Around the globe several observatories are seeking the first direct detection of gravitational waves (GWs). These
waves are predicted by Einstein's General Theory of Relativity [Einstein, A., Annalen der Physik 49, 769-822
(1916)] and are generated e.g. by black-hole binary systems [Sathyaprakash, B. S. and Schutz, B. F., Living
Rev. Relativity 12, 2 (2009)]. Current GW detectors are Michelson-type kilometer-scale laser interferometers
measuring the distance changes between in vacuum suspended mirrors. The sensitivity of these detectors at
frequencies above several hundred hertz is limited by the vacuum (zero-point) fluctuations of the
electromagnetic field. A quantum technology - the injection of squeezed light [Caves, C. M., Phys. Rev. D 23,
1693-1708 (1981)] - offers a solution to this problem. Here we demonstrate the squeezed-light enhancement
of GEO600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs
for the next 3-4 years. GEO600 now operates with its best ever sensitivity, which proves the usefulness of
quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy.