A gravitational wave observatory operating beyond the quantum shot-noise limit

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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 relativity1 and are generated, for example, by black-hole binary systems2. Present GW detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between mirrors suspended in vacuum. 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 light3—offers a solution to this problem. Here we demonstrate the squeezed-light enhancement of GEO 600, which will be the GW observatory operated by the LIGO Scientific Collaboration in its search for GWs for the next 3–4 years. GEO 600 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 astronomy4.

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Figure 1: Simplified optical layout of the squeezed-light enhanced GEO 600 observatory.
Figure 2: View into the GEO 600 central building.
Figure 3: Nonclassical reduction of the GEO 600 instrumental noise using squeezed vacuum states of light.


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The authors gratefully acknowledge the support of the United States National Science Foundation for the construction and operation of the LIGO Laboratory and the Science and Technology Facilities Council of the United Kingdom, the Max Planck Society, the Deutsche Forschungsgemeinschaft, the cluster of excellence QUEST (Centre for Quantum Engineering and Space-Time Research), the BMBF, the Volkswagen Foundation, and the State of Niedersachsen/Germany for support of the construction and operation of the GEO 600 detector. The authors also gratefully acknowledge the support of the research by these agencies and by the International Max Planck Research School (IMPRS), the SFB TR7, the FP7 project Q-ESSENCE, the Australian Research Council, the Council of Scientific and Industrial Research of India, the Istituto Nazionale di Fisica Nucleare of Italy, the Spanish Ministerio de Educación y Ciencia, the Conselleria d’Economia, Hisenda i Innovació of the Govern de les Illes Balears, the Royal Society, the Scottish Funding Council, the Scottish Universities Physics Alliance, The National Aeronautics and Space Administration, the Carnegie Trust, the Leverhulme Trust, the David and Lucile Packard Foundation, the Research Corporation, and the A. P. Sloan Foundation.

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A list of authors and their affiliations appear in the Supplementary Information The activities of the LIGO Scientific Collaboration (LSC) cover modelling astrophysical sources of gravitational waves, setting sensitivity requirements of observatories, designing, building and running observatories and researching new techniques to increase the sensitivity of these observatories, and performing searches for astrophysical signals contained in the data. The principal investigators of the advancement reported here are H.G. and R.S., being responsible for GEO 600 and for the squeezed-light laser during the past 3 years, in which this experiment was prepared and conducted, respectively. In this period a great number of the LSC members contributed directly to the success of this project. H.V. and H.G. supervised the integration of the squeezed-light laser into GEO 600. Together with A.K., they took and analysed the data shown. The initial manuscript was written by a team involving those mentioned above together with R.S. The manuscript went into a two-stage LSC-wide review process, which was organized and led by R.F., T.R.C., M.H., and D.Sigg. All authors approved the final version of the manuscript.

Correspondence to R. Schnabel.

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