A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit
Authors: Hartmut Grote, Roman Schnabel, Henning Vahlbruch
Affiliation: Institute for Gravitational Physics, Leibniz Universität Hannover and Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute, AEI), Hannover, Germany
Quantum-squeezed light has now gone beyond the development-phase in the confinement of laboratory, and is now improving the sensitivity of the German/British gravitational-wave (GW) observatory GEO600 located close to Hannover (Germany). This is what our recent publication in Nature Physics reports . Arguably -- for the first time -- a technology, which exploits the special findings of quantum mechanics, is put into a real application in metrology. The idea that squeezed states of light might be valuable in the field of GW detection is already 30 years old , but only now realized.
Gravitational waves are predicted by Einstein’s general theory of relativity, and are generated, for example, by black-hole binary systems. In principle, they can even be observed on earth by kilometer-scale Michelson-type laser interferometers measuring the changes in distance between mirrors suspended in vacuum. However, so far they have not been observed directly. Fig. 1 shows one of the suspended mirrors of the GW observatory GEO600 (mirror at bottom right). More details about GEO600 can be found in Ref. .
Fig. 1: One of four suspended 5 kg test-masses of the GEO600 gravitational wave detector (mirror at bottom right) together with reaction and suspension point masses. Changes in the 600 m distance between the test-masses are now measured with new quantum-squeezed laser light (image by courtesy of Harald Lück, AEI).
In the past, its measurement sensitivity at frequencies above several hundred hertz has been limited by the vacuum (zero-point) fluctuations of the electromagnetic field. Now, GEO600 incorporates an additional laser – a squeezed light laser – which has previously been presented in Ref. . This new laser operates below its laser threshold and is based on parametric down-conversion of 532nm light thereby producing quantum correlated quasi-monochromatic photon-pairs at 1064 nm.
Fig. 2: View into the GEO600 central building. The squeezed-light laser is shown in the front. Its optical table is surrounded by several vacuum chambers containing suspended interferometer optics such as the mirror shown in Fig.1.
This rather dim laser mode is matched into the GEO600 laser interferometer where it interferes with the observatory’s ordinary 3 kW laser beam. As a result, the vacuum fluctuations at the photo-diode in GEO600’s output port are reduced (“squeezed”). GEO600 now operates with its best ever sensitivity being 50% higher than before at signal frequencies above 1 kHz, as shown in Fig. 3. Our success has finally proven the qualification of squeezed light as a key technology for future GW astronomy.
Fig. 3: Nonclassical reduction of the GEO600 instrumental noise (calibrated to a space strain) using squeezed vacuum states of light. The black trace shows the observatory noise spectral density without the injection of squeezed light. An injection of squeezed vacuum states into the interferometer leads to a broadband noise reduction of up to 50% (3.5 dB in power, red trace). The peaks are not due to gravitational waves. They appear at well-known frequencies and are mainly due to violin modes of the mirror’s pendulum suspensions.
During the past years GEO600 has been made one of the most sensitive measuring devices ever built. Up to one hundred scientists from Germany, UK and other countries have contributed . All of them are also members of the LIGO Scientific Collaboration (LSC) . A number of new technologies have been developed, some of which are by now also implemented in the other gravitational wave observatories, namely the US LIGO and the Italian/French Virgo project. In this course of steady improvements, “classical” techniques have been driven to its extremes and GEO600 eventually became so sensitive that the squeezing technology became worth the effort.
Squeezed light has been generated in several research laboratories in the world before, however, it is a rather involved technique and leaving the conditions of a laboratory to an environment of continuous operation is difficult. All these aspects explain why only now the squeezed light is used for the first time in a GW observatory. In the past 5 years about a dozen of physicists have been working on the squeezed laser development in Hannover to enable this leap . A recent review article summarizes the progress on squeezed light generation over the past years .
We are convinced that squeezed light will be used in all GW observatories around the globe in near future. The squeezing technology is certainly not exhausted yet. We believe that improvements of up to 200% are feasible with current technology.
 The LIGO Scientific Collaboration, "A gravitational wave observatory operating beyond the quantum shot-noise limit", Nature Physics, doi:10.1038/nphys2083 (Published online September 11, 2011). Abstract. Free Download.
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 Willke, B. et al., "The GEO 600 gravitational wave detector", Class. Quantum Grav. 19, 1377 (2002). Abstract; Grote, H. et al., "The GEO 600 status", Class. Quantum Grav. 27, 084003 (2010). Abstract.
 H. Vahlbruch, A. Khalaidovski, N. Lastzka, C. Gräf, K. Danzmann, R. Schnabel, "The GEO600 squeezed light source", Class. Quantum Grav. 27, 084027 (2010). Article.
 The GEO600 team: www.geo600.org
 The LIGO scientific collaboration: www.ligo.org
 The Quantum Interferometry Group at the AEI: www.qi.aei-hannover.de
 R. Schnabel, N. Mavalvala, D. E. McClelland, P. K. Lam, "Quantum metrology for gravitational wave astronomy", Nature Communications, 1:121, doi: 10.1038/ncomms1122 (2010). Abstract.