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TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

Recent Progress on Adiabatic Quantum-flux-parametron Logic for Extremely Low-power Microprocessors

Naoki Takeuchi

Published Jan 1, 2017• 1 citation
Publisher

Abstract

We report recent progress on adiabatic quantum-flux-parametron (AQFP) logic, which is an energy-efficient superconductor logic based on quantum flux parametron (QFP). The switching energy of an AQFP gate can be arbitrarily reduced via adiabatic switching, in which the shape of the potential energy gradually varies between single-well and double-well shapes as the result of using ac excitation magnetic fluxes. First, we show the operation principle of AQFP and explain adiabatic switching in light of potential energy shapes. We also show calculated switching energy and bit error rates to demonstrate that the switching energy of AQFP can be significantly reduced while keeping a low error rate. Then, we report recently developed AQFP subsystems, which include an 8-bit carry look-ahead adder, a register file, and a long-interconnect driver. Our demonstration reveals that AQFP technology is ready to be applied in microprocessor design.

DOI
10.2221/jcsj.52.332

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We report recent progress on adiabatic quantum-flux-parametron (AQFP) logic, which is an energy-efficient superconductor logic based on quantum flux parametron (QFP). The switching energy of an AQFP gate can be arbitrarily reduced via adiabatic switching, in which the shape of the potential energy gradually varies between single-well and double-well shapes as the result of using ac excitation magnetic fluxes. First, we show the operation principle of AQFP and explain adiabatic switching in light of potential energy shapes. We also show calculated switching energy and bit error rates to demonstrate that the switching energy of AQFP can be significantly reduced while keeping a low error rate. Then, we report recently developed AQFP subsystems, which include an 8-bit carry look-ahead adder, a register file, and a long-interconnect driver. Our demonstration reveals that AQFP technology is ready to be applied in microprocessor design.
Ultra-low-power adiabatic quantum flux parametron (QFP) logic is investigated since it has the potential to reduce the bit energy per operation to the order of the thermal energy. In this approach, nonhysteretic QFPs are operated slowly to prevent nonadiabatic energy dissipation occurring during switching events. The designed adiabatic QFP gate is estimated to have a dynamic energy dissipation of 12% of I c Φ 0 for a rise/fall time of 1000 ps. It can be further reduced by reducing circuit inductances. Three stages of adiabatic QFP NOT gates were fabricated using a Nb Josephson integrated circuit process and their correct operation was confirmed.
Adiabatic quantum-flux-parametron (AQFP) circuits are very low-power due to zero static power consumption and adiabatic switching operations. However, compared to other superconducting circuits, such as rapid single-flux-quantum logic, the operation frequency is relatively low and the latency is relatively large. In this work, we propose multi-excitation AQFP (ME-AQFP) circuits for high-speed and low-latency operations. In ME-AQFP, the intrinsic periodicity of the potential energy of a Josephson device with respect to applied magnetic fluxes is utilized to multiply operation frequencies and reduce latency. In a numerical simulation, we demonstrated an ME-AQFP AND gate, which operates at 10 GHz or 20 GHz with 5-GHz excitation currents. We also evaluated the energy efficiency of ME-AQFP circuits. In an experiment, we successfully demonstrated an ME-AQFP XOR gate and an ME-AQFP full adder.
We have been investigating an ultra-low-power adiabatic quantum-flux-parametron (AQFP) logic, the energy dissipation of which can be decreased by changing its potential energy slowly or adiabatically. In the present study, we designed basic AQFP logic gates and an AQFP 1-bit full adder, and examined their operations through simulations and experiments. The AQFP 1-bit full adder is composed of 46 Josephson junctions, which is approximately one-quarter the number of junctions of a full adder using conventional rapid single-flux-quantum logic. The measurement results indicated that the AQFP 1-bit full adder has a wide current bias margin of as large as ±27.8%.
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.
Adiabatic quantum-flux-parametron (AQFP) is an energy-efficient superconductor logic. The advantage of AQFP is that the switching energy can be reduced by lowering operation frequencies or by increasing the quality factors of Josephson junctions, while keeping the energy barrier height much larger than thermal energy. In other words, both low energy dissipation and low bit error rates (BERs) can be achieved. In this paper, we report the first measurement results of the low BERs of AQFP logic. We used a superconductor voltage driver with a stack of dc superconducting-quantum-interference-devices to amplify the logic signals of AQFP gates into mV-range voltage signals for the BER measurement. Our measurement results showed 3.3 dB and 2.6 dB operation margins, in which BERs were less than 10−20, for 1 Gbps and 2 Gbps data rates, respectively. While the observed BERs were very low, the estimated switching energy for the 1-Gbps operation was only 2 zJ or 30kBT, where kB is the Boltzmann's constant and T is the temperature. Unlike conventional non-adiabatic logic, BERs are not directly associated with switching energy in AQFP.
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%.