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We study the resonant quantum behaviour of a macroscopic superconducting system interacting with an external field. First we consider the resonant tunneling assisted by microwave frequency in the moderate underdamped quantum regime. Then we consider the resonant process in the extreme underdamped limit. In both regimes we have investigated a small frequency range, where resonant phenomena take place. The transition probability W from the left to the right potential well has been numerically calculated for an rf-SQUID as function of the external flux φx for both regimes. Results indicate that, in a proper frequency and φx range, the transition probability W can present two or three resonant peaks, depending on the damping regime.

We present a fully integrated rf SQUID-based system as flux qubit with a high control of the flux transfer function of the superconducting transformer modulating the coupling between the flux qubit and the readout system. The control of the system is possible by including into the superconducting flux transformer a vertical two-Josephson-junctions interferometer (VJI) in which the Josephson current is precisely modulated from a maximum to zero by a transversal magnetic field parallel to the flux transformer plane. The proposed system can be also used in a more general configuration to control the off-diagonal terms in the Hamiltonian of the flux qubit and to turn on and off the coupling between two or more qubits.

We present a niobium-based Josephson device as prototype for quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer to tune the flux qubit allowing the control of the off-diagonal Hamiltonian terms of the system. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control by a small transversal magnetic field parallel to the rf superconducting loop plane. © 2006 IOP Publishing Ltd.

The growing amount of theoretical interest in the area of quantum computing have stimulated in recent years research with the aim of developing a corresponding technology. Superconducting Josephson systems appear to be among the most promising candidates. We present the characterization of a fully integrated Josephson device consisting of an RF SQUID coupled to a readout system based on a dc SQUID sensor. We report measurements of the dc SQUID performances showing a high intrinsic responsivity and a low flux noise, giving a good signal to noise ratio in the small signal mode. In the classical regime data on the decay rate from the metastable flux states of RF SQUID are also reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments. Work is in progress to improve the design of the device to increase its application capability toward the quantum regime.

We will report results about the noise and performance of magnetic nanosensors based on niobium nano superconducting quantum interference device (nanoSQUID) having a flux capture area of 0.5 μm2. A proper device design based on a loop inductance asymmetry has been developed in order to achieve a better magnetic flux resolution. The device fabrication procedure is based on the electron-beam lithography, thin film deposition, and the lift-off technique. The characterization of the nanodevice at T =4.2 K includes measurements of current–voltage, critical current vs. magnetic flux characteristic (I−Φ), and flux noise. The nanosensors have shown a hysteretic I–V characteristic and a triangular-shaped I−Φ pattern. Due to the hysteretic behavior, the devices have been employed as a magnetic flux to current transducer. In such a configuration, an overall magnetic flux resolution of about 0.1 mΦ0 has been estimated. A proper feedback circuit has been employed to increase the dynamic range of the nanosensor. Magnetization measurements at T=4.2 K on Fe3O4 nanoparticles having a size of 8 nm have been reported, proving that nanoSQUIDs reported here can be successfully employed to investigate the magnetism at a sub-micrometric scale.

In this paper, the performance of a first order planar SQUID gradiometer with a long baseline, with regard its application to biomagnetism, is presented. In particular, we report on the analysis of the device response to the current dipole which is the most fundamental model used in biomagnetism. The planar gradiometer is arranged to measure the field components. The results have been compared with those of an axial SQUID gradiometer having the same baseline, pickup coil's size and the same SQUID design parameters. The performance study involves the computation of the magnetic flux measured by the sensors in the presence of a current dipole and the minimum detectable dipole as function of both off-axis and on-axis distance from the gradiometer. A comparative study as a function of sensor baseline has been also performed. Furthermore, the spatial resolution has been investigated by computing the response of both sensor type to a pair of current dipoles. The results have shown that the dipole current sensitivity of a planar gradiometer, (which measures the tangential magnetic fields), is better than those of the axial gradiometer suggesting that it is possible to employ planar gradiometers in multichannel systems for biomagnetism.

Improved niobium-based SQUID sensors in a magnetometer and a gradiometer configuration are presented. The SQUID magnetometer has an integrated square superconducting sensing coil with an area of 9 mm2, much smaller than a typical SQUID magnetometers keeping a comparable magnetic field sensitivity but a higher spatial resolution. At T = 4.2 K, an intrinsic magnetic field noise spectral density of 5.8 fT/?Hz, has been measured in the flux locked loop configuration. The SQUID planar gradiometer with a long baseline (50 mm) has a pickup antenna consisting of a series of two integrated rectangular coils inductively coupled to the SQUID in a double washer configuration. At T = 4.2 K a magnetic flux noise spectral density of 3 ??0/?Hz has been measured. The spectral density of the magnetic field noise referred to one sensing coil, is 3.0 fT/?Hz resulting in a gradient spectral noise of 0.6 fT/cm · ?Hz.

We present a new theoretical approach to describe the quantum behavior of a macroscopic system interacting with an external irradiation field, close to the resonant condition. Here we consider the extremely underdamped regime for a system described by a double well potential. The theory includes both: transitions from one well to the other and relaxation processes. We simulate resonant phenomena in a rf-SQUID, whose parameters lie in the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system $\phi_x$ shows three resonance peaks. One peak is connected with the resonant tunneling and the two others with the resonant pumping. The relative position of the two peaks correlated to the resonant pumping depends on the pumping frequency $\nu$ and on the system parameters. The preliminary measurements on our devices show a low dissipation level and assure that they are good candidates in order to realize new experiments on the resonant phenomena in the presence of an external microwave irradiation of proper frequency., Comment: 10 figures

We calculate the flux threading the pick-up coil of a square SQUID magnetometer in the presence of a current dipole source. The result reproduces that of a circle coil magnetometer calculated by Wikswo [J.P. Wikswo, AIP Conf. Proc. 44 (1978) 145] with only small differences. However it has a simpler form so that it is possible to derive from it closed form expressions for the current dipole sensitivity and the spatial resolution. The results are useful to assess the overall performance of the device and allow the comparison of magnetometers with different detection coil size.

We present the design and the performances of a vertical Josephson interferometer, which can be inserted within superconducting loops in order to obtain controllable Josephson devices. The most interesting feature of the vertical interferometer is that the Josephson current is precisely controlled by a small transversal magnetic field parallel to the superconducting loop plane and can be modulated from a maximum to zero with high precision. This system can be used to realize a switchable flux transformer to couple quantum bits when desired as well as to vary the critical current of a flux qubit in situ. In the latter application, the vertical interferometer is controlled by a magnetic field orthogonal to the one applied to tilt the qubit potential, thus minimizing the cross talking of the two magnetic fields.

We study the resonant quantum behaviour of a macroscopic superconducting system interacting with an external field. First we consider the resonant tunneling assisted by microwave frequency in the moderate underdamped quantum regime. Then we consider the resonant process in the extreme underdamped limit. In both regimes we have investigated a small frequency range, where resonant phenomena take place. The transition probability W from the left to the right potential well has been numerically calculated for an rf-SQUID as function of the external flux ? x for both regimes. Results indicate that, in a proper frequency and ? x range, the transition probability W can present two or three resonant peaks, depending on the damping regime.

We investigate the properties of Josephson junction networks with inhomogeneous architecture. The networks are shaped as "quare comb" planar lattices on which Josephson junctions link superconducting islands arranged in the plane to generate the pertinent topology. Compared to the behavior of reference linear arrays, the temperature dependencies of the Josephson currents of the branches of the network exhibit relevant differences. The observed phenomena evidence new and surprising behavior of superconducting Josephson arrays as well as remarkable similarities with bosonic junction arrays., improved figures (added magnetic pattern and single junction switching) some changes in the text and in the title

We consider the theoretical problem of resonant phenomena in the macroscopic quantum tunneling for an rf superconducting quantum interference device in the presence of an external irradiating field. The transition probability between different flux states is studied by varying the external parameters of the double-well potential describing the system in a way that the pumping level in the left potential well is close to one of the levels in the right potential well. The dependence of the transition probability on the external drive of the system shows two resonance peaks, the former connected with the resonant tunneling and the latter with the resonant pumping. The relative position of the peaks depends on the pumping frequency $\ensuremath{\omega}$ and on the system parameters.

We present the experimental observation of the effects of macroscopic quantum tunnelling in a SQUID device, consisting of a rf SQUID coupled to a readout system based on a dc SQUID sensor. Data on the decay rate from the metastable flux states of a rf SQUID are reported, both in the classical and quantum regimes. The low dissipation level and the good insulation of the probe from external noise are encouraging in view of building a macroscopic quantum coherent system.

We present an experimental characterization of $\mathrm{Nb}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Nb}$ Josephson devices in the quantum regime. Josephson junctions and fully integrated Josephson devices consisting of an rf SQUID coupled to a read-out system based on a dc SQUID have been measured. Data on the decay rates from the metastable states show evidence of macroscopic quantum tunneling with a very low dissipation level and a high isolation of the devices from the external environment.

We present the characterization of a fully integrated Josephson device consisting of an rf-SQUID coupled to a readout system based on a dc-SQUID sensor. In the classical regime, data on the decay rate from the metastable flux states of rf-SQUID are reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments. Furthermore we examine the design of a quantum CNOT gate by adiabatic operations for rf-SQUID devices.

A technique to induce spatial modulation of critical current density in niobium based Josephson devices by using a selective thermal annealing is reported. By depositing a carbon film onto selected region of the Josephson element it is possible to induce a localized heating, with a spatial resolution less than 1 μm, exploiting the much higher absorbance coefficient of carbon than the niobium one. The effectiveness of such technique is demonstrated by experimental measurement of the critical current vs. magnetic field, measured at T = 4.2 K, showing that the change of critical current density occurs only in the region corresponding to the absorber film area. Furthermore, the theoretical behaviour, by modelling a suitable step-like junction barrier shape, has been carried out to fit the experimental data in order to verify the selective modulation of critical current. This technique can be very useful in view of quantum computing experiments, Majorana fermions detection and superconducting magnetic sensors.

Results about induced spatial modulation of critical current density in niobium based Josephson devices by a selective radiation heating are presented. The localized heating inside a single circuit element with a good spatial resolution (less than 1 μm) is made possible by depositing a carbon film on the desired region exploiting the larger absorbance coefficient of carbon (close to 1) with respect to niobium (about 0.2). Measurements of critical current as a function of the external magnetic field on high quality Josephson junctions at T = 4.2 K evidenced that the critical current density was changed only in the absorber covered region, leaving unchanged the critical current density outside that region and ensuring the capability to locally modify the Josephson critical current density. The experimental data have been compared with theoretical predictions obtained by using a suitable junction barrier model based on step-like barrier shape. Such results are very interesting in view of applications in quantum computing, Majorana fermions detection, and superconducting magnetic sensors.

In the present paper, performances of compact fully integrated superconducting quantum interference device (SQUID) magnetometers, recently developed, have been investigated in view of their employment in large multichannel systems for biomagnetic imaging. The analysis has been focused on SQUID sensors having a pickup loop side length of 3 and 4 mm based on a design aimed to maximize the magnetic flux transferred from the detection coil to the SQUID in comparison with a magnetometer with 9 mm side length having a suitable sensitivity for biomagnetic applications. The performance study has been consisted in the computation of the magnetic responses to a current dipole which is the most fundamental approach used in biomagnetism. The results have shown that the dipole current sensitivity of 4 mm long side compact magnetometers is suitable for application in multichannel systems for magnetoencephalography and magnetocardiography.

We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit).

We present an experimental study of the effective dissipation of a fully integrated ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}$ Josephson device consisting of a rf superconducting quantum interference device (SQUID) coupled to a readout system based on a dc SQUID sensor. In the classical regime data on the decay rate from the metastable flux states of rf SQUID are reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of building a macroscopic quantum device.

We have investigated the mechanical response of a magnetically shielded room that hosts a magnetoencephalography system that is subject to external vibrations. This is a superconducting quantum interference device, which are the most sensitive sensors for magnetic flux variations. When the magnetoencephalography operates with people inside the room, the spectrum of the flux of the magnetic field shows anomalous peaks at several frequencies between 1 Hz and 20 Hz, independent of the experiment that is being run. As the variations in the flux of the magnetic field through the sensors might not only be related to the electrical currents circulating inside the brain, but also to non-damped mechanical oscillations of the room, we installed seismic instrumentation to measure the effective motion inside the room and to compare it to the external motion. For this analysis, we recorded the ambient seismic noise at two very close stations, one inside the magnetically shielded room, the other one outside in the room in which the magnetically shielded room is itself located. Data were collected over four days, including a week-end, to study the response of the magnetically shielded room subjected to different energy levels of external vibrations. The root mean square, Fourier spectra and power spectral density show significant differences between the signal recorded inside and outside the magnetically shielded room, with several anomalous peaks in the frequency band of 1 Hz to 20 Hz. The normalized spectral quantities (horizontal to vertical spectral ratio, and ratio between the internal and external spectra) show large amplification at several frequencies, reaching in some cases one order of magnitude. We concluded that the magnetically shielded room does not dampen the external vibrations, but it instead appears to amplify these across a broad frequency range.