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IEEE Transactions on Applied Superconductivity

Low error operation of a 4 stage single flux quantum shift register built with Y-Ba-Cu-O bicrystal Josephson junctions

J.H. Park, J.H. Kang, Tae Bong Jung, Kuyoul Jung, Chang‐Hoon Kim, Y.H. Kim, Sang Sam Choi, T. S. Hahn

Published Mar 1, 2001• 3 citations
Publisher

Abstract

We fabricated a 4 stage single flux quantum shift register with YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) bicrystal Josephson junctions and demonstrated the circuit's low error operation by using a computer controlled digital measurement set-up. The circuit was operated at 65 K. Binary data sequences of "1000", "1010", "1011," and "1111" were successfully loaded and shifted in the circuit. The two read SQUIDs placed next to each side of the shift register were used to sense all the individual data states. By operating the circuit with the proper current pulses, we observed no errors during 16 hours, which is equivalent to 21,000 error-free data shifts. We also found that temperature dependent inductance and junction critical currents limit the operating temperature range of the circuit, and the effective thermal noise temperature can be lower than 100 K.

DOI
10.1109/77.919422

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We fabricated a 4 stage single flux quantum shift register with YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) bicrystal Josephson junctions and demonstrated the circuit's low error operation by using a computer controlled digital measurement set-up. The circuit was operated at 65 K. Binary data sequences of "1000", "1010", "1011," and "1111" were successfully loaded and shifted in the circuit. The two read SQUIDs placed next to each side of the shift register were used to sense all the individual data states. By operating the circuit with the proper current pulses, we observed no errors during 16 hours, which is equivalent to 21,000 error-free data shifts. We also found that temperature dependent inductance and junction critical currents limit the operating temperature range of the circuit, and the effective thermal noise temperature can be lower than 100 K.
Testing on a single-flux-quantum (SFQ) shift register fabricated with a SFQ Nb-AlO/sub x/Nb Josephson junction process indicates that a stable flux quantum can be stored in a rapid SFQ (RSFQ) shift register stage and be shifted by SFQ pulses. Design simulations showed that logic errors can occur in SFQ shift registers even though the circuit passes the average voltage test. Three-junction-per-stage shift registers had an advantage over the two-junction-per-stage shift register in handling data with successive 1's and in incorporating resistors to increase tolerance of trapped magnetic flux. Input and output buffers for data and clock pulses were added to get an RSFQ shift register to obtain correct performance. The highest error-free operation speed for the data with successive 1's is about half of the speed for propagation of a lone data 1. Error-free operation of SFQ shift registers with three junctions per stage could be obtained up to speeds of 30-60 GHz with Josephson junctions having I/sub c/R/sub n/ (critical-current-normal-resistance product) values of 600 mu V.>
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 single flux quantum RS flip-flop circuits by using ramp-edge Josephson junctions and tested their performance. The junctions were constructed with two Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) electrodes and a barrier layer. As barrier materials we used cobalt-doped YBCO. To insulate between YBCO layers we used Sr/sub 2/AlTaO/sub 6/ (SAT) thin films. We also fabricated RS flip-flop circuits by using YBCO ground planes to reduce the inductance of YBCO film. The fabricated Josephson junctions showed typical RSJ-like current-voltage (I-V) characteristics above 50 K. We could successfully demonstrate the correct operation of the RS flip-flop circuits at the temperatures near 50 K. We used a computer controlled digital measurement set-up to test the circuits. The RS flip-flop circuit fabricated on a ground plane showed more definite set and reset states on the voltage-flux (V-/spl phi/) modulation curves of the read SQUID than those of the circuit made without ground plane. This may be attributed to the shielding effect by a YBCO ground plane.
Single flux quantum (SFQ) circuit components such as an SFQ-dc converter and a confluence buffer have been fabricated by using an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// ramp-edge junction technology and their logic operations at temperatures up to near 60 K were investigated. The SFQ-dc converter was correctly operated in a wide temperature range from 4.2 K to 56 K and found to be useful for detecting output signals from other SFQ circuit components at any operating temperatures. The basic function that a signal from either of two input Josephson transmission lines (JTLs) was transmitted to an output JTL was confirmed for the confluence buffer and finite operating margins were obtained at temperatures from 42 K to 61 K. The narrowest margin of dc supply current obtained at temperatures from 55 K to 60 K was /spl plusmn/20% and was consistent with the simulation. Margin reduction due to thermal noise was also evaluated. According to the analytical calculation, the operating margin to keep the bit-error rate less than 10/sup -5/ was as large as /spl plusmn/20% even at 50 K when the value of junction critical-current I/sub c/ was kept near 0.4 mA.