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Japanese Journal of Applied Physics

Direct Measurement of the Switching Waveform in a DC–SQUID

Akira Fujimaki, Koji Nakajima, Yasuji Sawada

Published Jan 1, 1987• 8 citations
Publisher

Abstract

A high-resolution Josephson sampler using a single fluxoid is fabricated. The sampling pulse generator comprised a two-junction interferometer (DC–SQUID) with a damping resistance resistively coupled to a single junction as the sampling gate to reduce a turn-on delay. An ultra-fast sampling pulse was generated at a vortex-to-vortex transition in this sampler. The fabricated sampler had less than a 5ps time resolution and about a 5 µA current sensitivity. The dynamic waveform associated with two kinds of transitions was observed by the use of this sampler: a vortex-to-vortex transition and a vortex-to-voltage transition. The results show that the pulse corresponding to a single fluxoid has a nearly constant pulse height that is independent of the bias current and that setting the bias current near the critical point results in the long rise time of a DC–SQUID.

DOI
10.1143/jjap.26.74

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Applied Physics Letters•Jan 24, 2000•14 citations
A high-resolution Josephson sampler using a single fluxoid is fabricated. The sampling pulse generator comprised a two-junction interferometer (DC–SQUID) with a damping resistance resistively coupled to a single junction as the sampling gate to reduce a turn-on delay. An ultra-fast sampling pulse was generated at a vortex-to-vortex transition in this sampler. The fabricated sampler had less than a 5ps time resolution and about a 5 µA current sensitivity. The dynamic waveform associated with two kinds of transitions was observed by the use of this sampler: a vortex-to-vortex transition and a vortex-to-voltage transition. The results show that the pulse corresponding to a single fluxoid has a nearly constant pulse height that is independent of the bias current and that setting the bias current near the critical point results in the long rise time of a DC–SQUID.
A Josephson sampler with direct resistive coupling between sampling gate and sampling pulse generator has been designed and simulated. The sampling gate and the sampling pulse generator comprise two Josephson junction interferometers. A short sampling pulse of 3ps width is generated at a suitable vortex to vortex transition. Delays up to 200ps between the sampling pulse and the main trigger input could be generated with an electronically adjustable circuit on the same chip. A layout of the complete sampling circuit, i.e. the sampling gate, the sampling pulse generator and the electronic delay circuit on a single 36mm2chip has been achieved for the Nb-NbxOy-PbInAu technology. The minimum line width is 8μm. Forj_{\max} = 3kA/cm2a time resolution of 3.5ps is predicted.
Measurement of signals generated by superconducting Josephson junction (JJ) circuits require ultra-fast components located in close proximity to the generating circuitry. We report a detailed study of optimal design criteria for a JJ-based sampler which balances the highest sampler bandwidth (shortest 10\%--90\% rise time) with minimal sampled waveform distortion. We explore the impacts on performance of a sampler, realized using a single underdamped JJ as the logical sampling element (the comparator), due to the type of signal-comparator coupling scheme that is utilized (galvanic, inductive, or capacitive). In these simulations we emulate the entire waveform reconstruction sampling process, via comparator threshold detection, while sweeping the time location at which the waveform is being sampled. We extract the sampled waveform rise time (or FWHM) as a function of the comparator's Stewart-McCumber parameter and as a function of the coupling strength between the device under test and comparator. Based on our simulation results we design, fabricate, and characterize a cryocooled (3.6 K operating temperature) JJ sampler utilizing the NIST state-of-the-art Nb/amorphous-Si/Nb junctions. We separately sample a step signal and impulse generator co-located on-chip with the comparator and sampling strobe generator by implementing the same binary search comparator threshold detection technique during sampler operation as is used in simulation. With this technique the system is fully-digital and automated and operation of the fabricated device directly mirrors simulation. Our sampler technology shows a 10\%--90\% rise time of 3.3 ps and the capability to measure transient pulse widths of 2.5 ps FWHM. A linear systems analysis of sampled waveforms indicate a 3 dB bandwidth of 225 GHz, but we demonstrate effective measurement of signals well above this -- as high as 600 GHz.
Measurement of signals generated by superconducting Josephson junction (JJ) circuits require ultra-fast components located in close proximity to the generating circuitry. We report a detailed study of optimal design criteria for a JJ-based sampler that balances the highest sampler bandwidth (shortest 10%-90% rise time) with minimal sampled waveform distortion. We explore the impacts on the performance of a sampler, realized using a single underdamped JJ as the logical sampling element (the comparator), due to the type of signal-comparator coupling scheme that is utilized (galvanic, inductive, or capacitive). In these simulations, we emulate the entire waveform reconstruction sampling process, via comparator threshold detection, while sweeping the time location at which the waveform is being sampled. We extract the sampled waveform rise time [or full width at half maximum (FWHM)] as a function of the comparator's Stewart-McCumber parameter and as a function of the coupling strength between the device under test and comparator. Based on our simulation results, we design, fabricate, and characterize a cryocooled (3.6 K operating temperature) JJ sampler utilizing the NIST state-of-the-art Nb/amorphous-Si/Nb junctions. We separately sample a step signal and an impulse generator co-located on-chip with the comparator and sampling strobe generator by implementing the same binary search comparator threshold detection technique during sampler operation as is used in simulation. With this technique, the system is fully digital and automated, and the operation of the fabricated device directly mirrors simulation. Our sampler technology shows a 10%-90% rise time of 3.3 ps and the capability to measure transient pulse widths of 2.5 ps FWHM. A linear system analysis of sampled waveforms indicates a 3 dB bandwidth of 225 GHz, but we demonstrate effective measurement of signals well above this-as high as 600 GHz.
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.
We report our time-resolved measurements of switching dynamics of grain-boundary, 5-μm-wide Y–Ba–Cu–O (YBCO) Josephson junctions, excited by 2-ps-wide electrical pulses optically generated by a voltage-biased, 5-μm-wide, 10-μm-long YBCO microbridge. The time-resolved transients were recorded at 20 K using a subpicosecond cryogenic electro-optic sampling system. The intrinsic junction response was separated from the circuit-related feedthrough, and we observed both experimentally and in numerical simulations a 0.8-ps-wide, single-flux-quantum (SFQ) pulse, generated by the switching of the junction with the IcRn product equal to ∼2 mV. At the same time, the measured turn-on delay time was almost three times shorter than that obtained from simulations, questioning applicability of the resistively shunted tunnel junction model to nonhysteretic, high-temperature superconducting weak links. Our test structure's power consumption associated with the single SFQ pulse generation was ∼0.1 μW, leading to a (switching time)×(dissipated power) product equal to 0.08 aJ.