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The growing interest in quantum technologies, from fundamental physics experiments to quantum computing, demands for extremely performing electronics only adding the minimum amount of noise admitted by quantum mechanics to the input signal (i.e. quantum-limited electronics). Superconducting microwave amplifiers, due to their dissipationless nature, exhibit outstanding performances in terms of noise (quantum limited), and gain. However, bandwidth and saturation power still show space for substantial improvement. Within the DARTWARS* collaboration, we are developing state-of-the-art microwave superconducting amplifiers based on Josephson junction arrays and on distributed kinetic inductance transmission lines. Here we report the realization of a setup for the characterization of the performances of Josephson Traveling-Wave Parametric Amplifiers (JTWPA) at a temperature of 300 mK. Although in the final experimental setup these amplifiers will operate at a base temperature of about 20 mK, their characterization at 300 mK allows to evidence the main aspects of their performances, but the ultimate noise level. This represents a quick and relatively inexpensive way to test these superconductive devices that can be of help to improve their design and fabrication. IEEE

The recent developments in quantum technologies, as well as advanced detection experiments, have raised the need to detect extremely weak signals in the microwave frequency spectrum. To this aim, the Josephson travelling wave parametric amplifier, a device capable of reaching the quantum noise limit while providing a wide bandwidth, has been proposed as a suitable cryogenic front-end amplifier. This work deals with the numerical study of a Josephson travelling wave parametric amplifier, without approximations regarding the nonlinearity of the key elements. In particular, we focus on the investigation of the system of coupled nonlinear differential equations representing all the cells of the Josephson travelling wave parametric amplifier, with proper input and output signals at the boundaries. The investigation of the output signals generated by the parametric amplification process explores the phase-space and the Fourier spectral analysis of the output voltage, as a function of the parameters describing the pump and signal tones that excite the device. Beside the expected behavior, i.e., the signal amplification, we show that, depending on the system operation, unwanted effects (such as pump tone harmonics, incommensurate frequency generation, and noise rise), which are not accounted for in simple linearized approaches, can be generated in the whole nonlinear system. IEEE

We have developed microwave kinetic inductance detectors suitable for near-IR single-photon counting. Our films are made of titanium and titanium nitride, deposited in a multilayer structure Ti/TiN/Ti/TiN with a total thickness of 44nm. The film has a transition temperature of 1.2K and a surface kinetic inductance of 34pH/sq. The resonator was designed with lumped elements and consists of two blocks of interdigitated capacitors connected by a meandered stripe inductor. The resonator resonance frequency is 6.8GHz, and the internal quality factor is 125,000. The detector is read out with the usual homodyne scheme and calibrated with light pulses produced by a laser diode with wavelength 1550nm. For the 0- and 1-photon peaks, we measure a FWHM energy resolution of 0.44eV and 0.56eV, respectively. This resolution is sufficient to resolve events with up to 4 photons.

The QUAX proposal studies the interaction of dark matter axions with electrons. As the effect of axions can be described as an effective magnetic field, the signal is a magnetization oscillation of a sample. To detect it, the material is placed in a resonant cavity and polarized with a static magnetic field. Here we describe the operation of such a device, with a resonance frequency of 14 GHz, corresponding to an axion mass ma = 58 μeV. Eventually some recent developments on the resonant cavities design and test are detailed.

Titanium and titanium nitride layers were alternately sputtered on a high resistivity silicon wafer to obtain a multilayer film structure, for a total thickness ranging between 34 and 180 nm. The electrical resistance of the Ti/TiN film was characterized from room temperature down to the superconducting transition. Both the resistivity just above the transition and the critical temperature were investigated as a function of the total film thickness and the single TiN layer thickness, respectively. The obtained resistivity range is 67–312 μΩ cm. Exploiting the proximity effect, we were able to tune the critical temperature in the 0.29–4.5 K range. A comparison between experimental data and theoretical models is proposed in order to facilitate the a priori design of superconducting detectors.

We report on the development of thermal kinetic inductance detectors (TKIDs) suitable to perform X-ray spectroscopy measurements. The aim is to implement MKIDs sensors working in thermal quasi-equilibrium mode to detect X-ray photons as pure calorimeters. The thermal mode is a variation on the MKID classical way of operation that has generated interest in recent years. TKIDs can offer the MKIDs inherent multiplexibility in the frequency domain, a high spatial resolution comparable with CCDs, and an energy resolution theoretically limited only by thermodynamic fluctuations across the thermal weak links. Microresonators are built in Ti/TiN multilayer technology with the inductive part thermally coupled with a metal absorber on a suspended SiN membrane, to avoid escape of phonons from the film to the substrate. The mid-term goal is to optimize the single-pixel design in terms of superconducting critical temperatures, internal quality factors, kinetic inductance and spectral energy resolution. The final goal is to realize a demonstrator array for a next generation thousand pixels X-ray spectrometer. In this contribution, the status of the project after one year of developments is reported, with detailed reference to the microresonators design and simulations and to the fabrication process.

Over the last few years, there has been a growing interest toward the use of superconducting microwave microresonators operated in quasi-thermal equilibrium mode, especially applied to single particle detection. Indeed, previous devices designed and tested by our group with X-ray sources in the keV range evidenced that several issues arise from the attempt of detection through athermal quasiparticles produced within direct strikes of X-rays in the superconductor material of the resonator. In order to prevent issues related to quasiparticles self-recombination and to avoid exchange of athermal phonons with the substrate, our group focused on the development of thermal superconducting microresonators. In this configuration, resonators composed of multilayer films of Ti/TiN sense the temperature of an absorbing material. To maximize the thermal response, low-critical-temperature films are preferable. By lowering the critical temperature, though, the maximum probing power bearable by the resonators decreases abruptly because of the weakening of the electron–phonon coupling. A proper compromise between the value of critical temperature (and hence sensitivity to energy deposition) and readout power bearable by the device has to be found in order to avoid signal-to-noise ratio degradation. In this contribution, we report the latest measurement of the electron–phonon coupling.

To account for the dark matter content in our Universe, post-inflationary scenarios predict for the QCD axion a mass in the range $(10-10^3)\,\mu\mbox{eV}$. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5\,GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of Cu resonant cavities. In this paper we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a $36\,\mbox{cm}^3$ NbTi superconducting cavity. The cavity worked at $T=4\,\mbox{K}$ in a 2\,T magnetic field and exhibited a quality factor $Q_0= 4.5\times10^5$ for the TM010 mode at 9\,GHz. With such values of $Q$ the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup we set the limit $g_{a\gamma\gamma}

Superconducting microwave microresonators are low temperature detectors (LTDs) suitable for large-scale frequency domain multiplexing readout. A promising approach consists in operating the resonators in quasi-thermal equilibrium mode: the resonator acts as a thermometer sensing the temperature rise of an absorbing material caused by an energy release. Our aim is to develop such detectors to perform spectroscopy in the keV range, with a possible future application to a next generation experiment aimed at directly measuring the neutrino mass . Still, the best configuration in terms of detector design and material must be found. Resonators made of several Ti/TiN multilayer films, which is a high kinetic inductance superconducting material previously developed by our group, were recently produced. The resonators are deposited onto a silicon slab and the sensitive area, thermally coupled to a gold absorber, is kept suspended by means of a SiN x membrane. In this way we plan to exploit the excellent energy resolution of LTDs combined with the simple multiplexing scheme of the resonators. In this contribution we present the devices along with our project, with its status and perspectives.

Assessing the absolute neutrino mass scale is one of the major challenges in particle physics and astrophysics nowadays. A powerful tool to directly estimate the effective electron neutrino mass consists in the calorimetric measurement of the energy released within a nuclear beta decay. The progresses made over the last few decades on low-temperature detector technologies have permitted to design experiments with expected sensitivities on the neutrino mass below 1 eV/c2 with the calorimetric approach. Despite the remarkable performances in both energy (∼ eV at keV) and time resolutions $(\sim\! 1\ \mu\text{s})$ on the single channel, a large number of detectors working in parallel is required to reach a sub-eV sensitivity. Microwave frequency-domain readout provides a powerful technique to read out large arrays of low-temperature detectors, such as transition edge sensors (TESs) or microwave kinetic inductance detectors (MKIDs). In this way, the multiplex factor is only limited by the bandwidth of the available commercial fast digitizers. The microwave multiplexing system will be used to read out the TES array of the HOLMES experiment, which is made of 1000 163Ho-implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture decay of 163Ho with an expected sensitivity of the order of the eV/c2.

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows measuring the whole energy, except the fraction carried away by the neutrino: due to the energy conservation, a finite neutrino mass m ν causes the energy spectrum to be truncated at Q-m ν , where Q is the transition energy of the decay. The electron capture of 163 Ho (Q ~ 2.5 keV) is an ideal decay, due to the high fraction of events close to the endpoint (i.e., the maximum energy of the relaxation energy spectrum). In order to achieve enough statistics, a large number of detectors (~10 4 ) are required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency-domain readout, with theoretical energy and time resolution on the order of electronvolts and microseconds, respectively. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectrum of 163 Ho. Currently, a study aimed at the selection of the best design and material for the detectors is in progress. In order to obtain low-Tc detectors, with Tc ranging between ~0.5 and 2 K, different Ti/TiN (titanium nitride) multilayer films were produced. The reduced Tc was obtained by superposing thin layers of stoichiometric TiN to pure Ti layers, and the Tc was tuned by varying the ratio between the thickness of the layers. In this contribution, a comparison between the measurements (critical temperature, gap parameter, and X-ray energy spectra) made with stoichiometric and substoichiometric TiN and Ti/TiN multilayer film microresonators is presented.

Spontaneous collapse models predict that a weak force noise acts on any mechanical system, as a consequence of the collapse of the wave function. Significant upper limits on the collapse rate have been recently inferred from precision mechanical experiments, such as ultracold cantilevers and the space mission LISA Pathfinder. Here, we report new results from an experiment based on a high-Q cantilever cooled to millikelvin temperatures, which is potentially able to improve the current bounds on the continuous spontaneous localization (CSL) model by 1 order of magnitude. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.ler. Several physical mechanisms able to explain the observed noise have been ruled out., 10 pages, 12 figures

A search for a new scalar field, called moduli, has been performed using the cryogenic resonant-mass AURIGA detector. Predicted by string theory, moduli may provide a significant contribution to the dark matter (DM) component of our Universe. If this is the case, the interaction of ordinary matter with the local DM moduli, forming the Galaxy halo, will cause an oscillation of solid bodies with a frequency corresponding to the mass of moduli. In the sensitive band of AURIGA, some 100 Hz at around 1 kHz, the expected signal, with $Q=△f/f\ensuremath{\sim}1{0}^{6}$, is a narrow peak, $△f\ensuremath{\sim}1\text{ }\text{ }\mathrm{mHz}$. Here the detector strain sensitivity is ${h}_{s}\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$, within a factor of 2. These numbers translate to upper limits at 95% C.L. on the moduli coupling to ordinary matter $({d}_{e}+{d}_{{m}_{e}})\ensuremath{\lesssim}{10}^{\ensuremath{-}5}$ around masses ${m}_{\ensuremath{\phi}}=3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\text{ }\mathrm{eV}$, for the standard DM halo model with ${\ensuremath{\rho}}_{\mathrm{DM}}=0.3\text{ }\text{ }\mathrm{GeV}/{\mathrm{cm}}^{3}$.

We present our current progress on the design and test of Ti/TiN multilayer for use in kinetic inductance detectors. Sensors based on sub-stoichiometric TiN film are commonly used in several applications. However, it is difficult to control the targeted critical temperature $$T_c$$ , to maintain precise control of the nitrogen incorporation process and to obtain a production uniformity. To avoid these problems we investigated multilayer Ti/TiN films that show a high uniformity coupled with high quality factor, kinetic inductance and inertness of TiN. These features are ideal to realize superconductive microresonator detectors for astronomical instruments application but also for the field of neutrino physics. Using pure Ti and stoichiometric TiN, we developed and tested different multilayer configurations, in terms of number of Ti/TiN layers and in terms of different interlayer thicknesses. The target was to reach a critical temperature $$T_c$$ around $$(1{\div } 1.5)$$ K in order to have a low energy gap and slower recombination time (i.e. low generation–recombination noise). The results prove that the superconductive transition can be tuned in the $$(0.5{\div } 4.6)$$ K temperature range by properly choosing the Ti thickness in the $$(0{\div } 15)$$ nm range, and the TiN thickness in the $$(5{\div } 100)$$ nm range.

The European Research Council has recently funded HOLMES, a project with the aim of performing a calorimetric measurement of the electron neutrino mass measuring the energy released in the electron capture decay of 163Ho. The baseline for HOLMES are microcalorimeters coupled to transition edge sensors read-out with rf-SQUIDs, for microwave multiplexing purposes. A promising alternative solution is based on superconducting microwave resonators that have undergone rapid development in the last decade. These detectors, called Microwave Kinetic Inductance Detectors (MKIDs), are inherently multiplexed in the frequency domain and suitable for even larger-scale pixel arrays, with theoretical high energy resolution and fast response. The aim of our activity is to develop arrays of microresonator detectors for X-ray spectroscopy and suitable for the calorimetric measurement of the energy spectra of 163Ho. Superconductive multilayer films composed by a sequence of pure Titanium and stoichiometric TiN layers show many ideal properties for MKIDs, such as low loss, large sheet resistance, large kinetic inductance, and tunable critical temperature $$T_\mathrm{c}$$ . We developed Ti/TiN multilayer microresonators with $$T_\mathrm{c}$$ within the range from 70 mK to 4.5 K and with good uniformity. In this contribution, we present the design solutions adopted, the fabrication processes, and the characterization results.

Different approaches to quantum gravity, such as string theory and loop quantum gravity, as well as doubly special relativity and gedanken experiments in black-hole physics, all indicate the existence of a minimal measurable length of the order of the Planck length, Lp=??G/c3=1.6?10-35m. This observation has motivated the proposal of generalized uncertainty relations, which imply changes in the energy spectrum of quantum systems. As a consequence, quantum gravitational effects could be revealed by experiments able to test deviations from standard quantum mechanics such as those recently proposed on macroscopic mechanical oscillators. Here we exploit the sub-millikelvin cooling of the normal modes of the ton-scale gravitational wave detector AURIGA, to place an upper limit for possible Planck-scale modifications on the ground-state energy of an oscillator. Our analysis calls for the development of a satisfactory treatment of multi-particle states in the framework of quantum gravity models.

Starting from commercial chips, a two-stage dc superconducting quantum interference device (SQUID) was developed in order to use it as a low-noise amplifier on the resonant gravitational wave detector AURIGA. The SQUID was coupled to a high-Q electrical resonator, operating in the kilohertz frequency range, which was employed to simulate the real detector. The resonator was successfully stabilized by means of a capacitive damping network. SQUID additive noise and back-action noise were measured as functions of temperature. The best noise temperature of the SQUID amplifier, measured at 1.5 K, was better than 16 μK, and corresponds to a minimum detectable energy of 200 resonator quanta.

Superconducting Quantum Interference Device (SQUID) microsusceptometers have been widely used to study magnetic properties of materials at microscale. As intrinsically balanced devices, they could also be exploited for direct SQUID-detection of nuclear magnetic resonance (NMR) from micron sized samples, or for SQUID readout of mechanically detected NMR from submicron sized samples. Here, we demonstrate a double balancing technique that enables achievement of very low residual imbalance of a SQUID microsusceptometer over a wide bandwidth. In particular, we can generate ac magnetic fields within the SQUID loop as large as 1 mT, for frequencies ranging from dc up to a few MHz. As an application, we demonstrate direct detection of NMR from (1)H spins in a glycerol droplet placed directly on top of the 20 μm SQUID loops.

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows to measure all the released energy, except the fraction carried away by the neutrino: a finite neutrino mass m $$_\upnu $$ causes the energy spectrum to be truncated at Q $$-$$ m $$_\upnu $$ , where Q is the transition energy. The electron capture of $$^{163}$$ Ho (Q $$\sim $$ 2.5 keV) results to be an ideal decay. In order to achieve enough statistics, a large number of detectors ( $$\sim $$ 10 $$^4$$ ) is required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency domain readout, with theoretical energy and time resolution of $$\sim $$ eV and $$\sim \!\upmu $$ s. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectra of $$^{163}$$ Ho. Currently, a study aimed to the selection of the best design and material for the detectors is in progress. In this contribution, a comparison between the measurements (critical temperature, gap parameter, quasiparticle recombination time and X-ray energy spectra) made with stoichiometric, sub-stoichiometric TiN and Ti/TiN multilayer films are presented.

From the theory of the intrinsic noise in a dc SQUID by Tesche and Clarke, we derive the expressions of the current and voltage input noise spectral densities in a dc SQUID current amplifier operated in a flux locked mode. The expected current and voltage noises are compared, at audio frequencies, with the experimental results obtained with a low noise dc SQUID in which the input load (resonant and not) and the operating temperature (1–4 K) are changed. In order to evaluate the input voltage noise, which is directly related to the current noise around the SQUID loop and is usually neglected, we have used as the input circuit a LC resonator with a very high quality factor (≈106). Both the voltage and current input noises exceed the expected values by the same factor of about 8. This means that the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximately given by the product of the input coil inductance and the angular frequency. To explain the excess noise results, we propose a model in which the voltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.

For the first time a number of cryogenic resonant bar detectors of gravitational waves — ALLEGRO, AURIGA, EXPLORER, NAUTILUS, NIOBE — are on the air, in a continuous search for impulsive events. We present their capabilities, the foreseen upgrades and their role in a future global network with interferometric detectors.

The first run of the ultracryogenic resonant bar detector AURIGA is in progress. Diagnostics on the cryogenics, the data acquisition system and on the noise characteristics have been performed, with results in accord with the design. The bar reached 140 mK. In tests down to 2 K the detector noise was very close to `Brownian'.

The integrated version of a two stage dc SQUID-based low frequency amplifier has been designed, fabricated and tested. The value of the gain of the applied flux d/spl Phi//sub SQ2//d/spl Phi//sub SQ1/ in the range 30-70 and an overall flux-to-voltage transfer factor d/spl Phi//sub SQ2//d/spl Phi//sub SQ1/ as high as 3 mV//spl Phi//sub 0/ have been obtained. An effective input inductance L/sub IN/ equal to 1.8 /spl mu/H, a current sensitivity 1.33 /spl mu/A//spl Phi//sub 0/, and an effective coupling coefficient k/sup 2//sub IN/ close to 0.1 have been found in the accordance with the design.

A two-stage superconducting quantum interference device (SQUID), based on a commercial sensor, is strongly coupled to an electrical resonator at 11 kHz with a quality factor Q=600 000 and operated in the temperature range 1.33–4.17 K. From the analysis of the noise generated by this system, the back action noise of the SQUID amplifier is estimated. The minimum noise temperature, calculated from back action and additive noise measurements, is 63 μK at 1.33 K, and corresponds to 120 times the quantum-limited noise temperature. We discuss and experimentally verify a mechanism, which can limit the noise temperature and add losses to the system.

The design and performance of a compact and easy-to-build thermometer for the temperature range of 10 mK-4.2 K, which is wide enough to cover the entire temperature range of a typical dilution refrigerator, are described here. A commercial low-Tc dc superconducting quantum interference device (SQUID) is used to measure the thermal current noise of a resistor, i.e., an insulated copper wire, that scales linearly with the temperature, as expected from the Nyquist formula. Although, in principle, this thermometer is a primary thermometer, its performance was evaluated as a secondary thermometer. The relative precision of the temperature, which is independent of the temperature, was estimated at 0.4% in a measuring time of 130 s, in good agreement with the expectations. The accuracy evaluated from the comparison with other calibrated thermometers (i.e., superconducting reference point (SRP), cerium magnesium nitrate, and germanium) on different temperature ranges is better than 2% between 10 mK and 4.2 K. If only the SRP thermometer is considered for the comparison in the temperature range of 20-207 mK, the accuracy improves to 1%. The calibrations performed with a single fixed point have a typical accuracy value of about 1%.

The design and performance of a planar microwave RF-SQUID (superconducting quantum interference device) read out by a cryogenic high-electron-mobility-transistor (HEMT) preamplifier are described. The SQUID sensor consists of a planar half-wavelength microstrip resonator, into which the SQUID loop is integrated. The SQUID is operated at a pump frequency of 1.7 GHz. The cryogenic preamplifier uses a microwave HEMT, located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 K and 1.5 K. In an open loop, a flux noise of 2*10/sup -6/ Phi /sub 0// square root Hz was measured at 4.2 K, resulting in an energy resolution of about 100 h. The design of a 3-GHz magnetometer is also described, and the preliminary results of the measurements are reported. >

We report on the noise characterization of a two-stage dc superconducting quantum interference device (SQUID) amplifier developed for resonant gravitational wave detectors. The back action noise is estimated by coupling the SQUID to an electrical resonator at 1.6 kHz with Q=1.1×106. From measurements of back action and additive SQUID noise, performed in the temperature range 1.5–4.2 K, an upper limit is set on the noise temperature Tn of the device at the resonator frequency. The best value obtained at 1.5 K is Tn⩽16 μK and corresponds to 200 resonator quanta. The thermal component of the noise temperature is found in reasonable agreement with the predicted value.

We present here the results of a 515 days long run of the IGEC2 observatory, consisting of the four resonant mass detectors ALLEGRO, AURIGA, EXPLORER and NAUTILUS. The reported results are related to the fourfold observation time from Nov. 6 2005 until Apr. 14 2007, when Allegro ceased its operation. This period overlapped with the first long term observations performed by the LIGO interferometric detectors. The IGEC observations aim at the identification of gravitational wave candidates with high confidence, keeping the false alarm rate at the level of 1 per century, and high duty cycle, namely 57% with all four sites and 94% with at least three sites in simultaneous observation. The network data analysis is based on time coincidence searches over at least three detectors: the four 3-fold searches and the 4-fold one are combined in a logical OR. We exchanged data with the usual blind procedure, by applying a unique confidential time offset to the events in each set of data. The accidental background was investigated by performing sets of 10^8 coincidence analyses per each detector configuration on off-source data, obtained by shifting the time series of each detector. The thresholds of the five searches were tuned so as to control the overall false alarm rate to 1/century. When the confidential time shifts was disclosed, no gravitational wave candidate was found in the on-source data. As an additional output of this search, we make available to other observatories the list of triple coincidence found below search thresholds, corresponding to a false alarm rate of 1/month., Comment: 10 pages, 8 figures Accepted for publication on Phys. Rev. D

Ultrahigh frequency rf‐SQUID magnetometer employing a cryogenic preamplifier and a commercially available thin film rf‐SQUID sensor have been designed, manufactured, and tested. The system has been operated at 316 MHz, which is the maximum pump frequency accepted by the thin film sensor. The cryogenic preamplifier has been built around a microwave high‐electron‐mobility transistor which was made unconditionally stable at ultrahigh frequency band by means of an appropriate circuit design, namely, a cascode configuration. The total flux noise of the magnetometer has been found to be 8.4 × 10−6Φ0/√Hz at 4.2 K and 5.8× 10−6Φ0/√Hz at 1.4 K. The temperature dependence of the fractional step rise α of the current‐voltage rf characteristic has been measured to be proportional to the 2/3 power of the SQUID sensor temperature. The noise performance of the system at 4.2 K has been limited by the intrinsic SQUID noise only, in good agreement with theoretical predictions.

We analyze heat and work fluctuations in the gravitational wave detector AURIGA, modeled as a macroscopic electromechanical oscillator in contact with a thermostat and cooled by an active feedback system. The oscillator is driven to a steady state by the feedback cooling, equivalent to a viscous force. The experimentally measured fluctuations are in agreement with our theoretical analysis based on a stochastically driven Langevin system. The asymmetry of the fluctuations of the absorbed heat characterizes the oscillator's nonequilibrium steady state and reveals the extent to which a feedback cooled system departs from equilibrium in a statistical mechanics perspective.

An advanced version of a two stage dc SQUID-based low frequency amplifier and its measured signal and noise parameters are presented in this report. An effective input inductance of about 2.9 /spl mu/H, a SQUID's inductance of about 11 pH, an effective coupling k/sup 2/ of about 0.24 and a white noise level of about 0.87 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ at 4.2 K were achieved. The gradiometric scheme, a modified distributed damping filter and a modified geometry of coupling transformers have been used to suppress the influences of the external noise and resonances in the SQUID microstructure.

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., 4 pages, 4 figures

The network of resonant bar detectors of gravitational waves resumed coordinated observations within the International Gravitational Event Collaboration (IGEC-2). Four detectors are taking part in this collaboration: ALLEGRO, AURIGA, EXPLORER and NAUTILUS. We present here the results of the search for gravitational wave bursts over 6 months during 2005, when IGEC-2 was the only gravitational wave observatory in operation. The network data analysis implemented is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS, keeping the data from ALLEGRO for follow-up studies. With respect to the previous IGEC 1997-2000 observations, the amplitude sensitivity of the detectors to bursts improved by a factor about 3 and the sensitivity bandwidths are wider, so that the data analysis was tuned considering a larger class of detectable waveforms. Thanks to the higher duty cycles of the single detectors, we decided to focus the analysis on three-fold observation, so to ensure the identification of any single candidate of gravitational waves (gw) with high statistical confidence. The achieved false detection rate is as low as 1 per century. No candidates were found., Comment: 10 pages, to be submitted to Phys. Rev. D

The coupling effects of a commercial dc superconducting quantum interference device (SQUID) to an electrical LC resonator which operates at audio frequencies (≈1 kHz) with quality factors Q≈106 are presented. The variations of the resonance frequency of the resonator as functions of the flux applied to the SQUID are due to the SQUID dynamic inductance in good agreement with the predictions of a model. The variations of the quality factor point to a feedback mechanism between the output of the SQUID and the input circuit.

At the time when the giant flare of SGR1806-20 occurred, the AURIGA "bar" gw detector was on the air with a noise performance close to stationary gaussian. This allows to set relevant upper limits, at a number of frequencies in the vicinities of 900 Hz, on the amplitude of the damped gw wave trains, which, according to current models, could have been emitted, due to the excitation of normal modes of the star associated with the peak in X-rays luminosity., Comment: 4 pages, 3 figures

A high Q electrical resonator based on a superconducting coil and a low loss capacitor has been realized and characterized at temperatures down to 60 mK. The resonance frequency is near 10 kHz, and the quality factor is higher than 10(5). The main purpose of the experiment is to test the feasibility of cooling to ultracryogenic temperatures the readout of the gravitational wave detector AURIGA, which is based on a high Q resonant electrical matching network. The resonator current noise, measured by a superconducting quantum interference device amplifier, scales with temperature in the full range 60 mK-4.2 K, showing that the system is thermal noise limited and its dissipating elements are well thermalized. Some implications of these results and possible applications are discussed. (c) 2005 American Institute of Physics

We have developed a double dc SQUID based magnetometer with a direct read-out circuit and measured its signal and noise characteristics. The inductances of the input SQUID and SQUID-preamplifier were 120 and 250 pH. Thin-film flux transformers on the separate substrates had an inductance of the order of 1 /spl mu/H each and a current sensitivity of 0.54 and 0.32 /spl mu/A//spl Phi//sub 0/ for the first and the second SQUIDs, respectively. The intrinsic energy resolution of the fabricated magnetometer for an optimal gain of 58 for the first stage was close to 1.6/spl times/10/sup -32/ J/Hz. The dynamic range and the slew rate were about 126 dB and 4/spl times/10/sup 4/ /spl Phi//sub 0//s. >

A two-stage SQUID (Superconducting QUantum Interference Device) amplifier based on a modified commercial sensor is operated strongly coupled to an electrical resonator which simulates, as regards resonance frequency (≅1 kHz) and quality factor (≅106), the load of one of the mechanical modes of a resonant cryogenic gravitational wave detector. The behavior of the system is stable and, in the temperature range 1.5–4.2 K, the noise of the resonator is in agreement with the equipartition theorem. Thanks to the very high quality factor it is possible to evaluate the back action noise of the SQUID amplifier which is found to be thermal and in agreement with the theory. The open input broadband noise is also thermal and in agreement with the theory down to 300 mK and levels off to a constant value which corresponds to 35 h at lower temperatures. The noise temperature of the SQUID amplifier, obtained from the broadband and back action noise measurements, is 15 μK at 1.5 K. On the basis of these results the operat...

We report the status of the experimental effort devoted at improving the sensitivity and widening the band of the gravitational wave detector AURIGA. The focus is on an optimized setup of the capacitive resonant transducer, read by an improved dc-SQUID amplifier and on the implementation of an opto-mechanical resonant transducer. Both techniques, which are complementary, should lead to an improvement of the detector performances of at least two orders of magnitude in both energy sensitivity and bandwidth.

We have recently shown that the very low mechanical energy achieved and measured in the main vibration mode of gravitational wave bar detectors can set an upper limit to possible modifications of the Heisenberg uncertainty principle that are expected as an effect of gravity. Here we give more details on the data analysis procedure that allows one to deduce the energy of the bar mode (i.e., the meaningful parameter for our purpose). Furthermore, we extend the analysis of our results, discussing their implication for physical models that face quantum gravity from different points of view, e. g., proposing modified commutation relations or exploring spacetime discreteness.

Superconducting microwave microresonators are low temperature detectors compatible with large-scale multiplexed frequency domain readout. Our aim is to adapt and further advance the technology of microresonator detectors to develop new devices applied to the problem of measuring the neutrino mass. More specifically, we aim to develop detector arrays which can be applied to the calorimetric measurement of the energy spectra of 163Ho EC decay (Q ∼ 2-3 keV) for a direct measurement of the neutrino mass. In order to achieve this goal, a study aimed to the selection of the best design and material for the detectors is required. A recent advance in microwave microresonator technology was the discovery that some metal nitrides, such as TiN, possess properties consistent with very high detector sensitivity. In this contribution, our progress on the design and test of Ti/TiN multilayer films is presented. We report measurements made on stoichiometric TiN, sub-stoichiometric TiN and multilayer Ti/TiN films including the critical temperature, the gap parameter and the quasi-particle recombination time extrapolated from ∼keV X-ray pulses. © Published under licence by IOP Publishing Ltd.

We report on the design and performance of a planar microwave rf‐SQUID readout by a cryogenic preamplifier. The SQUID‐sensor consists of a planar half‐wavelength microstrip resonator, into which the SQUID‐loop is integrated. The pump frequency is 1.7 GHz. The cryogenic preamplifier employs a microwave high electron mobility transistor (HEMT), located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 and 1.5 K. In an open loop white flux noise values of 2×10−6 Φ0/√Hz were obtained at 4.2 K.

We have implemented likelihood testing of the performance of an optimal filter within the online analysis of AURIGA, a sub-Kelvin resonant-bar gravitational wave detector. We demonstrate the effectiveness of this technique in discriminating between impulsive mechanical excitations of the resonant-bar and other spurious excitations. This technique also ensures the accuracy of the estimated parameters such as the signal-to-noise ratio. The efficiency of the technique to deal with nonstationary noise and its application to data from a network of detectors are also discussed.

Sensitivity of presently working gravitational wave resonant detectors is limited by the transduction chain—resonant transducer, impedance matching circuits and SQUIDs. For this reason the Auriga collaboration has started in 1998 the construction of an ultracryogenic test facility where the entire transduction chain can be tested and improved. Mechanical isolation will easely allow for intrinsic thermal noise measurement. In this poster we present the design and first measurements of a new resonant capacitive transducer, intended to widen the detector bandwidth up to 50 Hz with the available new generation of SQUIDs.