J. P. Zendri

Wideband dual sphere detector of gravitational waves

We present the concept of a sensitive and broadband resonant mass gravitational wave detector. A massive sphere is suspended inside a second hollow one. Short, high-finesse Fabry-Perot optical cavities read out the differential displacements of the two spheres as their quadrupole modes are excited. At cryogenic temperatures, one approaches the standard quantum limit for broadband operation with reasonable choices for the cavity finesses and the intracavity light power. A molybdenum detector, of overall size of 2 m, would reach spectral strain sensitivities of 2x10(-23) Hz(-1/2) between 1000 and 3000 Hz.

Feedback Cooling of the Normal Modes of a Massive Electromechanical System to Submillikelvin Temperature

We apply a feedback cooling technique to simultaneously cool the three electromechanical normal modes of the ton-scale resonant-bar gravitational wave detector AURIGA. The measuring system is based on a dc superconducting quantum interference device (SQUID) amplifier, and the feedback cooling is applied electronically to the input circuit of the SQUID. Starting from a bath temperature of 4.2 K, we achieve a minimum temperature of 0.17 mK for the coolest normal mode. The same technique, implemented in a dedicated experiment at subkelvin bath temperature and with a quantum limited SQUID, could allow to approach the quantum ground state of a kilogram-scale mechanical resonator.

3-Mode Detection for Widening the Bandwidth of Resonant Gravitational Wave Detectors

Along with peak sensitivity, an important parameter of a resonant gravitational wave detector is its bandwidth. In addition to the obvious advantage of making the detector more sensitive to short bursts, a wider bandwidth would allow, for instance, details of the signal emitted during a supernova gravitational collapse or the merger of compact binaries to be resolved [1]. Moreover, a wider bandwidth reduces the uncertainty in the burst arrival time [2] and consequently, with a detector network, permits a more precise source location and a higher efficiency of spurious events rejection [3]. The introduction of a mechanically resonant transducer, a standard practice in actual resonant detectors, has greatly improved the coupling between the bar and the amplifier, but the bandwidth is intrinsically limited [4], and in practice, according to the full width at half maximum (FWHM) definition applied to the two minima of the Shh strain noise spectra, values of a few Hz have been achieved [5]. The use of multimode resonant transducers should permit further improvements of the detector bandwidth [6]. This approach has been studied [7] in depth and a few 2-mode transducer prototypes have been realized [8] or are under development [9] to obtain 3mode operation of the resonant mass detectors. This Letter describes how a wider detection bandwidth can be obtained with an alternative 2-mode transduction system in which the resonant amplification is realized by means of a resonant mechanical mode plus a resonant electrical matching network. It also describes the key tests performed on the components of the transduction system in order to verify the achievement of the requirements set by analysis of the detector model. Figure 1 shows the electromechanical scheme of a cryogenic detector with a resonant capacitive transducer read by a SQUID amplifier. The matching transformer couples the output impedance of the transducer (a capacitance of a few nF) to the input impedance of the SQUID (a small

Loss budget of a setup for measuring mechanical dissipations of silicon wafers between 300 and 4 K

A setup for measuring mechanical losses of silicon wafers has been fully characterized from room temperature to 4 K in the frequency range between 300 Hz and 4 kHz: it consists of silicon wafers with nodal suspension and capacitive and optical vibration sensors. Major contributions to mechanical losses are investigated and compared with experimental data scanning the full temperature range; in particular, losses due to the thermoelastic effect and to the wafer clamp are modeled via finite element method analysis; surface losses and gas damping are also estimated. The reproducibility of the measurements of total losses is also discussed and the setup capabilities for measuring additive losses contributed by thin films deposited on the wafers or bonding layers. For instance, assuming that additive losses are due to an 80-nm-thick wafer bond layer with Young modulus about ten times smaller than that of silicon, we achieve a sensitivity to bond losses at the level of 5x10(-3) at 4 K and at about 2 kHz.

First room temperature operation of the AURIGA optical readout

In the frame of the AURIGA collaboration, a readout scheme based on an optical resonant cavity has been implemented on a room temperature resonant bar detector of gravitational waves. The bar equipped with the optical readout has been operating for a few weeks and we report here the first results.

IGEC2: A 17-month search for gravitational wave bursts in 2005-2007

We present here the results of a 515 day search for short bursts of gravitational waves by the IGEC2 observatory. This network included 4 cryogenic resonant-bar detectors: AURIGA, EXPLORER, and NAUTILUS in Europe, and ALLEGRO in America. These results cover the time period from November 6th 2005 until April 15th 2007, partly overlapping the first long term observations by the LIGO interferometric detectors. The observatory operated with high duty cycle, namely, 57% for fourfold coincident observations, and 94% for threefold observations. The sensitivity was the best ever obtained by a bar network: we could detect, with an efficiency >50%, impulsive events with a burst strain amplitude h{sub rss} < or approx. 1x10{sup -19} Hz{sup -1/2}. The network data analysis was based on time coincidence searches over at least three detectors, used a blind search technique, and was tuned to achieve a false alarm rate of 1/century. When the blinding was removed, no gravitational wave candidate was found.

Status report of the gravitational wave detector AURIGA

We present the status of the ultracryogenic gravitational wave detector AURIGA, which is taking data since may 1997 with an energy sensitivity in the mK range and bandwidth greater than 1 Hz. The typical detector output is summarized in daily reports which are important tools for detector diagnostic and for checking the vetoes of periods of unsatisfactory operation of the detector.

INITIAL OPERATION OF THE INTERNATIONAL GRAVITATIONAL EVENT COLLABORATION

The International Gravitational Event Collaboration, IGEC, is a coordinated effort by research groups operating gravitational wave detectors working towards the detection of millisecond bursts of gravitational waves. Here we report on the current IGEC resonant bar observatory, its data analysis procedures, the main properties of the first exchanged data set. Even though the available data set is not complete, in the years 1997 and 1998 up to four detectors were operating simultaneously. Preliminary results are mentioned.

Dissipative feedback does not improve the optimal resolution of incoherent force detection

To the Editor — In 2012, Gavartin et al. reported a dissipative feedback scheme for an integrated optomechanical system1. The authors focus on detecting an incoherent force acting on a thermally driven resonator, and argue that dissipative feedback “can substantially improve the incoherent force resolution of nanoor micromechanical systems” and “enabling a significant decrease of measurement time and thus extending the range of applications for small-scale transducers.” However, in general, stationary linear feedback cannot improve the signalto-noise ratio of a force measurement, regardless of whether the force to be detected is coherent or incoherent. This is because feedback modifies the response of the resonator to both force signal and force noise in the same way. This concept is known to researchers involved in force detection experiments such as those that use gravitational wave detectors2, atomic force microscopes3 and optomechanical systems4. The results of Gavartin et al. seem to contradict this statement. In this respect, we observe that the force resolution depends on the specific estimator that is implemented. Gavartin et al. use an estimator based on the bare resonator energy (equation (4) in ref. 1), and conclude, coherently, that the force resolution is improved by feedback because of a shortening of the correlation time. However, energy is not the only possible estimator, and it is definitely not the most efficient. An optimal method for such a problem is well known in data processing and is based on Wiener’s theory of optimal filtering. For instance, its application to the specific problem of detecting an incoherent force acting on a resonator is discussed in ref. 5. The optimal filter can be implemented off-line in most resonator-based systems without the need for additional hardware. It is essentially the product of an inverting filter and a bandpass filter. The inverting filter removes the narrowband resonator dynamics by converting the measured displacement into an equivalent force, whereas the bandpass filter minimizes the contribution of wideband detection noise. The correlation time of filtered data τf is determined by the bandwidth over which the resonator noise overcomes the wideband detection noise, which is often (as in the work of Gavartin et al.) much larger than the intrinsic resonator bandwidth. Remarkably, τf does not change when the actual resonator correlation time τ is modified by a dissipative feedback. As a consequence, the force resolution, which is obtained by time-averaging the filtered data, is also feedbackindependent. Actually, it can be shown that the optimal force resolution for a given averaging time depends only on the signalto-noise ratio5. We note that the technique proposed by Gavartin et al., based on simple energy-averaging with a feedback-assisted reduction of the effective resonator time constant, can be considered as a way to approximate the optimal filter in hardware. However, it provides no additional improvement as soon as optimal filtering is applied. In conclusion, stationary dissipative feedback does not improve the resolution of incoherent force detection, provided that appropriate data processing is implemented. ❐

Search for gravitational wave bursts by the network of resonant detectors

The groups operating cryogenic bar detectors of gravitational waves are performing a coordinated search for short signals within the International Gravitational Event Collaboration (IGEC). We review the most relevant aspects of the data analysis, based on a time-coincidence search among triggers from different detectors, and the properties of the data exchanged by each detector under a recently-upgraded agreement. The IGEC is currently analysing the observations from 1997 to 2000, when up to four detectors were operating simultaneously. 10% and 50% of this time period were covered by simultaneous observations, respectively, of at least three or at least two detectors. Typical signal search thresholds were in the range 2–6 10−21/Hz. The coincidences found are within the estimated background, hence improved upper limits on incoming GW (gravitational wave) bursts have been set.