  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.
