Single-junction SQUID Research Articles

Bi-SQUID: a novel linearization method for dc SQUID voltage response

We show that the voltage response of a dc SQUID can be substantially linearized by theintroduction of a nonlinear inductance. The inductance tuning allows us to achieveresponse linearity close to 120 dB. Such a nonlinear inductance can be easily formed usingJosephson junction inductance. The additional junction and main inductance forma single-junction SQUID and hence the device can be called a bi-SQUID. Toobtain high dynamic range commensurate to the high response linearity, one canuse a serial array of nonlinear inductance dc SQUIDs. Experimental studies ofa single bi-SQUID and serial arrays of bi-SQUIDs are reported and discussed.

Stochastic-parametric amplification of narrow-band signals in a single-junction SQUID interferometer

The features of the response of a single-junction superconducting quantum interferometer to a low-frequency harmonic signal in the presence of noise and a high-frequency electromagnetic field are investigated through numerical solution of the equations of motion. It is shown that in this situation a system described by a double-well potential will display stochastic-parametric amplification of weak harmonic signals owing to the cooperative effects of noise and the high-frequency field. The gain is a nonmonotonic function of the amplitude of the high-frequency field and the variance of the noise flux and passes through a maximum. A detailed numerical analysis of the dependence of the gain on the noise intensity and on the frequency and amplitude of the high-frequency field is carried out in the stochastic, parametric, and stochastic-parametric amplification regimes. It is shown that at optimal amplitude of the high-frequency field the gain for a weak harmonic signal reaches rather high values (10–30). The specific properties of stochastic-parametric amplification are discussed, and the possible applications of this effect for constructing input circuits of detectors based on SQUIDs with registration of weak narrow-band information signals are considered.

A CHAOTIC GRAVITATIONAL WAVE DETECTOR

In this letter we signalize the possibility of applying a quantum chaos as an element of high sensitivity which serves to detect small changes in length generated by gravitational waves. We propose the construction of a double-bar antenna with a coupling Josephson junction in its center-of-mass. In fact the new antenna is a single Josephson junction with massive bulk contacts, like a single-junction SQUID but with free ends. Computer experiments demonstrate that very small changes generated by the variation of the distance between the bulk plates of the junction capacitance will produce a variety of very different intermittency routes to chaos. A concrete numerical example illustrates the smallness of a quantum of chaos and thus the extraordinary sensitivity of the proposed method.

Experimental operation of an RS flip-flop composed of nonlatching Josephson gates

We present the experimental implementation of an RS flip-flop (RS-FF) composed of dc-biased coupled-SQUID (C-SQUID) gates. The C-SQUID gate is a combination of a single-junction SQUID and a double-junction SQUID. This gate utilizes nonhysteretic Josephson junctions and it is operated in nonlatching mode with dc-biasing. Several logical functions are able to be realized with a C-SQUID gate by adjusting the input bias and the input signal levels. The speed performance of the gate is evaluated by simulation for ring oscillators, and the minimum switching delay of 6.5 ps/stage is obtained under Josephson critical current density of 10 kA/cm/sup 2/. We have fabricated the RS-FF composed of two C-SQUID NOR gates. The circuit is integrated using a Nb/AlO/sub x//Nb junction technology and its operation is demonstrated experimentally.

Microwave performance of YBa/sub 2/Cu/sub 3/O/sub 7-x/ resonators coupled to single-junction SQUIDs

The effects of microwave current and dc magnetic field on quality factor and resonant frequency at 3 GHz of planar resonators, which were differently coupled to hysteretic microwave SQUIDs, are discussed. The dissipative response is analyzed in terms of a single-junction SQUID, and its coupling to a resonant tank circuit. The temperature dependences and field periodicities of all relevant parameters are discussed, and conditions for optimized operation are deduced. The results indicate potential for further improvement of the performance of microwave SQUID magnetometers. >

Superconducting neural circuits using SQUIDs

Superconducting implementation of neural circuits is presented. A neuron element is composed of a coupled-SQUID, which is a combination of a single-junction SQUID and a double-junction SQUID. A synapse element is composed of multiple double-junction SQUIDs. To demonstrate the abilities of these circuits experimentally, a 8-bit neural-based A/D converter, which contains three neurons and three synapses, has been fabricated using a niobium integration technology. This is the first successful implementation of both neuron and active synapse elements into a superconducting neural network. >

Integrated microwave SQUID

We describe the properties of the single junction SQUID operated at microwave frequencies. The low inductance SQUID is fabricated in a thin film format and tightly coupled to an integrated superconducting transmission line and transformer. We discuss new modes of operation and opportunities for improved device and circuit performance.

Low noise microwave parametric amplifier

We report the lowest noise temperature of any microwave amplifier except the maser. This amplifier is a nearly degenerate superconducting parametric amplifier at X-band consisting of a low β, thin film, single junction SQUID, a monolithic 50 Ω to 1 Ω impedance matching network, and a cooled circulator. Nb/Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> /Pb alloy junctions were incorporated into Nb and Pb-in circuitry on 1×2 cm silicon chips. The SQUID was phase-biased near 90° and driven by an external pump at approximately twice the operating frequency. The self-resonance frequency of the SQUID inductance and junction capacitance was set at the operating frequency of 8.2GHz. The operating charac teristics depended on the pump amplitude, the junction phase bias, and the SQUID parameters. For small values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\beta &lt; 1</tex> , the maximum gain in both the signal and idler channels was ≈10dB, the bandwidth = 170MHz, and the single-sidehand noise temperature was 6K with an uncertainty of (+15 to -7)K. For higher β, the gain increased to nearly 30dB with increased noise and parametric oscillations occurring near the pump subharmonic frequency.

SQUID parametric amplifier

The single junction SQUID was previously shown by analysis and simulation to be an attractive candidate for a parametric amplifier. Further calculations of the noise and saturation behavior of the nearly degenerate parametric amplifier have now been performed by numerical simulation. These simulations clearly show that the amplifier noise temperature will be approximately the device temperature T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> , and that the amplifier will be completely saturated in the presence of white noise characteristic of 30T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . Signal saturation of the amplifier also occurs for an output power <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10^{-2}F_{o}\phi_{o}^{2}/2L</tex> , strongly limiting the dynamic range, However, a coherent array of N single junction SQUIDs is shown to have a signal saturation level increased by N relative to a single SQUID, with no increase in noise temperature, resulting in an N-fold improvement in dynamic range.

Electronic analogs of double-junction and single-junction SQUIDs

We describe electronic analogs of a double-junction (dc) SQUID and a single-junction (rf) SQUID that are simple to construct and operate, and are considerably more flexible than those previously reported. The Josephson junction analog is based on the Resistively-Shunted Junction (RSJ) model, and uses a novel method based on a sample-and-hold circuit to generate the sin ϑ ’’supercurrent.’’ Examples are shown of the single-junction I–V characteristics, threshold curves of critical current vs applied flux and I–V curves for the double-junction SQUID analog, and rf SQUID characteristics as a function of applied flux and rf drive current. Other applications of the analogs, including the incorporation of a nonsinusoidal current-phase relation, are discussed.

Comparison of the microscopic theory and the RSJ model of Josephson tunneling for calculating the amount of flux entry into a single-junction squid

Comparison of the microscopic theory and the RSJ model of Josephson tunneling for calculating the amount of flux entry into a single-junction squid

Analog-to-digital conversion with unlatched SQUID's

Digital Josephson circuits that do not latch into the voltage state provide an opportunity for high-speed serial processing. These fast logic gates must ultimately generate and record small fast pulses generated by single fluxoid events. This is demonstrated in a concept for a high-speed analog-to-digital (A/D) converter. Low β-value unlatched SQUID's are used throughout for ultimate speed, sensitivity, and simplicity. The analog signal is quantized by a single-junction SQUID and converted into a pulse train resulting from fluxoid transitions. These are counted in a chain of bistable two-junction SQUID's which successively scale the pulse train by a factor 2. These SQUID's perform both logic and memory functions at the single fluxoid level. The bit count stored in the array is gated out each sampling interval to provide the digitized signal. The performance of this device has been simulated and criteria established for the unlatched SQUID design. These include dimensionality, impedance, damping, intra- and inter-SQUID delays and β factor. Large analog dynamic range is available with the high intrinsic linearity of the SQUID quantizer and a long chain of scalers.