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Superconductor Science and Technology

An adiabatic quantum flux parametron as an ultra-low-power logic device

Naoki Takeuchi, Dan Ozawa, Yuki Yamanashi, Nobuyuki Yoshikawa

Published Jan 28, 2013• 347 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/26/3/035010

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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.
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%.
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.
Adiabatic quantum-flux-parametron (AQFP) logic devices can operate under extremely low bit energy approaching the thermal limit by varying their potential energy adiabatically using ac bias currents. We have proposed an on-chip ac power source, which is composed of a Josephson relaxation oscillator and a superconductingLCresonator in order to drive AQFP circuits at 5 GHz or higher frequencies. We herein experimentally confirmed the correct switching operation of an AQFP gate at 4.94 GHz using the proposed on-chip ac power source. Furthermore, we confirmed that the ac power source can drive approximately 800 AQFP gates.
We optimized the circuit parameters of a low-power adiabatic quantum-flux-parametron (AQFP) gate in terms of both bit energy and bias margin and confirmed that the bit energy of the optimized gate with a rise time of 200 ps can be decreased to 6% ofIcΦ0, whereIcis the critical current of the Josephson junctions and Φ0is the single flux quantum. In addition, we investigated the effect of thermal noise on the operation of the optimized AQFP gate. The bias margins and bit error rate of the AQFP gate were examined at finite temperature through circuit simulations, in which the thermal noise was taken into account using the Monte Carlo method. The bias margin of the AQFP gate with bias margins of ±26% at zero temperature shrunk to ±19.4% at 4.2 K, but is still large enough for circuit applications.
As we rapidly advance through the information age, the power consumed by computers, data centers, and networks grows exponentially. This has inspired a race to develop alternative low-power computational technologies. A new adiabatic configuration of a decades-old superconducting digital logic device has darted into the lead called quantum flux parametrons (QFP). QFP operate with dissipation so low that they seemingly violate the laws of thermodynamics. In just a short span of time, they have gone from simple single NOT gates to complex processors containing thousands of gates. They are fabricated from elemental niobium superconductors cooled to just a few degrees above absolute zero. However, their efficiency is so great that for large high-performance computers with several gates, the energy savings are immense. For smaller computational platforms QFPs from high-temperature superconductors (high-Tc) are highly desirable. In this work, we take the first steps towards this goal with the demonstration of a high-T C QFP shift register. Our device is fabricated using focused helium ion beam lithography where the material is modified with an ion beam at the nanoscale to directly pattern these circuits into a high-T C thin film. We validate the correct logical operation at 25 K, over 6 times higher than niobium devices with an estimated bit energy of 0.1 attoJoule at 10 GHz.
Adiabatic quantum-flux-parametron (AQFP) logic can achieve high energy-efficiency. The bit energy and bit-error-rate (BER) of AQFP gates with under-damped Josephson junctions were simulated. The results indicate that the bit energy reaches 12 yJ, which is only 20% of the thermal energy at 4.2 K, using unshunted 50-μA junctions and excitation currents with a rise/fall time of 2000 ps. Although the bit energy is extremely small, the bias margin, where the BER is less than 10−23, is approximately ±22%. These results indicate there is no minimum energy limitation for the operation of AQFP gates unless the entropy of the system decreases.