The sensitivity of laser interferometers used for the detection of
gravitational waves (GWs) is limited by quantum noise of light. An improvement
is given by light with squeezed quantum uncertainties, as employed in the GW
detector GEO600 since 2010. To achieve simultaneous noise reduction at all
signal frequencies, however, the spectrum of squeezed states needs to be
processed by 100m-scale low-loss optical filter cavities in vacuum. Here, we
report on the proof-of-principle of an interferometer setup that achieves the
required processed squeezed spectrum by employing Einstein-Podolsky-Rosen (EPR)
entangled states. Applied to GW detectors, the cost-intensive cavities would
become obsolete, while the price to pay is a 3dB quantum penalty
