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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 relativity and are generated, for example, by black-hole binary systems. 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 light--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

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We report preliminary results from a search for galactic axions in the frequency range 1.09 < f a < 1.22 GHz. For an axion linewidth V a < 200 Hz we obtain the experimental limit (g arr /m a ) 2 pa <1.4xl0" 41 . The theoretical prediction is (g ar y/m a ) 2 p a = 3.9x 10 ~4 4 with p a =300 MeV/cm 3 . We have also searched for the presence of a continuous spectrum of light pseudoscalar particles; if we assume that the above p a is contained between the upper and lower frequencies of our search, then we find thatg flyr <2xl0-30 MeV ,/2 cm 3/2^1 0" 11 GeV" 1 .PACS numbers: 98.60. Df, 14.80.Gt, 98.80.Es On the basis of rotation curves of galaxies and galactic clusters, it is widely believed today that the total mass in the Universe greatly exceeds the observable luminous matter. Also, arguments concerning primordial nucleosynthesis of the light elements limit the total baryonic matter to less than 0.2 times the critical density required to close the Universe. Little is known about the missing or "dark" matter but many candidate constituent particles have been proposed, some of them exotic.* A leading candidate is a very light pseudoscalar particle, the so-called "invisible axion." 2In 1977 Peccei and Quinn 3 (PQ) introduced a new global symmetry which is spontaneously broken in such a way as to cancel the CP-nonconserving terms that are present in the QCD Lagrangean but not observed. Weinberg 4 and Wilczek 4 pointed out that the breaking of the PQ symmetry must give rise to a pseudoscalar Goldstone boson named the axion. Accelerator searches 5 have failed to detect axions with mass greater than 200 keV, but models with light, weakly interacting axions have been proposed. 2 In these models, the mass of the axion and its coupling to fermions are inversely proportional to the vacuum expectation energy f a at which the PQ symmetry is broken. Specifically,where f n = 93 MeV and m^ = 135 MeV. While m a is not predicted by theory, observations on the cooling rate of ordinary stars 6 imply that m a < 1 eV, while neutron stars 7 imply that m a <4xl0 -2 eV; the best upper limit of m a < 1 x 10 ~2 eV comes from redgiant evolution. 8 At the same time cosmological considerations 9 place a lower limit of m a > 10 ~5 eV. In this window m a~~\ 0~s eV is preferred if the axions are to close the Universe. Axions should have been produced in the early stages of the Universe and later condensed into the galaxies with their present velocity being equal to the virial galactic velocity. Turner has calculated the expected axion density near the Earth on the assumption that galactic halos are due primarily to the presence of axions. 10 He finds that

=5xl0~2 5 g/cm 3^3 00 MeV/cm 3 .The axion couples to fermions and, through a triangle graph, to two photons [see Fig. 1(a)]. Sikivie 11 proposed that, in spite of the extremely weak coupling, axions could be converted to photons in the presence of an intense electromagnetic field via the PrimakofT effect 12 [ Fig. 1(b)]. The interaction Lagrangean is given by Lint = ~~ (#a yy /4;r)E-B(p...