Squeezing of light's quantum noise requires temporal rearranging of photons. This again corresponds to creation of quantum correlations between individual photons. Squeezed light is a nonclassical manifestation of light with great potential in high-precision quantum measurements, for example, in the detection of gravitational waves [C. M. Caves, Phys. Rev. D 23, 1693 (1981)]. Equally promising applications have been proposed in quantum communication [H. P. Yuen and J. H. Shapiro, IEEE Trans. Inf. Theory 24, 657 (1978)]. However, after 20 years of intensive research doubts arose whether strong squeezing can ever be realized as required for eminent applications. Here we show experimentally that strong squeezing of light's quantum noise is possible. We reached a benchmark squeezing factor of 10 in power (10 dB). Thorough analysis reveals that even higher squeezing factors will be feasible in our setup. DOI: 10.1103/PhysRevLett.100.033602 PACS numbers: 42.50.Dv, 03.65.Ta, 04.80.Nn, 42.65.Yj Theoretical considerations about the possible existence of light with squeezed quantum noise can be traced back to the 1920's [1]. However, only after applications for squeezed light were proposed in the 1980's squeezing was discussed in more detail [1][2][3][4][5]. In [2] it was suggested to use squeezed light to improve the sensitivity of kilometre-scale Michelson laser-interferometers for the detection of gravitational waves. Such detectors have now reached a technical standard at which squeezed light can contribute in a valuable way. For example squeezing the quantum noise can provide a sensitivity improvement equivalent to even higher laser powers, however, without increasing the already problematic thermal load inside the interferometer. This is of great relevance for cryogenically cooled detectors. Proof of principle experiments have been successfully conducted [6,7] and squeezed states have been generated also in the audio signal band of ground-based detectors [8,9]. Another field of application is continuousvariable (CV) quantum communication and information [3,10,11]. While discrete-variable quantum information typically relies on single-photon detectors, which are limited in terms of detection speed and quantum efficiency, squeezed light is detected with homodyne and heterodyne detectors which reveal quantum correlations by averaging over a vast number of detected photons. Because of this, high bandwidth and almost perfect detection efficiencies are possible. Squeezed states of light have been used to demonstrate several CV quantum information protocols. They have been used to construct entangled states of light and to demonstrate quantum teleportation [12 -14]. They are a possible resource for secure quantum key distribution protocols [15,16] and for generation of cluster states for universal quantum computing [17]. Recently, squeezed states of light have been used to prepare macroscopic quantum superposition states for quantum information networks [18,19].For all proof of principle experiments so far only modest...

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We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50 -1000 Hz and with the frequency's time derivative in the range ÿ1 10 ÿ8 Hz s ÿ1 to zero. Data from the fourth LIGO science run (S4) have been used in this search. Three different semicoherent methods of transforming and summing strain power from short Fourier transforms (SFTs) of the calibrated data have been used. The first, known as StackSlide, averages normalized power from each SFT. A ''weighted Hough'' scheme is also developed and used, which also allows for a multiinterferometer search. The third method, known as PowerFlux, is a variant of the StackSlide method in which the power is weighted before summing. In both the weighted Hough and PowerFlux methods, the weights are chosen according to the noise and detector antenna-pattern to maximize the signal-to-noise ratio. The respective advantages and disadvantages of these methods are discussed. Observing no evidence of periodic gravitational radiation, we report upper limits; we interpret these as limits on this radiation from isolated rotating neutron stars. The best population-based upper limit with 95% confidence on the gravitational-wave strain amplitude, found for simulated sources distributed isotropically across the sky and with isotropically distributed spin axes, is 4:28 10 ÿ24 (near 140 Hz). Strict upper limits are also obtained for small patches on the sky for best-case and worst-case inclinations of the spin axes.

We carry out two searches for periodic gravitational waves using the most sensitive few hours of data from the second LIGO science run. Both searches exploit fully coherent matched filtering and cover wide areas of parameter space, an innovation over previous analyses which requires considerable algorithm development and computational power. The first search is targeted at isolated, previously unknown neutron stars, covers the entire sky in the frequency band 160 -728.8 Hz, and assumes a frequency derivative of less than 4 10 ÿ10 Hz=s. The second search targets the accreting neutron star in the lowmass x-ray binary Scorpius X-1 and covers the frequency bands 464-484 Hz and 604-624 Hz as well as the two relevant binary orbit parameters. Because of the high computational cost of these searches we limit the analyses to the most sensitive 10 hours and 6 hours of data, respectively. Given the limited sensitivity and duration of the analyzed data set, we do not attempt deep follow-up studies. Rather we concentrate on demonstrating the data analysis method on a real data set and present our results as upper limits over large volumes of the parameter space. In order to achieve this, we look for coincidences in parameter space between the Livingston and Hanford 4-km interferometers. For isolated neutron stars our 95% confidence level upper limits on the gravitational wave strain amplitude range from 6:6 10 ÿ23 to 1 10 ÿ21 across the frequency band; for Scorpius X-1 they range from 1:7 10 ÿ22 to 1:3 10 ÿ21 across the two 20-Hz frequency bands. The upper limits presented in this paper are the first broadband wide parameter space upper limits on periodic gravitational waves from coherent search techniques. The methods developed here lay the foundations for upcoming hierarchical searches of more sensitive data which may detect astrophysical signals.

We propose and demonstrate a coherent control scheme for stable phase locking of squeezed vacuum fields. We focus on sideband fields at frequencies from 10 Hz to 10 kHz, which is a frequency regime of particular interest in gravitational-wave detection and for which conventional control schemes have failed so far. A vacuum field with broadband squeezing covering this entire band was produced using optical parametric oscillation and characterized with balanced homodyne detection. The system was stably controlled over long periods utilizing two coherent but frequency shifted control fields. In order to demonstrate the performance of our setup the squeezed field was used for a nonclassical sensitivity improvement of a Michelson interferometer at audio frequencies. DOI: 10.1103/PhysRevLett.97.011101 PACS numbers: 04.80.Nn, 42.50.Lc, 42.65.Yj, 95.55.Ym It was first proposed by Caves [1] that injected squeezed states may be used to improve the sensitivity of laser interferometers and might therefore contribute to the challenging effort of direct observation of gravitational waves [2]. The goal of that proposal was the reduction of the measurement's shot noise. Later Unruh [3] has found that squeezed light can be used to correlate interferometer shot noise and radiation pressure noise thereby breaking the socalled standard quantum limit and allowing for a quantum nondemolition measurement on the mirror test mass position, for an overview we refer to Ref. [4]. Harms et al. [5] have shown that advanced interferometer recycling techniques [6] that also aim for an improvement of the signalto-shot-noise ratio are fully compatible with squeezed field injection. Gravitational-wave detectors require squeezing in their detection band from about 10 Hz to 10 kHz. The majority of current squeezing experiments, however, have been performed in the MHz regime. Furthermore the orientation of the squeezing ellipse needs to be designed for every sideband frequency. The transformation from frequency independent squeezing to optimized frequency dependent squeezing can be performed by optical filter cavities as proposed in [4] and demonstrated in [7] for MHz frequencies. Also in the MHz regime, the combination of squeezed field injection and recycling techniques has been demonstrated [8,9]. Squeezing at audio frequencies has been demonstrated recently for the first time [10,11]. However, the phase of the squeezed vacuum could not be controlled by a coherent field.Controlling squeezed vacuum fields is the basic problem for squeezed field applications in gravitational wave (GW) detectors. Common control schemes rely on the injection of a weak, phase modulated seed field at the carrier frequency into the optical parametric oscillator (OPO) thereby turning the device into an optical parametric amplifier. It has been shown that even lowest carrier powers introduce large amounts of classical laser noise at audio frequencies and squeezing can no longer be achieved [10].On the other hand phase modulation sidebands are not present in a pure vac...