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IEEE Transactions on Circuits and Systems I Regular Papers

Superconducting Shuttle-Flux Shift Register for Race Logic and Its Applications

Meriam Gay Bautista, Patricia Gonzalez-Guerrero, Darren Lyles, George Michelogiannakis

Published Oct 10, 2022• 6 citations
Publisher

Abstract

This paper presents a superconducting, magnetically-coupled, shuttle-flux shift register (SF-SR) that stores single flux quantum (SFQ) pulses. This shift register has a DC bias operating margin of ±34% at 10 GHz, with a power dissipation of$3.6~\mu W$and 38% fewer Josephson junctions (JJs) when scaled up to multiple stages compared to a data flip-flop (DFF) based shift register. The clock input is inductively coupled and is independent from the data input. We then present three applications for our SF-SR. In the first application, we add two non-destructive readout (NDRO) cells to construct a buffer that temporarily stores the temporal information of a series of race logic (RL) pulses. The second application is a pseudo-random number generator based on a linear function shift register (LFSR). The third application is N parallel SF-SRs that can act similar to a deserializer or instead can emulate a single SF-SR of N times higher clock frequency. These three applications motivate deep shift registers with many shifting intervals, which our SF-SR can implement with fewer JJs and lower power consumption compared to DFF-based shift registers.

DOI
10.1109/tcsi.2022.3210023

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This paper presents a superconducting, magnetically-coupled, shuttle-flux shift register (SF-SR) that stores single flux quantum (SFQ) pulses. This shift register has a DC bias operating margin of ±34% at 10 GHz, with a power dissipation of3.6 μW3.6~\mu W3.6 μWand 38% fewer Josephson junctions (JJs) when scaled up to multiple stages compared to a data flip-flop (DFF) based shift register. The clock input is inductively coupled and is independent from the data input. We then present three applications for our SF-SR. In the first application, we add two non-destructive readout (NDRO) cells to construct a buffer that temporarily stores the temporal information of a series of race logic (RL) pulses. The second application is a pseudo-random number generator based on a linear function shift register (LFSR). The third application is N parallel SF-SRs that can act similar to a deserializer or instead can emulate a single SF-SR of N times higher clock frequency. These three applications motivate deep shift registers with many shifting intervals, which our SF-SR can implement with fewer JJs and lower power consumption compared to DFF-based shift registers.
This paper presents a superconducting magnetically coupled shift buffer based on a shuttle-flux shift register that stores single flux quantum (SFQ) pulses. The shift register has a DC bias operating margin of ±34% at 10 GHz, the power dissipation of 3.6µW at the clock bias, and 38% fewer Josephson junctions for 1024 stages compared to DFF-based shift registers. Clock bias is independent of the input pulse sequence through inductive coupling with a minimal current drive. Based on our shift register, we add two non-destructive readouts (NDRO) cells to construct a buffer that can inexpensively store the temporal information of race logic (RL) pulses or a pulse train for spiking neural networks. These applications motivate deep shift registers with many shifting intervals, which our shift register can implement with only a pair of coupled inductors and one JJ per shifting interval.
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 Josephson SFQ (single flux quantum) shift register circuit operating under a two-phase power was designed and tested. The test cell was fabricated using the Nb/AlO/sub x//Nb junction SNEAP process. The main data latch is DC biased; the data storage is accomplished by the transfer of a single flux quantum in or out of a SQUID (superconducting quantum interference device) superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended.>
Due to their high-speed operation and low power consumption, single flux quantum (SFQ) circuits are promising technologies for integrated circuits. However, current superconducting fabrication limitations impose strict area constraints, challenging the construction of large-scale systems. Unary computing (UC) encodes numerical values by concentrating all “1”s at the beginning of a bit sequence. This approach enables arithmetic operations with fewer logic gates, thereby improving the area efficiency. In this study, we propose a novel high-capacity D flip-flop (HC-DFF) cell for UC-based SFQ circuits that achieves high-density data registration while preserving UC encoding. The HC-DFF employs a cascaded storage loop structure in which each loop sequentially stores one flux quantum, avoiding the margin deterioration associated with storing multiple flux quanta in a single loop. Simulation shows that the proposed HC-DFF achieves a bias margin of –20.8% to +32.7% and supports read/write operations above 100 GHz. We fabricated the HC-DFF using a 10 kA/cm² Nb four-layer superconducting process and verified its operation through low-speed measurements. The HC-DFF offers enhanced scalability and stability while reducing the number of required Josephson junctions by 83.3% per bit compared to conventional DFF-based shift registers for saving UC signals. For short unary sequences of less than 31 bits, it retains an advantage over storing binary data with equivalent precision using conventional DFF-based shift registers.
Novel reconfigurable superconductive single flux quantum (SFQ) logic devices, the functions of which can be dynamically reconfigured by inputting control signals, have been investigated. The characteristics of single flux quantum circuits can be easily modulated by applying currents or magnetic fields because of the high sensitivity of superconductive circuits. We investigated several design approaches for the realization of dynamically reconfigurable SFQ logic devices. We employed direct current injection to the logic gate using a non-destructive read-out flip-flop. By changing the internal state of the non-destructive read-out flip-flop, the logic function can be dynamically reconfigured. We have designed and tested a dynamically reconfigurable Josephson transmission line (JTL)/delay flip-flop (DFF) circuit. The measured dc bias margin was 72.5-107.2%. Furthermore, we have demonstrated the operation of a variable bit-length shift register, composed of the dynamically reconfigurable JTL/DFF circuits. The investigated design approach can be applied to more sophisticated dynamically reconfigurable single flux quantum logic gates.
Recent progress on reciprocal quantum logic (RQL) has renewed interest in ac powering of superconductor digital circuits, which had been abandoned since the famous IBM project of 1970s. In this paper, we propose and demonstrate new ac-biased single flux quantum (SFQ) circuits, and search for synergy of ac and currently dominating dc biasing schemes. As the first step, we suggest an on-chip ac/dc converter capable of feeding a few dc-biased gates surrounded by their ac-biased counterparts. As the second step, we introduce and present the first successful demonstration of a new ac-powered circuit-an 8192-bit shift register with over 32800 Josephson junctions (JJs) and JJ density of about 6 · 105 JJ per cm2. We suggest a few niche applications for this type of ac-biased circuits, which require high clock rates. For example, these circuits, scalable to millions of JJs per chip, can serve as a convenient benchmark for new SFQ fabrication technology nodes, allowing the operating margins of individual cells to be extracted and thus “visualize” individual fabrication defects and flux trapping events. The circuit can also be developed into a megapixel imaging array for a magnetic field microscope.