  Squeezed states of light have been recently used to improve the sensitivity
of laser interferometric gravitational-wave detectors beyond the quantum limit.
To completely establish quantum engineering as a realistic option for the next
generation of detectors, it is crucial to study and quantify the noise coupling
mechanisms which injection of squeezed states could potentially introduce. We
present a direct measurement of the impact of backscattered light from a
squeezed-light source deployed on one of the 4 km long detectors of the Laser
Interferometric Gravitational Wave Observatory (LIGO). We also show how our
measurements inform the design of squeezed light sources compatible with the
even more sensitive advanced detectors currently under construction, such as
Advanced LIGO.
