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Superconductor Science and Technology

Directly coupled adiabatic superconductor logic

Naoki Takeuchi, Kota Arai, Nobuyuki Yoshikawa

Published Apr 8, 2020• 12 citations
Publisher

Abstract

Abstract Adiabatic superconductor logic (ASL) families are energy-efficient because they can operate with a switching energy much less than the I cΦ0 product, where I c is the critical current for Josephson junctions, Φ0 is the flux quantum, and I cΦ0 is generally on the order of 10–19 J. A switching energy of 0.03I cΦ0 has been demonstrated by adiabatic quantum-flux-parametron (AQFP) logic, which indicates the possibility of extremely energy-efficient digital circuits using ASL. ASL uses signal transformers to propagate and invert the signal current; otherwise, it is difficult to design inverters. However, signal transformers are generally difficult to miniaturize. Therefore, to realize high-density and energy-efficient superconductor circuits, ASL that does not use signal transformers is required. In this paper, we propose ASL without signal transformers, which we call directly coupled quantum-flux-parametron (DQFP). DQFP logic is based on AQFP, but it can invert signal current without using signal transformers. We conducted numerical simulation of a DQFP inverter chain to show that DQFP logic can invert signal current without signal transformers, and that DQFP logic can operate adiabatically. Then, we designed and fabricated basic DQFP circuits, such as an inverter chain and a full adder. All the circuits were found to have wide operating margins. Our results indicate that DQFP logic is suitable for high-density and energy-efficient superconductor circuits.

DOI
10.1088/1361-6668/ab87ad

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IEEE International Superconductive Electronics Conference (ISEC)•Jul 1, 2015•13 citations
Abstract Adiabatic superconductor logic (ASL) families are energy-efficient because they can operate with a switching energy much less than the I cΦ0 product, where I c is the critical current for Josephson junctions, Φ0 is the flux quantum, and I cΦ0 is generally on the order of 10–19 J. A switching energy of 0.03I cΦ0 has been demonstrated by adiabatic quantum-flux-parametron (AQFP) logic, which indicates the possibility of extremely energy-efficient digital circuits using ASL. ASL uses signal transformers to propagate and invert the signal current; otherwise, it is difficult to design inverters. However, signal transformers are generally difficult to miniaturize. Therefore, to realize high-density and energy-efficient superconductor circuits, ASL that does not use signal transformers is required. In this paper, we propose ASL without signal transformers, which we call directly coupled quantum-flux-parametron (DQFP). DQFP logic is based on AQFP, but it can invert signal current without using signal transformers. We conducted numerical simulation of a DQFP inverter chain to show that DQFP logic can invert signal current without signal transformers, and that DQFP logic can operate adiabatically. Then, we designed and fabricated basic DQFP circuits, such as an inverter chain and a full adder. All the circuits were found to have wide operating margins. Our results indicate that DQFP logic is suitable for high-density and energy-efficient superconductor circuits.
We have been developing the adiabatic quantum-flux-parametron (AQFP), which is an ultra-low-power superconductor logic family. To date, somewhat large AQFP circuits, such as a 4-bit microprocessor and an 8-bit adder, have been demonstrated. To design much larger devices, space-efficient logic circuits are needed. Therefore, in a previous study, we proposed the directly coupled quantum-flux-parametron (DQFP). DQFP circuits propagate data via a direct connection, whereas AQFP circuits propagate data via transformers, which can occupy a large part of the gate area. Thus, DQFP circuits are more space-efficient than AQFP circuits. In this study, we optimized the circuit parameters for DQFP logic cells using numerical simulations from the viewpoint of excitation current margins, component parameter margins, and energy dissipation. We also designed the physical layout of the optimized DQFP cells, fabricated chips, and demonstrated basic DQFP circuits with the optimized cells.
Adiabatic superconductor logic (ASL), including adiabatic quantum-flux-parametron (AQFP) logic, exhibits high energy efficiency because its bit energy can be decreased below the thermal energy through adiabatic switching operations. In the present paper, we present the general scaling laws of ASL and compare the energy efficiency of ASL with those of other energy-efficient logics. Also, we discuss the minimum energy-delay product (EDP) of ASL at finite temperature. Our study shows that there is a maximum temperature at which the EDP can reach the quantum limit given by ħ /2, which is dependent on the superconductor material and the Josephson junction quality, and that it is reasonable to operate ASL at cryogenic temperatures in order to achieve an EDP that approaches ħ /2.
Superconductor/ferromagnet/superconductor Josephson junctions across which the phases of superconducting order parameters are shifted by π in the ground state are called π Josephson junctions (π-JJs). Recently, several groups have established the fabrication technology of π-JJs so that superconductor logic families exploit π-JJs to improve circuit performance. In this paper, we investigate the advantages of π-JJs in adiabatic quantum-flux-parametron (AQFP) logic, which is an adiabatic superconductor logic based on the quantum-flux-parametron. First, we show that AQFP logic with π-JJs (π-AQFP) can operate using only π-JJs and can be powered and clocked in the same way as the conventional AQFP logic. Then, we show that π-AQFP logic can realize inverters without using signal transformers, which generally occupy a large area in AQFP gates and are difficult to miniaturize. We demonstrate π-AQFP inverters using numerical simulation to establish that π-AQFP logic operates adiabatically and robustly. Our results show that π-JJs can be easily incorporated into AQFP logic and enhance the circuit density of AQFP logic.
Abstract Adiabatic quantum-flux-parametron (AQFP) logic is an energy-efficient superconductor logic with zero static power and very small dynamic power due to adiabatic switching operations. In order to build large-scale digital circuits, we built AQFP logic cells using superconductor magnetic shields, which are necessary in order to avoid unwanted magnetic couplings between the cells and excitation currents. In preliminary experimental tests, we confirmed that the unwanted coupling became negligibly small thanks to the superconductor shields. As a demonstration, we designed a four-to-one multiplexor and a 16-junction full adder using the shielded logic cells. In both circuits, we confirmed correct logic operations with wide operation margins of excitation currents. These results indicate that large-scale AQFP digital circuits can be realized using the shielded logic cells.
Abstract Adiabatic quantum-flux-parametron (AQFP) is an adiabatic logic based on the quantum-flux-parametron. It has extremely low switching energy, as well as high signal gain and high robustness. In this work, we present the development of an AQFP cell library using the MIT LL 100 μ A μ m −2 superconductor electronics fabrication process, SFQ5ee. Four types of basic sub-cells, i.e. buffer, inverter, constant, and branch, are designed as the building blocks for combinational AQFP logic cells. Taking advantage of the 8 metal layers, we place the dc superconducting quantum interference device (SQUID) part of a buffer above the ground plane and the signal transformer part below the ground plane to mitigate unwanted couplings. In addition, by vertically overlapping the signal transformer and the dc SQUID, the dimension of a buffer is reduced to 15 μ m × 20 μ m, which is only 30% of that of the conventional design. More complex AQFP logic cells, such as the majority gate, AND gate, and OR gate, are designed by arraying the sub-cells together. Correct operations and wide excitation current margins are confirmed for the established AQFP logic cells by both simulation and experiments.
Adiabatic quantum flux parametron (AQFP) is a very energy-efficient superconductor logic. In AQFP logic, dynamic energy dissipation can be drastically reduced due to adiabatic switching operations using ac excitation currents. In the present study, we evaluated circuit yields using a large-scale AQFP circuit composed of as many as 10,000 AQFP gates with approximately 20,000 Josephson junctions. We measured eleven chips from four wafers and obtained circuit yields of 48% for 1,000-gate circuits and 22% for 10,000-gate circuits, respectively. In all the circuits, which worked correctly, the margins of excitation currents were as wide as approximately ±20%.