  Nearly a century after Einstein first predicted the existence of
gravitational waves, a global network of earth-based gravitational wave
observatories is seeking to directly detect this faint radiation using
precision laser interferometry. Photon shot noise, due to the quantum nature of
light, imposes a fundamental limit on the attometer level sensitivity of the
kilometer-scale Michelson interferometers deployed for this task. Here we
inject squeezed states to improve the performance of one of the detectors of
the Laser Interferometer Gravitational-wave Observatory (LIGO) beyond the
quantum noise limit, most notably in the frequency region down to 150 Hz,
critically important for several astrophysical sources, with no deterioration
of performance observed at any frequency. With the injection of squeezed
states, this LIGO detector demonstrated the best broadband sensitivity to
gravitational waves ever achieved, with important implications for observing
the gravitational wave Uni- verse with unprecedented sensitivity.
