Point Absorber Limits to Future Gravitational-Wave Detectors.

High-quality optical resonant cavities require low optical loss, typically on the scale of parts per million. However, unintended micron-scale contaminants on the resonator mirrors that absorb the light circulating in the cavity can deform the surface thermoelastically and thus increase losses by scattering light out of the resonant mode. The point absorber effect is a limiting factor in some high-power cavity experiments, for example, the Advanced LIGO gravitational-wave detector. In this Letter, we present a general approach to the point absorber effect from first principles and simulate its contribution to the increased scattering. The achievable circulating power in current and future gravitational-wave detectors is calculated statistically given different point absorber configurations. Our formulation is further confirmed experimentally in comparison with the scattered power in the arm cavity of Advanced LIGO measured by in situ photodiodes. The understanding presented here provides an important tool in the global effort to design future gravitational-wave detectors that support high optical power and thus reduce quantum noise.

Approaching the motional ground state of a 10-kg object.

The motion of a mechanical object, even a human-sized object, should be governed by the rules of quantum mechanics. Coaxing them into a quantum state is, however, difficult because the thermal environment masks any quantum signature of the object's motion. The thermal environment also masks the effects of proposed modifications of quantum mechanics at large mass scales. We prepared the center-of-mass motion of a 10-kilogram mechanical oscillator in a state with an average phonon occupation of 10.8. The reduction in temperature, from room temperature to 77 nanokelvin, is commensurate with an 11 orders-of-magnitude suppression of quantum back-action by feedback and a 13 orders-of-magnitude increase in the mass of an object prepared close to its motional ground state. Our approach will enable the possibility of probing gravity on massive quantum systems.

Quantum correlations between light and the kilogram-mass mirrors of LIGO.

The measurement of minuscule forces and displacements with ever greater precision is inhibited by the Heisenberg uncertainty principle, which imposes a limit to the precision with which the position of an object can be measured continuously, known as the standard quantum limit1-4. When light is used as the probe, the standard quantum limit arises from the balance between the uncertainties of the photon radiation pressure applied to the object and of the photon number in the photoelectric detection. The only way to surpass the standard quantum limit is by introducing correlations between the position/momentum uncertainty of the object and the photon number/phase uncertainty of the light that it reflects5. Here we confirm experimentally the theoretical prediction5 that this type of quantum correlation is naturally produced in the Laser Interferometer Gravitational-wave Observatory (LIGO). We characterize and compare noise spectra taken without squeezing and with squeezed vacuum states injected at varying quadrature angles. After subtracting classical noise, our measurements show that the quantum mechanical uncertainties in the phases of the 200-kilowatt laser beams and in the positions of the 40-kilogram mirrors of the Advanced LIGO detectors yield a joint quantum uncertainty that is a factor of 1.4 (3 decibels) below the standard quantum limit. We anticipate that the use of quantum correlations will improve not only the observation of gravitational waves, but also more broadly future quantum noise-limited measurements.

Frequency-Dependent Squeezing for Advanced LIGO.

The first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 launched the era of gravitational-wave astronomy. The quest for gravitational-wave signals from objects that are fainter or farther away impels technological advances to realize ever more sensitive detectors. Since 2019, one advanced technique, the injection of squeezed states of light, is being used to improve the shot-noise limit to the sensitivity of the Advanced LIGO detectors, at frequencies above ∼50 Hz. Below this frequency, quantum backaction, in the form of radiation pressure induced motion of the mirrors, degrades the sensitivity. To simultaneously reduce shot noise at high frequencies and quantum radiation pressure noise at low frequencies requires a quantum noise filter cavity with low optical losses to rotate the squeezed quadrature as a function of frequency. We report on the observation of frequency-dependent squeezed quadrature rotation with rotation frequency of 30 Hz, using a 16-m-long filter cavity. A novel control scheme is developed for this frequency-dependent squeezed vacuum source, and the results presented here demonstrate that a low-loss filter cavity can achieve the squeezed quadrature rotation necessary for the next planned upgrade to Advanced LIGO, known as "A+."

Quantum-Enhanced Advanced LIGO Detectors in the Era of Gravitational-Wave Astronomy.

The Laser Interferometer Gravitational Wave Observatory (LIGO) has been directly detecting gravitational waves from compact binary mergers since 2015. We report on the first use of squeezed vacuum states in the direct measurement of gravitational waves with the Advanced LIGO H1 and L1 detectors. This achievement is the culmination of decades of research to implement squeezed states in gravitational-wave detectors. During the ongoing O3 observation run, squeezed states are improving the sensitivity of the LIGO interferometers to signals above 50 Hz by up to 3 dB, thereby increasing the expected detection rate by 40% (H1) and 50% (L1).

First Demonstration of Electrostatic Damping of Parametric Instability at Advanced LIGO.

Interferometric gravitational wave detectors operate with high optical power in their arms in order to achieve high shot-noise limited strain sensitivity. A significant limitation to increasing the optical power is the phenomenon of three-mode parametric instabilities, in which the laser field in the arm cavities is scattered into higher-order optical modes by acoustic modes of the cavity mirrors. The optical modes can further drive the acoustic modes via radiation pressure, potentially producing an exponential buildup. One proposed technique to stabilize parametric instability is active damping of acoustic modes. We report here the first demonstration of damping a parametrically unstable mode using active feedback forces on the cavity mirror. A 15 538 Hz mode that grew exponentially with a time constant of 182 sec was damped using electrostatic actuation, with a resulting decay time constant of 23 sec. An average control force of 0.03 nN was required to maintain the acoustic mode at its minimum amplitude.

Effects of transients in LIGO suspensions on searches for gravitational waves.

This paper presents an analysis of the transient behavior of the Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) suspensions used to seismically isolate the optics. We have characterized the transients in the longitudinal motion of the quadruple suspensions during Advanced LIGO's first observing run. Propagation of transients between stages is consistent with modeled transfer functions, such that transient motion originating at the top of the suspension chain is significantly reduced in amplitude at the test mass. We find that there are transients seen by the longitudinal motion monitors of quadruple suspensions, but they are not significantly correlated with transient motion above the noise floor in the gravitational wave strain data, and therefore do not present a dominant source of background noise in the searches for transient gravitational wave signals. Using the suspension transfer functions, we compared the transients in a week of gravitational wave strain data with transients from a quadruple suspension. Of the strain transients between 10 and 60 Hz, 84% are loud enough that they would have appeared above the sensor noise in the top stage quadruple suspension monitors if they had originated at that stage at the same frequencies. We find no significant temporal correlation with the suspension transients in that stage, so we can rule out suspension motion originating at the top stage as the cause of those transients. However, only 3.2% of the gravitational wave strain transients are loud enough that they would have been seen by the second stage suspension sensors, and none of them are above the sensor noise levels of the penultimate stage. Therefore, we cannot eliminate the possibility of transient noise in the detectors originating in the intermediate stages of the suspension below the sensing noise.

Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO and Advanced Virgo.

We present a possible observing scenario for the Advanced LIGO and Advanced Virgo gravitational-wave detectors over the next decade, with the intention of providing information to the astronomy community to facilitate planning for multi-messenger astronomy with gravitational waves. We determine the expected sensitivity of the network to transient gravitational-wave signals, and study the capability of the network to determine the sky location of the source. We report our findings for gravitational-wave transients, with particular focus on gravitational-wave signals from the inspiral of binary neutron-star systems, which are considered the most promising for multi-messenger astronomy. The ability to localize the sources of the detected signals depends on the geographical distribution of the detectors and their relative sensitivity, and 90% credible regions can be as large as thousands of square degrees when only two sensitive detectors are operational. Determining the sky position of a significant fraction of detected signals to areas of 5 deg2 to 20 deg2 will require at least three detectors of sensitivity within a factor of ∼ 2 of each other and with a broad frequency bandwidth. Should the third LIGO detector be relocated to India as expected, a significant fraction of gravitational-wave signals will be localized to a few square degrees by gravitational-wave observations alone.

Improved upper limits on the stochastic gravitational-wave background from 2009-2010 LIGO and Virgo data.

Gravitational waves from a variety of sources are predicted to superpose to create a stochastic background. This background is expected to contain unique information from throughout the history of the Universe that is unavailable through standard electromagnetic observations, making its study of fundamental importance to understanding the evolution of the Universe. We carry out a search for the stochastic background with the latest data from the LIGO and Virgo detectors. Consistent with predictions from most stochastic gravitational-wave background models, the data display no evidence of a stochastic gravitational-wave signal. Assuming a gravitational-wave spectrum of Ω_{GW}(f)=Ω_{α}(f/f_{ref})^{α}, we place 95% confidence level upper limits on the energy density of the background in each of four frequency bands spanning 41.5-1726 Hz. In the frequency band of 41.5-169.25 Hz for a spectral index of α=0, we constrain the energy density of the stochastic background to be Ω_{GW}(f)<5.6×10^{-6}. For the 600-1000 Hz band, Ω_{GW}(f)<0.14(f/900 Hz)^{3}, a factor of 2.5 lower than the best previously reported upper limits. We find Ω_{GW}(f)<1.8×10^{-4} using a spectral index of zero for 170-600 Hz and Ω_{GW}(f)<1.0(f/1300 Hz)^{3} for 1000-1726 Hz, bands in which no previous direct limits have been placed. The limits in these four bands are the lowest direct measurements to date on the stochastic background. We discuss the implications of these results in light of the recent claim by the BICEP2 experiment of the possible evidence for inflationary gravitational waves.

Squeezed light for advanced gravitational wave detectors and beyond.

Recent experiments have demonstrated that squeezed vacuum states can be injected into gravitational wave detectors to improve their sensitivity at detection frequencies where they are quantum noise limited. Squeezed states could be employed in the next generation of more sensitive advanced detectors currently under construction, such as Advanced LIGO, to further push the limits of the observable gravitational wave Universe. To maximize the benefit from squeezing, environmentally induced disturbances such as back scattering and angular jitter need to be mitigated. We discuss the limitations of current squeezed vacuum sources in relation to the requirements imposed by future gravitational wave detectors, and show a design for squeezed light injection which overcomes these limitations.

Search for gravitational waves associated with γ-ray bursts detected by the interplanetary network.

We present the results of a search for gravitational waves associated with 223 γ-ray bursts (GRBs) detected by the InterPlanetary Network (IPN) in 2005-2010 during LIGO's fifth and sixth science runs and Virgo's first, second, and third science runs. The IPN satellites provide accurate times of the bursts and sky localizations that vary significantly from degree scale to hundreds of square degrees. We search for both a well-modeled binary coalescence signal, the favored progenitor model for short GRBs, and for generic, unmodeled gravitational wave bursts. Both searches use the event time and sky localization to improve the gravitational wave search sensitivity as compared to corresponding all-time, all-sky searches. We find no evidence of a gravitational wave signal associated with any of the IPN GRBs in the sample, nor do we find evidence for a population of weak gravitational wave signals associated with the GRBs. For all IPN-detected GRBs, for which a sufficient duration of quality gravitational wave data are available, we place lower bounds on the distance to the source in accordance with an optimistic assumption of gravitational wave emission energy of 10(-2)M⊙c(2) at 150 Hz, and find a median of 13 Mpc. For the 27 short-hard GRBs we place 90% confidence exclusion distances to two source models: a binary neutron star coalescence, with a median distance of 12 Mpc, or the coalescence of a neutron star and black hole, with a median distance of 22 Mpc. Finally, we combine this search with previously published results to provide a population statement for GRB searches in first-generation LIGO and Virgo gravitational wave detectors and a resulting examination of prospects for the advanced gravitational wave detectors.

Constraints on cosmic strings from the LIGO-Virgo gravitational-wave detectors.

Cosmic strings can give rise to a large variety of interesting astrophysical phenomena. Among them, powerful bursts of gravitational waves (GWs) produced by cusps are a promising observational signature. In this Letter we present a search for GWs from cosmic string cusps in data collected by the LIGO and Virgo gravitational wave detectors between 2005 and 2010, with over 625 days of live time. We find no evidence of GW signals from cosmic strings. From this result, we derive new constraints on cosmic string parameters, which complement and improve existing limits from previous searches for a stochastic background of GWs from cosmic microwave background measurements and pulsar timing data. In particular, if the size of loops is given by the gravitational backreaction scale, we place upper limits on the string tension Gµ below 10(-8) in some regions of the cosmic string parameter space.

Squeezed quadrature fluctuations in a gravitational wave detector using squeezed light.

Squeezed states of light are an important tool for optical measurements below the shot noise limit and for optical realizations of quantum information systems. Recently, squeezed vacuum states were deployed to enhance the shot noise limited performance of gravitational wave detectors. In most practical implementations of squeezing enhancement, relative fluctuations between the squeezed quadrature angle and the measured quadrature (sometimes called squeezing angle jitter or phase noise) are one limit to the noise reduction that can be achieved. We present calculations of several effects that lead to quadrature fluctuations, and use these estimates to account for the observed quadrature fluctuations in a LIGO gravitational wave detector. We discuss the implications of this work for quantum enhanced advanced detectors and even more sensitive third generation detectors.

Loss in long-storage-time optical cavities.

Long-storage-time Fabry-Perot cavities are a core component of many precision measurement experiments. Optical loss in such cavities is a critical parameter in determining their performance; however, it is very difficult to determine a priori from independent characterisation of the individual cavity mirrors. Here, we summarise three techniques for directly measuring this loss in situ and apply them to a high-finesse, near-concentric, 2 m system. Through small modifications of the cavity's length, we explore optical loss as a function of beam spot size over the 1-3 mm range. In this regime we find that optical loss is relatively constant at around 5 ppm per mirror and shows greater dependence on the positions of the beam spots on the cavity optics than on their size. These results have immediate consequences for the application of squeezed light to advanced gravitational-wave interferometers.

Directional limits on persistent gravitational waves using LIGO S5 science data.

The gravitational-wave (GW) sky may include nearby pointlike sources as well as stochastic backgrounds. We perform two directional searches for persistent GWs using data from the LIGO S5 science run: one optimized for pointlike sources and one for arbitrary extended sources. Finding no evidence to support the detection of GWs, we present 90% confidence level (C.L.) upper-limit maps of GW strain power with typical values between 2-20×10(-50) strain(2) Hz(-1) and 5-35×10(-49) strain(2) Hz(-1) sr(-1) for pointlike and extended sources, respectively. The latter result is the first of its kind. We also set 90% C.L. limits on the narrow-band root-mean-square GW strain from interesting targets including Sco X-1, SN 1987A and the Galactic center as low as ≈7×10(-25) in the most sensitive frequency range near 160 Hz.

An upper limit on the stochastic gravitational-wave background of cosmological origin.

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations. Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 x 10(-6) at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models. This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100 Hz.

All-sky LIGO search for periodic gravitational waves in the early fifth-science-run data.

We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50-1100 Hz and with the frequency's time derivative in the range -5 x 10{-9}-0 Hz s{-1}. Data from the first eight months of the fifth LIGO science run (S5) have been used in this search, which is based on a semicoherent method (PowerFlux) of summing strain power. Observing no evidence of periodic gravitational radiation, we report 95% confidence-level upper limits on radiation emitted by any unknown isolated rotating neutron stars within the search range. Strain limits below 10{-24} are obtained over a 200-Hz band, and the sensitivity improvement over previous searches increases the spatial volume sampled by an average factor of about 100 over the entire search band. For a neutron star with nominal equatorial ellipticity of 10{-6}, the search is sensitive to distances as great as 500 pc.

Effects of pulsed electric fields on ovalbumin solutions and dialyzed egg white.

Ovalbumin solutions (2%, pH 7.0, 200 ohm.cm) and dialyzed fresh egg white (pH 9.2, 200-250 ohm.cm) were subjected to 50-400 exponential decay pulses with an electric field strength of 27-33 kV/cm. The pulse width was ca. 0.3 micros (at a capacitance of 20 nF) or 0.9 micros (at 80 nF), and the corresponding dissipated energy was 0.7 or 2.3 J/(pulse.mL) of solution. The sample temperature was maintained below 29 degrees C. While the four sulfhydryl groups of native ovalbumin did not react with DTNB, they became reactive immediately after pulse processing, indicating either partial protein unfolding or enhanced SH ionization. The extent of SH reactivity increased with dissipated energy, 3.7 SH groups becoming reactive after 100 or 200 pulses at 31.5 kV/cm and 80 nF. However, SH reactivity was reversible, since only 0.79 or 0.2 SH group was found to remain reactive 30 min or 8 h after pulse processing. The fourth derivatives of UV spectra of ovalbumin were determined, before and 15-30 min after pulse processing, to assess possible polarity and conformation changes in the environment of tyrosine and tryptophan. No differences were observed. Thermal gels prepared from fresh or dialyzed egg white had markedly different mechanical and water retention characteristics. Pulse processing of dialyzed egg white (200 pulses, 30 kV/cm, 80 nF) only slightly reduced its gelling properties. Thus electric pulses known to induce significant microbial inactivation did not cause notable changes in the proteins investigated.

[Simvastatin and ischemia-reperfusion damage: its effects on apoptotic myocyte death and on the endothelial expression of nitric-oxide synthetase in an experimental model of the isolated rat heart]. / Simvastatina e danno da ischemia-riperfusione: effetti sulla morte apoptotica dei miociti e sull'espressione dell'ossido nitrico-sintetasi endoteliale nel modello sperimentale del cuore isolato di ratto.

Recent studies have suggested that simvastatin may exert endothelial-protective and anti-ischemic effects via nitric oxide (NO) mechanisms. The aim of this study was to evaluate, in isolated working rat hearts, the effect of acute simvastatin administration on endothelial and inducible NO-synthase (eNOS and iNOS) mRNA and on myocytic apoptosis after ischemia-reperfusion. We used isolated working rat hearts submitted to 15 min global, no-flow, normothermic ischemia and 180 min reperfusion. To detect myocytic apoptosis we used DNA agarose gel electrophoresis and Tunel technique; eNOS and iNOS expression were evaluated by multiplex reverse transcriptase-polymerase chain reaction; glyceraldehyde-3-phosphate dehydrogenase (G3PDH) was used as standard. The eNOS and iNOS mRNAs were expressed as G3PDH/eNOS and G3PDH/iNOS densitometric ratio (BioRad Gel Doc 1000). Hearts were divided into four groups: A) hearts excised and used as histological controls; B) untreated hearts submitted to ischemia and reperfusion; C) actinomicin D-treated (1.5 mg/kg) hearts, perfused with 25 microM simvastatin, subjected to ischemia and reperfusion; D) hearts treated with simvastatin 25 microM and submitted to ischemia and reperfusion. In Group B we evidenced a significant myocytic apoptotic damage, reduced in groups C and D. In Group B an increase in G3PDH/eNOS ratio vs Group A was detected; in Group D a reduction in G3PDH/eNOS ratio vs Group B occurred; no significant changes were observed between groups C and D. As for G3PDH/iNOS ratio, it was significantly increased in Group D with respect to groups A and B. Our data suggest that simvastatin in acute may modulate NO-synthase mRNA expression (induction of eNOS mRNA by means of post-transcriptional mechanisms and inhibition of iNOS postischemic overexpression) and reduce myocytic apoptosis.