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We have measured the rate of thermally induced escape from the zero-voltage state in long Josephson junctions of both overlap and in-line geometry as a function of applied magnetic field. The statistical distribution of switching currents is used to evaluate the escape rate and derive an activation energy ⌬U for the process. Because long junctions correspond to the continuum limit of multidimensional systems, ⌬U is in principle the difference in energy between stationary states in an infinite-dimensional potential. We obtain good agreement between calculated and measured activation energies for junctions with lengths a few times the Josephson penetration depth J . ͓S0163-1829͑96͒01145-9͔

This paper presents the results of the observations of the detectors participating in the International Gravitational Event Collaboration ͑IGEC͒ from 1997 to 2000 and reviews the data analysis methods. The analysis is designed to search for coincident excitations in multiple detectors. The data set analyzed in this article covers a longer period and is more complete than that given in previous reports. The current analysis is more accurate for determining the false dismissal probability for a time coincidence search and it optimizes the search with respect to a target amplitude and direction of the signal. The statistics of the accidental coincidences agrees with the model used for drawing the results. The observations of this IGEC search are consistent with no detection of gravitational wave burst events. A new conservative upper limit has been set on the rate of gravitational wave bursts with a Fourier component HϾ2ϫ10 Ϫ21 Hz Ϫ1 , both for searches with and without a filter for the galactic center direction. This study confirms that the false alarm rate of the observation can be negligible when at least three detectors are operating simultaneously.

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 implemented network data analysis is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS; ALLEGRO data was reserved for follow-up studies. The network amplitude sensitivity to bursts improved by a factor 3 over the 1997-2000 IGEC observations; the wider sensitive band also allowed the analysis to be tuned over a larger class of waveforms. Given the higher single-detector duty factors, the analysis was based on threefold coincidence, to ensure the identification of any single candidate of gravitational waves with high statistical confidence. The false detection rate was as low as 1 per century. No candidates were found.

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We experimentally demonstrate the coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable via the amplitude of the flux pulse. The demonstrated operation mode allows to realize quantum gates which take less than 100 ps time and are thus much faster compared to other superconducting qubits. An other advantage of this type of qubit is its insensitivity to both thermal and magnetic field fluctuations.PACS numbers: 03.67. Lx, 85.25.Dq Superconducting qubits stand between the most promising systems for the realization of quantum computation. Coherent quantum evolution and manipulation have been demonstrated and extensively studied for single [1,2,3,4,5] and coupled superconducting qubits [6,7,8,9,10,11]. In most cases, the state of superconducting qubits are manipulated by means of microwave pulses, with a technique similar to the NMR manipulation of atoms. An alternative way to manipulate qubits is based on modifying their energy potential without applying any microwave signals [1,5]. The latter approach requires a much simpler experimental technique and offers the possibility of using classical logic signals to control a quantum processor in situ, which is advantageous for the large scale implementation of a quantum circuits.In this Letter, we report the observation of tunable coherent oscillations in a SQUID-based flux qubit. These oscillations are obtained by manipulating the qubit with nanosecond-long pulses of magnetic flux rather than microwaves. By this technique, we could increase the oscillation frequency up to 21 GHz, which allows to perform very fast logical quantum gates. Since the relevant quality factor of a qubit is the number of gate operations which can be performed during its coherence time, this result is of particular interest towards the realization of a solid-state quantum computer.The investigated circuit, shown in Fig. 1(a), is a double SQUID consisting of a superconducting loop of inductance L = 85 pH, interrupted by a small dc SQUID of loop inductance l = 6 pH. This dc SQUID is operated as a single Josephson junction (JJ) whose critical current is tunable by an external magnetic field. Each of the two JJs embedded in the dc SQUID has a critical current I 0 = 8µA and capacitance C = 0.4 pF. The qubit is manipulated by changing two magnetic fluxes Φ x and Φ c , applied to the large and small loops by means of two coils of mutual inductance M x = 2.6 pH and M c = 6.3 pH, respectively. The readout of the qubit flux is performed by measuring the switching current of an unshunted dc SQUID, which is inductively coupled to the qubit [12]. The circuit was manufactured by Hypres [13] using standard Nb/AlO x /Nb technology in a 100 A/cm 2 critical current density process. The dielectric material used for junction isolation is SiO 2 . The whole circuit is designed gradiometrically in order...

We report the result from a search for bursts of gravitational waves using data collected by the cryogenic resonant detectors EXPLORER and NAUTILUS during the year 2001, for a total measuring time of 90 days. With these data we repeated the coincidence search performed on the 1998 data (which showed a small coincidence excess) applying data analysis algorithms based on known physical characteristics of the detectors. With the 2001 data a new interesting coincidence excess is found when the detectors are favorably oriented with respect to the Galactic Disk.