Resonant Gravitational Wave Detectors Research Articles

Optimal reconstruction of the input signal in resonant gravitational wave detectors: Data processing algorithm and physical limitations.

We discuss a method of data filtering for a resonant gravitational wave detector that allows an optimal reconstruction of the input signal. The method consists of the estimate, by optimal Wiener filtering, of the amplitude of the Karhunen-Lo\`eve components of the signal at the antenna input, and only needs the assumption that the signal has a finite duration. After discussing an application of the method to a simplified model for a resonant antenna, we present a practical application to the reduction of the data from a room temperature antenna.

Resonant gravitational wave detectors: a progress report

We report on recent advances on resonant-mass detectors for gravitational radiation. After a review of the concepts that underlie the development of antennas and the enhancement of their sensitivity, the most recent results on coincidence observation between two cryogenic antennas, Explorer and Allegro, are presented. We then describe the efforts currently under way to bring into operation a third generation, ultracryogenic detector, and finally present a review of new ideas and techniques, currently being studied or developed in laboratories around the world, that could lead to substantial improvements in antenna design.

Fundamental noise, electromechanical transduction and their role in resonant gravitational-wave detectors

The basic principles of electromechanical transduction are reviewed with an emphasis on applications relevant to gravitational-wave detectors. In any apparatus where feeble forces are measured, certain fundamental limits are imposed by the quantum nature of the measurement. After filtering out all sources of noise whose origins are external to the apparatus, the noise sources that remain are intrinsic to the measurement itself. The origin of three major noise sources are identified. These are: (i) Brownian noise due to the energy exchange with the surrounding thermal reservoir, (ii) the noise injected into the apparatus by the amplifier, and (iii) the noise of the amplifier itself. It is shown how the contribution of these noise sources are affected in the transduction mechanism. A general strategy is suggested where a parametric transducer strongly coupled to a gravity-wave antenna reduces the back-action of noisy amplifiers to negligible amounts. A strong electromechanical coupling and the attendant increase in the bandwidth facilitates rapid energy transfer between antenna and transducer. As a result, with a sufficiently low dissipative antenna, the remaining Brownian noise can be reduced by using short sampling intervals. This strategy is illustrated using, as an example, a tunnelling transducer. Along with other technological improvements, quite possibly a similar strategy may be necessary to lower the sensitivity of future Weber-type gravity-wave detectors.

High Q and cryogenic resonant gravitational wave detectors

Gravitational wave detectors, aimed at detecting the momentary gravitational perturbation that might come from the gravitational collapse of a star, share almost every attribute of a very sensitive acoustic detector, except the requirement that they be completely decoupled from any true acoustic source. There are now four multiton cryogenic resonant detectors around the world that have operated with burst amplitude sensitivity in the attometer (10−18-m) range, including the ALLEGRO detector at LSU. Even at liquid helium temperature, one possible fundamental noise source is the Langevin force, whose spectral density is determined by the mechanical Q of the relevant antenna resonances. Two practical materials, aluminum–magnesium alloy and high-purity niobium, have proven to have Q’s greater than 10 million under some conditions. Now that many other noise sources have been eliminated, further improvements in sensitivity will come from further increases in Q.

Applications of low temperature physics to gravitational astronomy

For twenty years a dedicated community of physicists has been developing resonant mass gravitational wave detectors which exploit the unique advantages of low temperature physics. Several tonne masses are suspended in large scale cryostats. Superconducting transducers monitor vibrations to a sensitivity ≈10 -19 metres. Suspension systems and materials properties allow the acoustic Q-factor of the antenna to exceed 10 8, corresponding to acoustic ring down times of several days. Massive bars have been cooled to less than 100 mK using dilution refrigerators. Using these methods thermal Nyquist noise is reduced such that the antenna effective temperature is reduced to a few microdegrees. A world-wide array of detectors should now allow any gravitational collapse which gives rise to the birth of a neutron star or black hole to be detected anywhere in our galaxy.

The ultracryogenic gravitational wave antenna AURIGA

AURIGA is a resonant gravitational wave detector under development at the INFN laboratories of Legnaro, Italy. Its main liquid Helium cryostat houses vibration isolation stages, the dilution refrigerator, the 2.3 tons cylindrical antenna and the resonant displacement transducer, in a design derived from NAUTILUS, the other INFN ultracryogenic antenna. The expected working temperature of the antenna, transducer and inner vibration isolation stage is about 50 mK. We implemented additional features to increase the long term reliability of AURIGA, particularly in the dilution refrigerator. The design provides also for a separate cryostat with short thermal cycle, which will house the signal amplification stage. This allows independent temperature control of the cryogenic electronics and helps its optimization and maintenance.

Upper limit for nuclearite flux from the Rome gravitational wave resonant detectors.

Flux limits for nuclearites at sea level are given, making use of the resonant gravitational wave detector Explorer. It is found, at the 90% confidence level, that there are less than 1.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ ${\mathrm{sr}}^{\ensuremath{-}1}$ ${\mathrm{s}}^{\ensuremath{-}1}$ with $\ensuremath{\beta}$ ranging from 1 to 0.001.

Long-term operation of the Rome "Explorer" cryogenic gravitational wave detector.

We describe the cryogenic resonant gravitational wave detector of the Rome group, named Explorer, and report on its long term operation with sensitivity for short bursts in the range $h\ensuremath{\simeq}7\ensuremath{-}10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$. Explorer has mass $M=2270$ kg and is equipped with a resonant capacitive transducer followed by a dc superconducting quantum interference device amplifier. It has been operated at $T\ensuremath{\simeq}2.6$ K in a cryostat cooled with superfluid helium. With a transducer voltage bias of 320 V ($E=6.15$ MV/m) the two resonant modes have frequencies of 904.7 and 921.3 Hz with coupled quality factors, respectively, of 0.77\ifmmode\times\else\texttimes\fi{}${10}^{6}$ and 1.0\ifmmode\times\else\texttimes\fi{}${10}^{6}$. The description of the experimental apparatus and of its calibration is followed by the analysis of the noise and the calculation of the expected sensitivity of the detector: $h\ensuremath{\simeq}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ (under the assumption of bursts with duration of 1 ms). We then describe the data acquisition system and the techniques of data analysis, discussing the filtering algorithms. The last section reports the experimental results obtained during the operation of the detector from May 1990 to December 1991. During this period the data were recorded for more than two-thirds of the total time: we show the distributions of the data and the hourly averages of the sensitivity. The data taken from May 1991 to December 1991 have also been used to establish a new improved upper limit for the rate and strength of gravitational wave pulses; at $h=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$, for example, there are no more than 0.5 events/day as averaged over a period of 134 days.

The Rochester gravitational wave detector

The authors present the first detailed report of the Rochester cryogenic resonant gravitational wave detector. They believe that the approach used in designing the detector will bring substantial contributions to the solutions of many unsolved problems. They describe in detail their transducer which makes use of several features (superconducting, wideband, noncontacting) in a unique combination that already has made it possible to achieve the highest mechanical Q for aluminum in a gravitational wave detector (Q=2*107). They also present encouraging results of preliminary tests and they calculate the values of the physical parameters needed to achieve a competitive ultimate sensitivity. They show how reasonable parameters lead to such sensitivity, even though the detecting mass is smaller than what is commonly used. The authors explain why dealing with a smaller mass presents important advantages. Finally they include a detailed analysis of the sensitivity and a description of the procedure for the calibration.

Sensitivity analysis of a two-mode resonant gravitational-wave detector with arbitrary tuning

We present a very general characterization of a resonant gravity wave detector and an explicit algorithm to evaluate its noise temperature when a passive transducer is used. This procedure also provides quantitative results for the sensitivity of such a detector in applications yet to be analysed, like insufficient or excessive tuning of the second resonator and use in detection of monochromatic radiation, both on and off resonance.

Design of a resonant gravitational-wave detector with quantum-limited sensitivity

It is shown that aresonant transducer is indispensable in a wide range of experimental conditions in order to accomplish optimum matching between a cryogenic antenna and a low-noise amplifier. Noise characteristics of a d.c. SQUID are analyzed and an improved design of a point contact d.c. SQUID is described. Conditions for transducer matching and detectability of a gravitational-wave signal are derived for a system employing a d.c. SQUID. It appears that a d.c. SQUID operating at the quantum-limited sensitivity could be realized in our proposed design and matched to a massive aluminum antenna cooled to 0.05 K by using a resonant superconducting inductive transducer. Presently achievable detector parameters are shown to be sufficient to allow a detection sensitivity corresponding to a resolution of a few photons in the antenna, the ultimate quantum-mechanical limit derived for an antenna instrumented with a linear-motion detector.

The Prospects for High Sensitivity Gravitational Antennae

The sensitivity of a resonant gravitational wave detector which is necessary for the detection of pulses from asymmetric stellar collapses or from black-hole collisions in nearby galaxies is examined. Limitations on this sensitivity due to the resonating quality of the detector, thermal noise, and reaction of the detection system upon the detector are studied. No severe difficulties for the detection of the above-mentioned pulses are expected. For the detection of pulses from a cluster at the galactic center, a nonresonant system based on Doppler ranging to a drag-free satellite would be more appropriate. At present the sensitivity of Doppler-ranging is still two orders of magnitude below the requirements.