unknown area was whether the addition of a squeezer would introduce noise couplings that degrade the crucial low frequency sensitivity. The results demonstrate that injected squeezed states are compatible with low frequency gravitational-wave measurement. The characterisation of squeezing-injection optical losses and fluctuations of the squeezing angle are also reported. The knowledge and processes gained, from both the squeezed light source development work and the LIGO Squeezed Light Injection Experiment, will inform the design, planning and implementation of squeezed states in future gravitational-wave detectors
