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The recent discovery of gravitational waves (GW) by Advanced LIGO (Laser Interferometric Gravitational-wave Observatory) has impressively launched the novel field of gravitational astronomy and allowed us to glimpse exciting objects about which we could previously only speculate. Further sensitivity improvements at the low-frequency end of the detection band of future GW observatories must rely on quantum non-demolition (QND) methods to suppress fundamental quantum fluctuations of the light fields used to readout the GW signal. Here we present a novel concept of how to turn a conventional Michelson interferometer into a QND speed-meter interferometer with coherently suppressed quantum back-action noise. We use two orthogonal polarizations of light and an optical circulator to couple them. We carry out a detailed analysis of how imperfections and optical loss influence the achievable sensitivity. We find that the proposed configuration significantly enhances the low-frequency sensitivity and increases the observable event rate of binary black-hole coalescences in the range of \\[10^2 - 10^3\\,M_ \\odot\\] by a factor of up to ~300.

A microelectromechanical system (MEMS) gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s. This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of 20.7 m, and up and down a local hill of 275 m. These tests demonstrate that the device has the potential to be a useful field-portable instrument. The development of an even smaller device is underway, with a total package size similar to that of a smartphone. This article is part of a discussion meeting issue 'The promises of gravitational-wave astronomy'.

Terrestrial gravity perturbations caused by seismic fields produce the so-called Newtonian noise in gravitational-wave detectors, which is predicted to limit their sensitivity in the upcoming observing runs. In the past, this noise was seen as an infrastructural limitation, i.e., something that cannot be overcome without major investments to improve a detector’s infrastructure. However, it is possible to have at least an indirect estimate of this noise by using the data from a large number of seismometers deployed around a detector’s suspended test masses. The noise estimate can be subtracted from the gravitational-wave data, a process called Newtonian noise cancellation (NNC). In this article, we present the design and implementation of the first NNC system at the Virgo detector as part of its AdV+ upgrade. It uses data from 110 vertical geophones deployed inside the Virgo buildings in optimized array configurations. We use a separate tiltmeter channel to test the pipeline in a proof-of-principle. The system has been running with good performance over months.

A microelectromechanical system (MEMS) gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s. This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of 20.7 m, and up and down a local hill of 275 m. These tests demonstrate that the device has the potential to be a useful field-portable instrument. The development of an even smaller device is underway, with a total package size similar to that of a smartphone. This article is part of a discussion meeting issue ‘The promises of gravitational-wave astronomy’.

In this paper we describe a feasible length sensing and control scheme for the low frequency interferometers of the Einstein Telescope (ET-LF) along with the techniques used to optimise several optical parameters, including the length of the recycling cavities and the modulation frequencies, using two numerical interferometer simulation packages: Optickle and Finesse. The investigations have suggested the use of certain combinations of sidebands to obtain independent information about the different degrees of freedom.

The Ninth Edoardo Amaldi conference on gravitational waves (Amaldi 9) and the 2011 Numerical Relativity – Data Analysis meeting (NRDA 2011) were held on July 10–15, 2011 in Cardiff, UK. The summer of 2011 marked the beginning of a crucial time for the field of gravitational-wave astronomy. After the successful completion of long-duration data taking, the initial LIGO and Virgo detectors were shut down and the era of first-generation laser interferometric gravitational-wave detectors came to an end. With the analysis of the last set of first-generation data approaching completion, the field now faces the challenge of preparing for the era of second-generation detectors, and, hopefully, the regular detection of gravitational waves. The first of the advanced detectors should begin operation in 2014, but the intervening years are far from a time of sitting and waiting. This will be a hectic time for experimenters, who have to fight the limits of current technology to make their theoretical noise curves a reality. During Amaldi 9 we were led through the upgrades to advanced detectors, and the challenges that the experimenters face. We also heard about proposals for going yet further with third-generation and space-based detectors. This is also a crucial time for theoretical work. With the increased sensitivity of advanced detectors, we hope to not only make the first detections of gravitational waves, but to learn about their sources, and interpret what this means for astrophysics. We need more complete source modelling, more sophisticated and efficient search pipelines and parameter estimation tools, as well as a greater understanding of what we can learn about the universe. Amaldi 9 included a number of talks on the status of these efforts, and the open questions that will be the focus in the coming years. Sunday July 10th was devoted entirely to NRDA 2011. In recent years the NRDA meetings have brought together numerical relativists who model compact-binary sources, and gravitational-wave data analysts, who want to use the numerical results to aid gravitational-wave detection and parameter estimation. This work has coalesced around the Numerical INJection Analysis (NINJA) project, which is now in its second incarnation, and will be used to test and refine data-analysis procedures on black-hole binary waveforms injected into real LIGO–Virgo data. This NRDA meeting focused on preparing the set of numerical waveforms, and paving the way for data analysis projects. The rest of the week consisted of Amaldi and NRDA-specific sessions, which contained talks on the most pressing issues facing gravitational-wave science. The participants enjoyed a successful meeting that included plenty of time scheduled for informal discussions, and popular poster sessions. Most evenings featured public lectures by prominent Cardiff academics following wine and cheese receptions. The Wednesday afternoon social excursions saw participants exploring the fairy-tale Castle Coch, the imposing Caerphilly Castle, and downtown Cardiff itself. The conference banquet on Thursday evening was hosted in the National Gallery, which included viewing of works by Renoir, Monet, and Cezanne in the Gallery's Impressionist wing. The organization of these meetings was overseen by the Gravitational Wave International Committee (GWIC), the NRDA Scientific Organizing Committee, and the Amaldi Local Organizing Committee. The organisers wish to extend special thanks for the generous support of Cardiff University, in particular to Samantha Emmott, Cardiff & Co., and the students and staff of the Cardiff University School of Physics and Astronomy, who bore the brunt of the effort at ground level. We thank IUPAP for financial support. We also wish to thank Adam Day and Ben Sheard of IOP for their assistance in the preparation of this Special Issue. Finally, to all the participants, we say thank you for making these meetings a success. These proceedings represent a small fraction of the science presented, discussed, and imagined at Amaldi 9 / NRDA 2011. Additional proceedings are published in the accompanying edition of Classical and Quantum Gravity. It is our hope that these articles will be a resource for the field for years to come. The Editors

This article gives an overview of potential upgrades of second generation gravitational wave detectors and the required key technologies to improve the limiting noise sources. In addition the baseline design of the Einstein Telescope, a European third generation gravitational wave observatory, is briefly discussed.

Gravitational wave detector technology provides high-precision measurement apparatuses that, if combined with a modulated particle source, have the potential to measure and constrain particle interactions in a novel way, by measuring the pressure caused by scattering particle beams off the mirror material. Such a measurement does not rely on tagging a final state. This strategy has the potential to allow us to explore novel ways to constrain the presence of new interactions beyond the Standard Model of Particle Physics and provide additional constraints to poorly understood cross sections in the non-perturbative regime of QCD and Nuclear Physics, which are limiting factors of dark matter and neutrino physics searches. Beyond high-energy physics, if technically feasible, the proposed method to measure nucleon-nucleon interactions can lead to practical applications in material and medical sciences.

The AEI 10 m Prototype is in its designing phase and will provide a test-bed for very sensitive interferometric experiments, such as the sub-SQL interferometer. It will test new techniques to reach – and even surpass – the Standard Quantum Limit. The experience and knowledge that can be gained from this experiment can be applied to large-scale interferometric gravitational detectors to improve the detector sensitivities. In order for the sub-SQL interferometer to achieve the required sensitivity all limiting noise sources need to be suppressed sufficiently. Noise sources can include seismic noise, thermal noise, and laser noise; laser frequency noise will be the main focus of this document. The laser frequency noise will be suppressed to a level of 10−4 Hz/ at 20 Hz dropping to below 10−6 Hz/ at 1kHz. The proposed design to suppress the laser frequency noise with a ring cavity is described in this paper.

In this article we propose a new method for reducing Newtonian noise in laser-interferometric gravitational-wave detectors located on the Earth's surface. We show that by excavating meter-scale recesses in the ground around the main test masses of a gravitational wave detector it is possible to reduce the coupling of Rayleigh wave driven seismic disturbances to test mass displacement. A discussion of the optimal recess shape is given and we use finite element simulations to derive the scaling of the Newtonian noise suppression with the parameters of the recess as well as the frequency of the seismic excitation. Considering an interferometer similar to an Advance LIGO configuration, our simulations indicate a frequency dependent Newtonian noise suppression factor of 2 to 4 in the relevant frequency range for a recesses of 4m depth and a width and length of 11m and 5m, respectively. Though a retrofit to existing interferometers seems not impossible, the application of our concept to future infrastructures seems to provide a better benefit/cost ratio and therefore a higher feasibility.