DC SQUID Amplifier Research Articles

Development of a dc-SQUID Amplifier with Intra-Coil Resistors

Development of a dc-SQUID Amplifier with Intra-Coil Resistors

Resonance analysis using a high frequency simulation method for dc-SQUID amplifiers

We performed high-frequency simulation to optimize the geometric size of dc-SQUID amplifiers and to remove their intrinsic resonance characteristics from the operating range. The aim is to eliminate distortions in the IVΦ characteristics that ensures low noise and stable operation over a large range of bias current. With this method, we designed and fabricated a dc-SQUID amplifier with two series-connected octagonal loops to achieve a robust operating performance. The measured results not only show smooth IVΦ characteristics without any distortion, but also demonstrate that the swing voltage is much larger than the corresponding voltage realized without this method at the same bias current. In addition, the white flux noise level as low as 1.7 μΦ0/√Hz was achieved at 0.1 K corresponding to a low current noise level of 15 pA/√Hz.

SQIF-based dc SQUID amplifier with intrinsic negative feedback

We report on a two-stage system consisting of a dc SQUID magnetometer inductivelycoupled to an amplifying second stage realized by a SQIF. The SQIF is a series or parallelconnection of dc SQUIDs with various inductances, which facilitates a unique globalminimum in the voltage–flux characteristic and ensures a stable operating point, a highvoltage swing and a large transfer function of the system. The noise of the amplified signalis dominated by the input stage and its voltage swing exceeds 1 mV, which reduces thedemands on the readout electronics. A part of the current of the SQIF can optionally befed back to the feedback coil of the first-stage SQUID, so that an on-chip linearizationof the voltage–flux characteristic is realized. The short feedback path enables asubstantially higher bandwidth compared to a flux-locked loop using a roomtemperature amplifier. From experimental results it can be expected that thisset-up is capable of measuring magnetic fields with a high slew rate and dynamicrange.

Optimization of a Low-Tc DC SQUID Amplifier With Tightly Coupled Input Coils

We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.

Backaction Noise in Strongly Interacting Systems: The dc SQUID and the Interacting Quantum Point Contact

We study the backaction noise and measurement the efficiency (i.e., noise temperature) of a dc SQUID amplifier and, equivalently, a quantum point-contact detector formed in a Luttinger liquid. Using a mapping to a dissipative tight-binding model, we show that these systems are able to reach the quantum limit even in regimes where several independent transport processes contribute to the current. We suggest how this is related to the underlying integrability of these systems.

Experimental determination of HTS dc-SQUID amplifier inductance and noise performance

The inductance L/sub sq/, transimpedance Z, transfer function V/sub /spl Phi// and flux noise S/sub /spl Phi// have been experimentally determined, in open loop mode, for a series of simple dc-SQUID amplifiers which varied only in the length of the loop. The SQUID design had no input flux transformer but included a current path for directly injecting flux into the loop. Correlations between these parameters were obtained which compare favorably with theoretical estimates. We confirm that the best device performance (small S/sub /spl Phi//) is obtained if L/sub sq/ is kept small whilst V/sub /spl Phi// is maximized; the latter was achieved in one device by a novel ion-beam trimming technique.

Dynamic characteristics of S-band DC SQUID amplifier

A low-noise RF amplifier based on a dc SQUID (SQA) has been tested in the frequency range 3.0-4.6 GHz in the open-loop configuration. The following parameters have been measured for the single-stage balanced type SQA at 4.0 GHz: gain (12/spl plusmn/1) dB, 3 dB bandwidth of 500 MHz and noise temperature (1.0/spl plusmn/0.25) K. For the nonbalanced type SQA at 4.0 GHz gain was (15/spl plusmn/1) dB, 3 dB bandwidth 200 MHz and noise temperature (0.5/spl plusmn/0.25) K. The improved performance is obtained due to the increased characteristic voltage (/spl ap/420 /spl mu/V) of the small-area (down to 0.7-0.9 /spl mu/m/sup 2/) high-quality Nb-AlO/sub x/-Nb SIS junctions. The saturation power (normalized to 1 GHz) referred to the input at 1 dB gain compression is estimated as /spl ap/55 K*GHz at a bias voltage of 60 /spl mu/V. The reasons for saturation of the SQA are discussed.

Very low frequency SQUID NMR measurements of two-dimensional solid [formula omitted] films

Pulsed NMR measurements have been performed on 3 He films adsorbed on an exfoliated graphite sample of area 2 m 2 . The spectrometer is constructed using a DC SQUID amplifier, with additional positive feedback, and bandwidth 3.4 MHz . The input circuit is a superconducting flux transformer and so is intrinsically broadband. The static magnetic field is provided by a superconducting solenoid enclosed in a niobium shield. We report measurements at the “ferromagnetic anomaly” at frequencies below 50 kHz . A narrow 13 C line is also observed, which can be used to directly measure the temperature of the substrate.

Measurements for an experiment of macroscopic quantum coherence with SQUIDs

We have realized a fully integrated chip containing all the different devices (realized with trilayer Nb/AlO x /Nb technology) necessary to perform the MQC measurements. On the chip we have a double rf-SQUID inductively coupled to two different dc-SQUIDs: a standard dc-SQUID amplifier to monitor the state of the rf-SQUID, and an underdamped dc-SQUID, operating as a two-state system, to perform stroboscopic measurements of the rf-SQUID flux states. On this system we have realized a set of measurements to characterize the different devices at temperatures of the order of tens of mK. We have also performed the first measurements with the dc-SQUID amplifier and with the dc-SQUID switch, making stroboscopic measurements on the rf-SQUID.

A high- Tc L-band SQUID amplifier combined with superconductive thin-film filters

An experimental investigation of high-temperature dc SQUID amplifier connected in series with a high- T c superconductor (HTS) thin-film band-pass filter is presented. The SQUID amplifiers are designed with microstrip input coils and fabricated on bicrystal substrates of zirconia and sapphire with different misorientation angles of 24° and 37°. SQUID parameters are optimized to attain 50- Ω input impedance for the 1500 MHz central frequency range. The maximum power gain of the amplifier with six turn input coil was measured to be 6 dB at 800 MHz and 4.5 dB at 1750 MHz. Thin-film HTS YBCO filters are based on a band-pass quasi-elliptic prototype consisting of a set of parallel tank circuits connected to a shunt and separated by admittance inverters. The central frequency of the band-pass filter was adjusted close to 1750 MHz amplifier frequency with 4% bandwidth. The natural combination of a high- T c SQUID amplifier with a HTS band-pass filter allows the reduction of out-of-band noise and optimization of amplifier performance, and demonstrates compatibility with practical components of front-end receiver subsystems.

Two-stage S-band DC SQUID amplifier

A 6 mm/spl times/6 mm chip comprising two identical DC SQUID based amplifiers (SQAs) has been designed, fabricated and tested as a two-stage RF amplifier in a frequency range 3.5-4.0 GHz. Each SQA consists of a double washer type DC SQUID with novel integrated input resonant circuit. The reflection coefficient of both input and output of the SQA has been measured in the two stage configuration. To avoid SQA saturation at the wide band noise tests a tunable 40 MHz bandpass YIG-filter has been used. The following parameters of the two-stage SQA have been measured at the 3.65 GHz: gain of (17.5/spl plusmn/1) dB, 3 dB bandwidth of about 250 MHz, and noise temperature (4.0/spl plusmn/0.5)K what corresponds to intrinsic flux noise S/spl phi//sup 1/2//spl ap/0.6 /spl mu//spl Phi//sub 0/Hz/sup -1/2/ and energy sensitivity /spl epsiv//sub i//spl ap/75 /spl planck/ (7.8/spl middot/10/sup -33/ J/Hz). If the measured RF mismatch between the input/output of SQA and source signal/post-amplifier is taken into account a gain of (20.5/spl plusmn/1.5) dB and noise temperature as low as (2.5/spl plusmn/1.0) K (intrinsic flux noise S/spl phi//sup 1/2//spl ap/0.4 /spl mu//spl Phi//sub 0/Hz/sup -1/2/, energy sensitivity /spl epsiv//sub i//spl ap/47 /spl planck/) can be estimated.

Modelling of the coupling structure of a HTS DC SQUID amplifier

In order to estimate self- and mutual inductances of the input coil and the pick-up loop of a YBCO SQUID amplifier operating at 77 K, a simple method based on the determination of the current and flux distributions into the superconductors is presented. The cases of inductors (single loop and spiral) and induced circuits (closed and open loop) are considered. The authors stress the difference between a SQUID pick-up loop and a directly coupled magnetometer depending upon their action on the input inductance. Finally, the coupling efficiency and the ground plane effect are calculated and measured for various patterns.

First steps towards a high-Tc squid amplifier

A high- T c DC SQUID amplifier has been prepared and tested. The principle of operation of the amplifier was first proved by using low- T c Nb-Al 2 O 3 -Nb SQUIDs with integrated Nb input coils. A flux gain factor of about 20 was demonstrated at 4.2 K with the low- T c SQUIDs. The operation of the high- T c SQUID amplifier was demonstrated by using step edge and/or bicrystal DC SQUIDs. The input coils of the amplifier were wire wound copper coils. A flux gain of about 8 has been measured at 77 K. A flip chip arrangement with a thin film coil has also been tried. In this case, one input coil was a one turn high T c superconducting strip. The second coil consisted of a 22-turn high- T c superconducting stripline with a gold strip crossover. The measurements showed a high contact resistance between Au and YBa 2 Cu 3 O 7−δ which was detrimental for a proper operation of the amplifier.

Direct detection of NMR at low frequencies using a DC SQUID amplifier

We have constructed a broadband (DC to 50 kHz) pulsed NMR spectrometer which uses a DC SQUID to detect precessing magnetization directly. The superconducting receiver coil and the SQUID input coil form a flux transformer, the most sensitive input coupling scheme for NMR detection at frequencies below a few hundred kHz. Using this spectrometer free induction decay signals have been obtained from platinum powder in the temperature range 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal to noise corresponds to that calculated, with the noise being dominated by flux noise in the SQUID. Signals from 1021 Pt195 spins have been observed in a single shot at 37 kHz and 1.4 K. In addition free precession signals have been obtained from 1020 He3 gas spins at 4.2 K and at frequencies between 0.2 and 40 kHz.

DC SQUID Amplifier for NMR Experiments on Small Samples

Low noise preamplifiers are required for the detection of the magnetic resonance of small size samples or at low magnetic fields. According to calculations based on the e n , i n model, SQUID based amplifiers are more suitable to NMR experiments on small samples than semiconductor amplifiers. We propose an amplification method by frequency transposition using a DC SQUID. Such a system can be run over a broad range of frequency as the noise impedance of the stages subsequent to the SQUID is continually optimised. The measurements are in good agreement with our theoretical model, based on a simplified voltage to flux SQUID characteristic.

Analytical model of a resonant gravitational wave antenna.

We report on the development of a model for a resonant gravitational wave antenna. This model considers a resonant bar coupled with a resonant capacitive transducer connected to a dc SQUID amplifier, i.e., a system with three coupled oscillators. The model is used to derive an analytical expression of the apparatus sensitivity in terms of an effective temperature for burst signals. This leads to new and more general mechanical and electrical matching conditions for improving the sensitivity: when they are satisfied the Giffard limit is obtained.

Superconducting kinetic inductance radiometer

A bolometer that is based on a differential thermometer that senses temperature changes through changes in the kinetic inductance of a superconducting thin film is being developed. The temperature transducer is an inductance bridge patterned as an integrated circuit on a 1-cm/sup 2/ Si substrate. Two inductors from opposite arms of the bridge are patterned on a 2-mm/sup 2/ thermally isolated Si island which is supported by a 9- mu m thick Si:B membrane. The bridge is excited with audio-frequency current, and the bridge imbalance is detected with a commercial DC SQUID amplifier. The bridge is balanced by applying power to the thermally isolated island. This thermometer is the sensor for a prototype radiometer that will provide an absolute measure of IR power. The radiometer, which is designed for a NEP of about 10/sup -11/ W/ square root Hz, is intended to measure the spectrally dispersed power of a 300-K black body. This absolute radiometer is being developed for use at the Low Background Infrared (LBIR) Facility at NIST, Gaithersburg, MD. The noise floor of the temperature transducer for the radiometer has been measured to be 0.7 pW for a 100-s integration time. This is approximately 150 times lower noise than that of the commercial absolute radiometer currently used at the LBIR facility.

The gravitational wave experiment of the Rome group

The scientific objectives of the gravitational wave (g.w.) experiment of the Rome group are briefly reviewed: (a) detection of g.w. bursts emitted during supernovae explosions in the Virgo cluster, and (b) detection of monochromatic g.w. possibly emitted by fast pulsars. The interaction of a g.w. burst with a resonant antenna is considered. It is shown that in order to observe the supernovae in the Virgo cluster the resonant antenna should detect a Fourier transform value H( ν R) of the metric tensor perturbation H( t) of the order of: H( ν R) ∼ 3 × 10 −24/Hz at the antenna resonance frequency ν R ∼ 900 Hz. The Rome group has installed a cryogenic antenna at CERN that has started to operate from November 1985. It consists of an aluminum cylinder, 3 m long, weighing 2270 kg, cooled to 4.2 K and equipped with a resonant capacitive transducer and a dc SQUID amplifier. The present sensitivity expressed in terms of the minimum detectable value of H( ν R) is H min( ν R) ∼ 9 × 10 −22Hz −1. In order to reach the final goal it is necessary to cool the antenna than 0.1 K and to shield it from cosmic ray muons.

Low-temperature nuclear magnetic resonance with a dc SQUID amplifier

We report the detection of nuclear magnetic resonance at 4 K and 1.9 MHz with a receiver based on a thin-film dc superconducting quantum interference device. The noise temperature of the spectrometer is 300 mK, limited by the second stage amplifier.

Coupling of a high-sensitivity superconducting amplifier to a gravitational-wave antenna.

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.