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Applied Physics Letters

Rapid single-flux-quantum balanced comparator based on high-T c bicrystal Josephson junctions

B. Oelze, Ben J. Ruck, M. Roth, Regina Dömel, M. Siegel, Anna Kidiyarova-Shevchenko, Timur V. Filippov, M. Yu. Kupriyanov, Gernot Hildebrandt, Hannes Töpfer, F. H. Ihlmann, W. Prusseit

Published May 6, 1996• 39 citations
Publisher

Abstract

A balanced current comparator based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The operation of the balanced comparator has been demonstrated in the temperature range from 4.2 to 70 K. The correct sequence of switching of the Josephson junctions has been determined by dc measurements for different values of operating frequency and signal current Ix. The threshold uncertainty ΔIx of the balanced comparator, measured at 40 K, was 60 μA at 40 GHz and 300 μA at 240 GHz.

DOI
10.1063/1.115580

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A balanced current comparator based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The operation of the balanced comparator has been demonstrated in the temperature range from 4.2 to 70 K. The correct sequence of switching of the Josephson junctions has been determined by dc measurements for different values of operating frequency and signal current Ix. The threshold uncertainty ΔIx of the balanced comparator, measured at 40 K, was 60 μA at 40 GHz and 300 μA at 240 GHz.
The use of high-T/sub c/ Josephson junctions for digital applications requires a careful study of the influence of thermal noise on circuit performance. We investigated a balanced current comparator, which contains a basic component of all RSFQ circuits: two overdamped Josephson junctions connected in series. The dependence of the dc-voltage across the Josephson junctions on signal current was measured to determine the threshold uncertainty of switching of the comparator. The experimental data obtained at different sampling pulse frequencies, in a temperature range from 10 to 50 K, were compared with a theoretical model, taking into account the influence of thermal noise as well as of sampling pulse frequency.
A 3 bit single flux quantum (SFQ) shift register based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The circuit consists of 26 bicrystal Josephson junctions and includes the shift register itself, two dc-SFQ converters, one readout superconducting quantum interference device, serving as a SFQ-dc converter, and three Josephson transmission lines. The correct operation of all circuit components has been demonstrated by low frequency testing at a temperature of 50 K.
We fabricated and tested high-temperature superconducting (HTS) rapid-single-flux-quantum (RSFQ) circuits using interface-engineered Josephson junction (IEJ). The characteristics of high-speed operation were examined by voltage-divider operation based on the toggle-flip-flop circuit. An operation at 155 GHz was obtained at 15 K and the proper functioning of the voltage divider was observed up to 27 K. Temperature dependence of the maximum operation speed was analyzed by circuit simulation with thermal-noise sources. To achieve higher speed operation using a Josephson junction with a higher I/sub c/R/sub n/ product, we also fabricated an RSFQ balanced comparator using IEJ; proper switching characteristics of the circuit was confirmed up to 190 GHz at 30 K. The bias condition dependencies were studied based on the circuit simulations in the relationship of the I/sub c/R/sub n/ product. The results of circuit simulation indicate that a Josephson junction with an I/sub c/R/sub n/ product of more than 0.7 mV is required for high-speed operation (100 GHz) in high-temperature range (30-40 K). We believe that the IEJ is a promising junction candidate for realizing high-speed operation in a high-temperature range.
The Josephson balanced comparator, a decision-making circuit comprising two Josephson junctions (JJs) connected in series from a clocking perspective and in parallel for the current to be measured, is ubiquitous in single flux quantum (SFQ) logic. Its noise properties are crucial for the performance of logic devices. The characteristics of the balanced comparator can also be used to monitor fab process and design implementation as an indicator for excess noise, overheating, linearity, dynamic effects, etc. We designed several test structures to measure gray zone of various comparators fabricated in different nodes of process at MIT-LL. We used digital circuitry to measure comparator characteristics at low frequencies and an analog testbed to perform high frequency characterization. Experimental results for gray zone of comparators designed for fabrication nodes with different current densities, sheet resistances, critical damping are presented and studied as a function of clock frequency.
The application of high temperature superconductor (HTS) Josephson junctions in digital rapid single flux quantum circuits requires a careful study of the influence of thermal noise on the bit error rate (BER). We have determined experimentally, for the first time, the BER of a HTS rapid single flux quantum circuit. A comparator, formed by two Josephson junctions, was integrated in a Josephson transmission line ring oscillator, allowing us to perform high speed testing of the comparator at GHz frequencies. For fabrication, focused-electron-beam-irradiated junctions have been used because of their small parameter spread and excellent alignment possibilities. A BER of less than 10−11 was obtained at 39 K.
We have designed and tested a Rapid Single-Flux-Quantum (RSFQ) circuit for experimental measurement of the fluctuation-induced threshold uncertainty of switching of SFQ comparators, which are essential components of RSFQ logic devices, analog-to-digital converters, and digital SQUIDs. Statistical density of the switching events in comparators using externally-overdamped Nb-trilayer Josephson junctions has been found to be in agreement with the distribution which follows from a simple theory based on a time-dependent harmonic-oscillator model of the device. Width of the distribution (i.e, the single-shot current resolution of the comparator) measured as a function of temperature in the range 1.7-4.2 K corresponds to fundamental (thermal/quantum) fluctuations, with no evidence of excess noise sources.>