Long term operation of low noise DC-SQUID coupled to a very high Q gravitational radiation detector

We have coupled a very low noise dc-SQUID to the gravitational radiation detector of the Rome group at CERN laboratories. The SQUID used is a multiloop thin-film device with an input inductance of 1.6 μH, loop inductance of 5 pH and coupling coefficient of 0.5. The gravitational radiation detector is composed by a 2.3 tons Aluminum cylinder mechanically coupled to a resonant capacitive transducer; this is matched to the SQUID by means of a large superconducting transformer. The signal to be detected is essentially composed by the two mode frequencies at about 1 kHz and with quality factors of the order of 4×10⁶. To operate in a closed feedback loop mode we have used a particular setup in order not to degrade the performance of the system. The system operated for seven months with some interruptions due to refilling of liquid helium and various tests on the apparatus. The flux noise obtained was 1.5 to3times10^{-6} Phi_{o}/sqrt{Hz}at 1 kHz with a linearity over 6 orders of magnitude and a long term stability of1.5 times 10^{-8} Phi_{o}/hour.     

Fast new technique for measuring the resonance frequencies of high Q systems

A technique for measuring the resonance frequencies of an aluminum bar (used as a gravitational antenna) with high accuracy is presented. This method is based on the positive feedback of the bar, and is much faster than the others commonly in use.     

Cryogenic Wide-Area Light Detectors for Neutrino and Dark Matter Searches

Large-mass arrays of bolometers proved to be good detectors for neutrinoless double beta decay (0 \(\nu \) DBD) and dark matter searches. CUORE and LUCIFER are bolometric 0 \(\nu \) DBD experiments that will start to take data in 2015 at Laboratori Nazionali del Gran Sasso in Italy. The sensitivity of CUORE could be increased by removing the background due to \(\alpha \) particles, by detecting the small amount of ?erenkov light ( \(\sim \) 100 eV) emitted by the \(\beta \) s’ signal and not by \(\alpha \) s. LUCIFER could be extended to detect also dark matter, provided that the background from \(\beta \) / \(\gamma \) particles ( \(\sim \) 100 eV of scintillation light) is discriminated from nuclear recoils of about 10 keV energy (no light). We have recently started to develop light detectors for CUORE, LUCIFER and similar bolometric experiments. The aim is to obtain detectors with an active area of \(5\times 5~\mathrm{cm}^2\) (the face of bolometric crystals), operating at 10 mK, and with an energy resolution at the baseline below 20 eV RMS. We have chosen to develop phonon-mediated detectors with KID sensors. We are currently testing the first prototypes.     

Estimation of neural dynamics from MEG/EEG cortical current density maps: application to the reconstruction of large-scale cortical synchrony

There is a growing interest in elucidating the role of specific patterns of neural dynamics--such as transient synchronization between distant cell assemblies--in brain functions. Magnetoencephalography (MEG)/electroencephalography (EEG) recordings consist in the spatial integration of the activity from large and multiple remotely located populations of neurons. Massive diffusive effects and poor signal-to-noise ratio (SNR) preclude the proper estimation of indices related to cortical dynamics from nonaveraged MEG/EEG surface recordings. Source localization from MEG/EEG surface recordings with its excellent time resolution could contribute to a better understanding of the working brain. We propose a robust and original approach to the MEG/EEG distributed inverse problem to better estimate neural dynamics of cortical sources. For this, the surrogate data method is introduced in the MEG/EEG inverse problem framework. We apply this approach on nonaveraged data with poor SNR using the minimum norm estimator and find source localization results weakly sensitive to noise. Surrogates allow the reduction of the source space in order to reconstruct MEG/EEG data with reduced biases in both source localization and time-series dynamics. Monte Carlo simulations and results obtained from real MEG data indicate it is possible to estimate non invasively an important part of cortical source locations and dynamic and, therefore, to reveal brain functional networks.     

Multivariate reconstruction of functional networks from cortical sources dynamics in MEG/EEG

In this paper, we present a simple method to find networks of time-correlated brain sources, using a singular value decomposition (SVD) analysis of the source matrix estimated after any linear distributed inverse problem in magnetoencephalography (MEG) and electroencephalography (EEG). Despite the high dimension of the source space, our method allows for the rapid computation of the source matrix. In order to do this, we use the linear relationship between sensors and sources, and show that the SVD can be calculated through a simple and fast computation. We show that this method allows the estimation of one or several global networks of correlated sources without calculating a coupling coefficient between all pairs of sources. A series of simulations studies were performed to estimate the efficiency of the method. In order to illustrate the validity of this approach in experimental conditions, we used real MEG data from a visual stimulation task on one test subject and estimated, in different time windows of interest, functional networks of correlated sources.     

Experimental Evaluation of the Intrinsic Dissipation from Energy-Level Quantization in Josephson Devices

The main problem in realizing an experiment of macroscopic quantum coherence, namely, an experiment where the nonclassical behavior of a macroscopic system must be detected, is the fulfillment of many experimental constraints, in principle very difficult to achieve. One of the most critical parameters is the decoherence time of the system. The Rabi oscillations of a two-level system, in fact, are canceled if the quality factor associated with the oscillation is less than unity. In particular, it can be shown that the decoherence time for a SQUID system, once the temperature is given, depends only on the effective resistance. To evaluate the effective resistance of our system we have measured the energy-level quantization (ELQ) under stationary conditions at a temperature between 13 and 35 mK, for a Josephson junction and for an rf SQUID using the same type of junction. For both systems we can clearly see ELQ, because of the very low level of the intrinsic dissipation. From these measurements we can then set a lower limit for the effective system dissipation and then infer the decoherence time related to the overall setup of our experiment.