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 (1) and are generated e.g. by black-hole binary
systems (2). Current GW detectors are Michelsontype
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 (3)
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 (4).