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It has recently been shown that the relaxation time of a graphene sheet is the crucial parameter that governs the radiation performance in graphene THz antennas based on either plasmonic or nonplasmonic leaky waves. Moreover, the radiating properties of these devices have always been derived assuming an ideal dipole-like source, and no full-wave and experimental results on realistic feeders have been reported, yet. To this purpose, in this work we aim at bringing the designs of graphene-based Fabry-Perot cavity leaky-wave antennas (FPC-LWAs) towards an experimental stage. First, the antenna design is validated with full-wave simulations, by modeling not only the antenna structure, but also a realistic THz source, namely an open-ended waveguide. Another considerable difference with the previous designs is the use of a cylindrical geometry. As a result, the radiation patterns now exhibit a remarkable omnidirectionality over the azimuthal plane, as the beam is scannable from broadside up to 45°. Second, THz measurements are carried out to characterize graphene samples deposited over silicon substrates. The relaxation time is estimated by combining the transmittance measurements of a sample with well-established theoretical models for the graphene surface conductivity. These results shed several issues for the future developments of graphene-based FPC-LWAs.

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2-7 K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (mu s) terahertz pulses. The NEP above 100 Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1 Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450 ns.

The electrical performances and the optical response of a superconducting hotspot air-bridge bolometer (SHAB) coupled through a planar antenna to Terahertz waves and operating in vacuum at about 4.2 K, have been measured. The SHAB consists of a narrow Nb air bridge, voltage biased to form, in the middle of the suspended Nb stripe, a resistive hotspot, whose volume is modulated by the incoming radiation. The signal resulting from the coupled optical power and the device noise performance have been measured by using a home made transimpedance amplifier operating at room temperature, resulting in an electrical NEP of about 40 fW/Hz(1/2). Passive detection of the 300 K blackbody radiation at frequency lower than 1.6 THz, band limited by a low-pass optical filters, has been successfully carried out by using a SHAB.

We describe a simple and efficient scheme for the readout of a tunable flux qubit, and present preliminary experimental tests for the preparation, manipulation and final readout of the qubit state, performed in incoherent regime at liquid Helium temperature. The tunable flux qubit is realized by a double SQUID with an extra Josephson junction inserted in the large superconducting loop, and the readout is performed by applying a current ramp to the junction and recording the value for which there is a voltage response, depending on the qubit state. This preliminary work indicates the feasibility and efficiency of the scheme., Comment: 10 pages, 5 figures

A new generation of astronomical detectors has recently been developed, on the basis of superconducting tunnel junctions (STJs). STJs are capable of detecting photons in a wide range of wavelengths, from X-rays to infrared; also the counting of single photons in the optical range has been demonstrated. Since these detectors operate at very low temperature (down to 100 mK), a natural candidate for the read-out electronics is the dc superconducting quantum interference device (SQUID), an extremely sensitive magnetometer that is fabricated with a similar technology to the STJ junctions. Biasing the STJ at a constant voltage, the current pulse caused by the incoming radiation is passed through the tightly coupled input coil of the SQUID and converted into a magnetic flux signal which can be detected. In this paper we present measurements on a home-made dc SQUID, showing the response of the device to current pulses, which simulate the real signals from a STJ. We show how the SQUID performance can be i...

We report on measurements on superconductor-insulator-normal tunnel junctions at temperatures below 1K. The junctions have been fabricated by shadow mask technique and e-beam lithography and have submicron dimensions. The measurements on our junctions show that their characteristics are very sensitive to temperature changes making them suitable for sensing temperature in a bolometer. Measurements on arrays of junctions designed to test the on chip electronic refrigeration will be also shown.

We propose a novel device, based on a series of dc SQUIDs (direct current Superconducting QUantum Interference Devices), each one with different sensitive area, that can be used to perform absolute measurements of magnetic field. With this array, we overcome the limit of a single dc SQUID of being sensitive only to variations of the linked magnetic flux. The overall voltage-flux characteristic of our device, in fact, has a non-periodic shape that allows a clear identification of the point with zero magnetic field. We present a discussion of the working principle and of the main characteristics, which have been partially tested on a prototype.

We propose a new scheme for a flux/phase qubit, based on a double-SQUID modified to achieve a direct readout of the flux state. The readout is performed by inserting a large Josephson junction in the SQUID loop; the junction is biased by an external current and switches from the zero-voltage to the running state depending on the flux state of the qubit. We report recent results concerning the theoretical behavior and the experimental characterization at 4.2 K of the device, which has been realized with Nb/AlOx/Nb trilayer technology.

We present a fast spectrometer working in the 0.7–4.8 THz range. Broadband radiation from a blackbody source is focused first on a rotating silicon wafer, whose surface was patterned with 18 metal band-pass filters, then on the sample under test and finally is detected by a superconducting microbolometer with microsecond time constant. The bolometer sensor is coupled to a spiral antenna whose frequency band matches the spectral range of the filters. The spectral resolution is set by the filters quality factor of about 3. A dynamic range of 100 and a S/N ratio of 20 are achieved by integrating for less than 10 second. The detector can operate up to 6 K in a closed-cycle cooler, hence making the present apparatus suitable for building up a simple terahertz video-rate spectrometer.

We have fabricated and characterized a superconducting planar gradiometer sensitive to the second‐order spatial derivative of the magnetic field. Our device has been patterned on a 12.8×12.8 mm silicon chip on which the sensitive area of the gradiometer and the SQUID inductance are generated by the same four 0.5‐mm‐square holes. We measured the sensitivity both in flux‐locked‐loop (FLL) and in open‐loop mode. With the second method we used another dc‐SQUID as amplifier and in this case we obtained the best noise performance. The gradiometer sensitivity was 18 fT/cm2 √Hz. The magnetic isoflux line distribution generated by a dipolar source was measured by the gradiometer in FLL mode.

Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating clock from 100MHz to 1GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep well condition. By using this scheme with a Nb/AlOx/Nb system we obtained coherent oscillations with sub-nanosecond period (tunable from 50ps to 200ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10ns, of the order of what obtained with similar devices and technologies but using microwave manipulation. We introduce the ultrafast manipulation presenting experimental results, new issues related to this approach (such as the use of a feedback procedure for cancelling the effect of "slow" fluctuations), and open perspectives, such as the possible use of RSFQ logic for the qubit control., 9 pages, 7 figures

We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations., Comment: Proceedings of Nobel Symposium "Qubits for future quantum computers", Goeteborg, Sweden, May 25-28, 2009; to appear in Physica Scripta

Superconducting meanders of NbN thin films have applications as single-photon detectors with high sensitivity in the infrared region. We report here a detailed analysis of the electrical characteristics of such meanders, by studying structures where each wire of the meander is separately contacted. The effect of heating on the superconducting-normal transition of adjacent stripes is evidenced. Moreover, the analysis of the switching current distribution of each wire highlights the high-critical current uniformity achieved by our meander process.

We have proposed and realized a possible flux qubit based on a Niobium fully gradiometric double SQUID, with a direct readout scheme based on the insertion in the loop of a large Josephson junction. The first characterization performed at 4.2 K shows the capability of our device to discriminate the flux states in a very efficient way using a very simple scheme. Further work is required to study and optimize the effects of the readout on the flux state, together with a low temperature characterization, and finally, a study of the coherent behavior of this system.

We describe the present status of a new read-out for the EXPLORER and NAUTILUS gravitational wave detectors. The read-out is based on a double-gap capacitive transducer and a double-SQUID amplifier. The transducer has been tested at liquid helium temperature and a Q of 1.0.106 has been measured with a biasing field of 20 MV/m. The double- SQUID amplifier has been tested down to 2 K with a high-Q resonant input load, showing very good stability and energy resolution, of about 70. With the new read-out, NAUTILUS, cooled to 100 mK, could reach a peak sensitivity of 3 . 10-22Hz-½ and a bandwidth, at the level of 10-21Hz-½, of about 35 Hz.

An rf-SQUID under appropriate temperature and bias conditions can be used to perform tests on macroscopic quantum coherence, or as a qubit element for quantum computing. For both these purposes it is necessary a readout system allowing to discriminate stroboscopically the rfSQUID state with high fidelity. This task can be performed by means of a hysteretic dc-SQUID inductively coupled to the rf-SQUID and turned on by a fast bias current pulse. We have proposed and realized an integrated system where all the devices, the rf-SQUID, the hysteretic dc-SQUID, and a SQUID magnetometer are on the same chip. This system can perform single-shot detection of the rf-SQUID state with a fidelity better than 98%. The rf-SQUID has been coupled to a high-frequency line working in the microwave region (2–26 GHz), and has been tested and characterized from 4.2 K to the mK region of temperatures.

We have realized and tested a fast stroboscopic detector for magnetic flux measurements. The key element of our detector is a hysteretic dc superconducting quantum interference device (SQUID). Stroboscopic read-out of the magnetic flux coupled with the SQUID is accomplished by biasing the SQUID with fast current pulses. The shorter these pulses, the more stroboscopic and less invasive the measurement we are performing. In order to reduce the duration of the current pulses, we take advantage of the superconducting-normal transition induced by laser light in thin superconducting films. The interaction of laser light with superconducting thin films has been investigated thoroughly in the past and many applications have been proposed which rely on the fast typical times with which superconductivity is broken and a resistive behavior arises. We have measured a threshold resolution of 6.9mPhi(0) at 4.2 K, and this value corresponds to the thermodynamic limit of the SQUID. The detector has been accurately characterized: An improved and more sensitive version might prove useful for quantum mechanics and quantum computation experiments, for example, in detecting the state of flux qubits.

Nell'articolo vengono discusse le proprieta' termiche di strutture mesoscopiche metalliche e isolanti, di interesse per applicazioni bolometriche. Sono stati dapprima caratterizzati sperimentalmente dei termometri a film sottile, basati su giunzioni tunnel metallo normale-isolante-superconduttore, operanti a temperature sotto il Kelvin e fabbricati usando Niobio o Alluminio come elettrodi superconduttori. E' stato poi misurato l'effetto di vari metodi di polarizzazione sulle caratteristiche corrente-tensione, cosi' come la risposta delle strutture metalliche mesoscopiche all'assorbimento o alla rimozione di potenza. La caratterizzazione e' stata fatta in un refrigeratore a 3He, compatibile con la strumentazione generalmente disponibile in esperimenti a terra o nello spazio.

The flux states of a rf superconducting quantum interference device (SQUID) can be used to investigate macroscopic quantum effects, the most challenging of which is the detection of quantum coherent oscillations, and are promising candidates for the implementation of quantum computing. The use of a proper readout system is of utmost importance for these purposes. In this letter we present experimental evidence of the possibility of using an underdamped, hysteretic dc SQUID to read out stroboscopically the flux states of a rf SQUID, allowing a single shot discrimination with an efficiency of 98%. The devices are all integrated on chip.

A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used in a cavity quantum electrodynamic experiment displaying ultrastrong coupling to the cyclotron transition of two-dimensional electron gas. A finite element modeling is developed and its results are in good agreement with the experimental data. In this system a normalized coupling ratio of $\frac{\Omega }{{{\omega }_{c}}}=0.27$ is measured and a clear modulation of the polaritonic states as a function of the temperature is observed., New Journal of Physics, 16, ISSN:1367-2630

The observation and study of macroscopic quantum coherence (MQC), i.e. the coherent superposition and evolution of macroscopically distinct quantum states, is of great importance for both our understanding of nature and for quantum engineering, for example in quantum computing. Recently a first indication of MQC in a system of SQUIDs has been indirectly observed by means of spectroscopic techniques. Applications like quantum computing, however, require the manipulation and the observation of the time evolution of the MQC states. We have realized a system of SQUIDs for the study of MQC. Our system, based on trilayer Nb/AlOx/Nb technology, consists of a double rf-SQUID, source of the MQC state, inductively coupled to a non-hysteretic dc-SQUID magnetometer and to a hysteretic dc-SQUID threshold detector, used for the read-out of the magnetic flux state. The magnetometer is used to test and monitor the rf-SQUID behaviour, while the hysteretic dc-SQUID is used to perform fast measurements with a reduced back action on the rf-SQUID. We present measurements performed at 4.2 K and at 20 mK to characterize the single devices and their overall behaviour, and a scheme for the use of a set of SQUIDs as a qubit system.

We report on a hot-electron micro-bolometer fabricated by electron beam lithography consisting of a small normal-metal absorber capacitively coupled to a planar antenna. Both the thermometer and the coupling capacitors are made by sub-micron normal metal–insulator–superconductor junctions. By adding two extra Andreev contacts to the absorber it is possible to characterize the bolometer by dc measurements.

A brief review on the state of the art of high sensitivity DC-Superconducting quantum interference devices (SQUIDs) will be given here, restricted to low-temperature devices. These devices can have a sensitivity to the external magnetic flux close to the limit posed by quantum mechanics. Progress, made in the last years to get instrumentation capable of reaching such a limit, will be described.

We present noise measurements made at temperatures down to 200 mK on a tunnel junction d.c. SQUID, to be used as amplifier for the gravitational-wave experiment Nautilus of the Rome group. We find that the flux noise decreases with temperature, as expected, with a best measured value of\(\Phi _n \simeq 7 \cdot 10^{ - 7} \Phi _0 /\sqrt {Hz} \) at 0.2 K. The presence of an excess noise term independent of temperature is discussed.

We report on the observation of phase-locking and radiation enhancement from two dc series-biased Josephson fluxon oscillators ac-coupled by a thin film-integrated capacitor. The radiation is detected by a small area Josephson junction displaying, under the effect of the radiation, critical current suppression and Shapiro steps at a voltage of 220μV. The phenomena that we observe are the higher frequency counterpart of the superradiant effects measured by means of X-band room temperature receivers detecting the radiation emitted by series arrays of long Josephson junctions.

A contouring technique based on multiple-source sandwich holography is describedfor evaluation of the corrosion rate of a metal or alloy. An image processing system for automatic fringe analysis is used as a real-time inspection technique. Some experimental results obtained on models are reported.

Noise measurements on multiloop dc SQUIDs, fabricated with Nb‐NbOx‐PbAuIn and Pb‐alloy Josephson tunnel junctions and well coupled with the external world by an input coil, we performed in flux locked loop mode. To achieve this task, we built a reliable low‐cost control electronics to detect the voltage output of the SQUID. The results show that for multiloop dc SQUIDs with single‐junction critical currents larger than 20 μA and an inductive parameter βL less than 4.5, an intrinsic white‐energy spectral density of about 100h can be obtained independently of the technology used to fabricate the junctions. Furthermore, the Nb‐NbOx‐PbAuIn dc SQUIDs have a lower 1/f noise than the Pb‐alloy ones.

A prototype inspection system using a holographic contouring method with computer image processing has been developed for measuring the surface materials lost from art objects by weathering and acid rain. Holographic patterns relating to marble and limestone models exposed to dilute sulfuric acid and methods that can be used to retrieve metrological information are discussed. Les AA. ont imaginé un protocole expérimental utilisant une méthode holographique de repérage des contours avec traitement informatique de l'image, pour mesurer la perte des produits de surface par altération climatique ou effet de pluie acide dans les objets d'art faits de différents matériaux. On commente les images holographiques de modèles en marbre et en calcaire, exposés à une solution d'acide sulfurique diluée pour lesquels les informations météorologiques peuvent être facilement disponibles

An electro-optical system for the detection and quantitative measurements of surface or subsurface defects is presented; it is based on a holographic contouring technique and digital image processing for an automatic analysis of fringe patterns. Several approaches have been followed on models, so that the defects can be unambiguously detected and possibly quantitatively analyzed. Some examples of applications in deterioration investigations of ancient artworks are reported.

In November 1985 the gravitational-wave antenna of the Rome group, installed at CERN, has started operating. It consists of a 5056 aluminium cilinder 3 m long, 2270 kg heavy, cooled at 4.2 K. The antenna vibrations are detected by means of a resonant capacitive transducer that together with the bar makes a two-coupled-oscillator system. The low-noise amplification is obtained with a d.c. SQUID amplifier. The frequencies of the two resonant modes are:v-=907.116 Hz andv+=923.083 Hz, with merit factorsQ−=3.2·106 andQ+=5.6·106. The sensitivity to short gravitational bursts, expressed in terms of effective noise temperature, is 18 mK. This corresponds to a change in the metric tensor whose Fourier transform isH=1.1·10−21 Hz−1. For monochromatic waves the antenna is sensitive (SNR=1) toh∼2·10−25 in a band width of about 1/3 Hz, with a three months integration time.

An investigation of the spontaneous brain activity from normal subjects has been carried out by means of the neuromagnetic method. Two different systems were used for magnetic measurements. Both used 2nd-order gradiometers specifically designed for brain studies: 2.9 cm diameter, 5 cm baseline. A r.f. SQUID and a d.c. SQUID were coupled to the gradiometers. The measured noise levels are\(40fT/\sqrt {Hz}\) and\(25fT/\sqrt {Hz}\), respectively. Thorough magnetic mapping of the spontaneous activity was performed on a few subjects. By mapping the average amplitude of the α-rhythm signals two maxima of activity over two symmetric regions of the scalp were clearly observed. Despite a general symmetry between hemispheres, however, the complexity of the measured maps suggests a corresponding complexity of the generators responsible for α-activity.

A new method for measurements of ultralow currents in the audio‐frequency range (from dc up to over 104 Hz) has been developed. The combined use of a ferromagnetic current transformer and of a superconducting flux transformer together with a rf‐biased SQUID allows measurements of currents as low as 10−14 A. A detailed circuital analysis and considerations on the input impedance of the device are presented. The agreement between predictions and experiments is quite good. The measured noise level beyond the low‐frequency region keeps at values typical of commercial SQUID’s.

Some levitation tests using superconducting coils on superconducting sheets are reported. These tests are used to provide preliminary data for the design of a levitation system for a gravitational antenna. Levitation tests on a small 60 kg bar are also reported. An empirical relation which enables displacement to be predicted as a function of the geometrical size, the weight, and the number of Ampere-turns of the coils is obtained from these tests.

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 to 3\times10^{-6} \Phi_{o}/\sqrt{Hz} at 1 kHz with a linearity over 6 orders of magnitude and a long term stability of 1.5 \times 10^{-8} \Phi_{o} /hour.

We report on the cooling at liquid-helium temperature of our 2270 kg 5056 Al bar at CERN. The liquid-helium container had been filled up to 1500 liters ensuring to keep the antenna cold for more than one month. The antenna is equipped with a resonant capacitive transducer operating at constant electrical charge with a FET low-noise amplifier. The transducer is tuned to the antenna within less than I Hz and the two normal-mode frequencies are ν−=908.160 Hz and ν+=924.234 Hz with an applied electrical field in the transducer of 106 V/m. The corresponding overall merit factors areQ −=5.20·106 andQ +=7.25·106. The transducer has been tested up to an electrical field of 6·106 V/m: in this condition we have βQ≈104. The antenna has been in operation for several weeks giving, for the Brownian noise, values in agreement with the calculated values. We report also on the results of tests performed on a DC SQUID, whose input was connected to a commercial capacitor via a transformer with turn ratio of 1000.

To detect gravitational waves it is necessary to separate the signals due to these waves from thermal noise in the antenna. A possible way of achieving this would be to operate at liquid helium temperatures. The behaviour of aluminium bar antenna was studied at liquid helium temperatures with particular reference to Q values.

The Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetic flux to voltage converters. Large efforts have been devoted in the last years to fabricate very low noise SQUIDs. The developing of high reliability lithographic techniques has allowed the development of dc-SQUIDs of very good performances. In the literature several dc-SQUIDs are reported with a noise temperature value close to the quantum limit for an amplifier.

A novel type of multiloop DC SQUID (superconducting quantum interference device) has been fabricated. The device is made of six different layers deposited on a 2-in silicon wafer; the layers are patterned by means of optical lithography. Every chip is a square 6.35 mm on a side and contains two SQUIDs and their input coils; the typical coil inductance is about 1 mu H. The junctions are 2 mu m/sup 2/ planar windows on SiO made of Nb-NbO/sub x/-PbAuIn with a current density of 500 A/cm/sup 2/ and V/sub m/ approximately=20 mV. The device was repeatedly cooled and stored in air for six months; after these thermal cycles no variations in critical current were observed. The measured mutual inductance between the input coil and SQUID is 2.1 nH, and the SQUID inductance is about 28 pH. The energy sensitivity referred to the input and in the flux-locked loop is approximately 1400 h in the white region with a 1/f corner frequency at 10 Hz. These SQUIDS are already operating in the gravitational wave experiment of the Rome group in Geneva. >

The experimental results obtained with low noise dc superconducting quantum interference devices (dc‐SQUIDs) are sometimes not well understood. In particular, when the critical current is high, very irregular voltage versus flux characteristics can be measured. On the other hand, when the critical current is low, the characteristics follow perfectly the resistively shunted junction model. We have studied the dynamics of a dc‐SQUID in the presence of stray capacitance and shunting resistors imbalance. The equations describing a circuit model for this physical system are integrated numerically and the results are compared with voltage versus flux characteristics obtained from fabricated devices. We find that the theoretical analysis accounts very well for previously misunderstood experimental features of the dc‐SQUID. This model can give more physical sense to the various regimes of behavior of the dc‐SQUID giving more insight into the experimental results.

We report the first measurements on a dc superconducting quantum interference device (dc-SQUID) coupled to a gravitational wave detector cooled down to liquid-helium temperature. The overall system is composed of a 2.3-ton aluminum bar and a resonant capacitive transducer coupled to the dc-SQUID amplifier by means of a high-ratio superconducting transformer. The SQUID used in this experiment is a planar device with a multiple-loop geometry having both low noise and good coupling with the external world. The electromechanical equivalent circuit of the system, which is essentially a set of three coupled oscillators, is analyzed in detail in its normal modes and an expression for the effective noise temperature for each mode is given. Measurements are reported on the SQUID alone, on the SQUID plus the high-ratio transformer, and on the SQUID coupled to the antenna in parallel with a field-effect transistor preamplifier.

We constructed two identical planar dc-SQUIDs coupled to different input coils on a single silicon chip. The inductances of the coils differ by two orders of magnitude. The ultimate noise performances do not depend significantly on the input circuit if, as in our case, the coupling constant is of the order of 0.5. In a flux locked loop operating mode the energy sensitivity of the more tightly coupled device is sligtly higher than for the less tightly coupled one. We coupled the device to a second-order gradiometer used for biomagnetic measurements obtaining our best performances to date in such a configuration.

We discuss our electron cooling experiments and the integration of the refrigerator with a bolometer.

We report on the design, fabrication and performance of a thin-film multiloop d.c.-SQUID, built in such a way to realize an efficient shielding against uniform ambient fields. The multiloop geometry is a design which allows one to reduce the SQUID inductance. This is an important item to improve the device sensitivity towards the quantum limit, but it has the drawback of a lower coupling coefficient with the input coil, compared to other geometries such as the washer one. In this new design, we improve the design of the input coil to overcome the problem of the coupling and arrange the SQUID loops in a gradiometric configuration to obtain self-shielding of the device.

The noise performance of dc-SQUIDs (Superconducting Quantum Interference Devices), which nowadays can reach values close to the quantum limit, makes them very attractive as amplifiers for experiments using low temperatures and demanding for extremely high sensitivity. After reviewing the working principle of the dc-SQUID, we describe a thin-film device with a new structure, which allowed us to reach an energy sensitivity of 5.5 ∞ at 0.9 K and a good coupling with a large input coil. At last we discuss some fundamental physics experiments which take advantage from the SQUID use.

Josephson devices and, in particular, RF-SQUIDs are ideal systems to test quantum phenomena at macroscopic level. In the view of an experiment of macroscopic quantum coherence, as a preliminary test we developed and fabricated on the same chip a set of thin-film RF-SQUIDs shunted with thin-film resistors of different values, to investigate the behavior of the RF-SQUIDs in different dissipation regimes. Here we present measurements on a moderately damped RF-SQUID, shunted with an 11 Ω resistor. In function of the temperature, we recorded the RF-SQUID characteristics (total magnetic flux vs applied magnetic flux) and studied the statistical distribution of the flux values at which the RF-SQUID switches from one metastable flux state to another. The RF-SQUID is read-out by a DC-SQUID integrated on the same chip.

Top-gate hydrogenated amorphous silicon thin-film transistors have been fabricated which show electrical characteristics suitable for application in the field of chemical sensors. These devices have been specialized to two different types of sensors: (a) Pd-gate hydrogen sensors; (b) K+ ion sensors. The obtained results show that the present technology can be successfully applied to the fabrication of gas-sensitive and ion-sensitive field-effect transistors.

An Electron Beam MicroFabricator (EBMF) has been used to write directly on a silicon wafer to obtain many equal chips containing dc-SQUIDs and superconducting test circuitry. We developed a complete lift-off technique on an electronic resist for all the seven needed layers of the process. The first layer, consisting of Au-Pd alloy, is used as resistor and for patterning the markers required to align the various layers. The calibration on such markers avoid the stitching problem due to the large chip dimensions (6.30×6.30 mm) as compared with the maximum range of the electron beam deflection. The direct writing permits alignment between the various layers better than 1um in any chip. The first test on the developed devices shows high reliability and flexibility of the overall process.

We show a tunable flux qubit with built-in readout, realized with a double SQUID with a supplementary Josephson junction. State preparation and manipulation of the qubit are achieved by applying pulses of magnetic flux in two externally coupled coils, a feature that suits very well with a future integration with RSFQ integrated logic. We show how the system can be read out and prepared in a definite flux state, moreover we show the results of manipulation (lowering of the potential barrier between states) in incoherent regime at liquid helium temperature.