This lecture covers the use of squeezed light in gravitational wave detectors, focusing on the modulation of continuous-wave light, spectrum of modes in squeezed states, and the first squeezed-light source for a GW detector. It explains the higher sensitivities achieved with squeezed light, the 'quantum weirdness' associated with it, and its applications in reducing shot noise in optical measurements and quantum teleportation. The lecture also discusses the potential of the Einstein Telescope to enhance sensitivity in observing gravitational waves.