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AIP conference proceedings

Noise-mediated cooperative behavior and signal detection in dc SQUIDs

Mario E. Inchiosa

Published Jan 1, 2000• 2 citations
Publisher

Abstract

We study the detection of very weak time-periodic magnetic signals via a double-junction (dc) Superconducting Quantum Interference Device (SQUID). The device, represented by two coupled nonlinear differential equations for the quantum mechanical junction phases, admits of long-time static or oscillatory solutions, the transition being controllable by experimental parameters. Signal detection is optimal when the device is “tuned” to the onset of the oscillatory solutions; i.e., when the minima in the 2D potential function disappear. Modeling the device via a derived input-output transfer characteristic yields a response (quantified via the signal-to-noise ratio at the signal frequency) in good agreement with recent experiments. We also present some preliminary results pertaining to coupled dc SQUIDs, taking into account the (incoherent) thermal noise sources in each of the Josephson junctions. As might be expected from previous work, coupling the SQUIDs and/or summing their outputs enhances the response to the external signal.

DOI
10.1063/1.1302439

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We study the detection of very weak time-periodic magnetic signals via a double-junction (dc) Superconducting Quantum Interference Device (SQUID). The device, represented by two coupled nonlinear differential equations for the quantum mechanical junction phases, admits of long-time static or oscillatory solutions, the transition being controllable by experimental parameters. Signal detection is optimal when the device is “tuned” to the onset of the oscillatory solutions; i.e., when the minima in the 2D potential function disappear. Modeling the device via a derived input-output transfer characteristic yields a response (quantified via the signal-to-noise ratio at the signal frequency) in good agreement with recent experiments. We also present some preliminary results pertaining to coupled dc SQUIDs, taking into account the (incoherent) thermal noise sources in each of the Josephson junctions. As might be expected from previous work, coupling the SQUIDs and/or summing their outputs enhances the response to the external signal.
We study the detection of very weak time-periodic magnetic signals via a double-junction (dc) Superconducting Quantum Interference Device (SQUID). The device, represented by two coupled nonlinear differential equations for the quantum mechanical junction phase differences, admits long-time static or oscillatory solutions, the transition between them being easily controlled by experimentally accessible parameters. Gain is maximal when the device is tuned to the onset of the oscillatory solutions; i.e., when the minima in the 2D potential function disappear. We concentrate on the SQUID dynamics near this critical point and compute the oscillation frequency via a center manifold reduction of the full 2D dynamics. Knowing this frequency permits its exploitation as a detection/classification tool in magnetic remote sensing applications.
Abstract We study the detection of very weak time‐periodic magnetic signals via a double‐junction (dc) Superconducting Quantum Interference Device (SQUID). The device, represented by two coupled nonlinear differential equations for the quantum mechanical junction phase differences, admits long‐time static or oscillatory solutions, the transition between them being easily controlled by experimentally accessible parameters. Gain is maximal when the device is tuned to the onset of the oscillatory solutions; i.e., when the minima in the 2D potential function disappear. We concentrate on the SQUID dynamics near this critical point and compute the oscillation frequency via a center manifold reduction of the full 2D dynamics. Knowing this frequency permits its exploitation as a detection/classification tool in magnetic remote sensing applications.
Superconducting quantum interference devices (SQUIDs) are promising for detection of extremely weak power regimes for the Internet of Things and long-distance satellite networks. In low-frequency magnetic signal measurements, it has demonstrated superior performance over the traditional systems. We review the resistively and capacitively shunted junction model of dc SQUIDs and investigate its transfer function to magnetic flux signals while taking thermal fluctuation into account. We numerically evaluate the influence of bias current and bias flux signal to determine the operating parameters of the device. The performances related to SQUID ring inductance operating at 4.2 K and 77 K are demonstrated. Combined with common radio frequency front-end modules, including low-noise amplifier and low-pass filter, we evaluate the bit-error-rate performance of the system under different inductance values and inferred that the sensitivity of the system can reach −99 dBm at 100 MHz bandwidth.
A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.>
We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room-temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small-signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux-locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux-locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.
We have designed, fabricated, and tested a Double Transformer (DT) coupled dc SQUID (Superconducting Quantum Interference Device) with low noise, an input inductance of 1μH and a smooth input-output characteristic. A transmission line model is presented to explain a resonance in the input-output characteristic of early versions of this device. Guided by the results of numerical simulations a new version of this device has been built and tested. Experimental results are presented that show that the resonance can be moved to a higher voltage by reducing the area of the SQUID loop. The voltage-external flux characteristic of some of these new devices agrees to within 10% with computer simulations. The minimum detectable energy per unit bandwidth (MDE) referred to the SQUID loop, is 10h, where h is Planck's constant. Computer simulations indicate an MDE of 6h.