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arXiv:2305.14184

High-Temperature Superconductor Quantum Flux Parametron for Energy-Efficient Logic

Cai, Han, Jay C. LeFebvre, Hao Li, Ethan Y. Cho, Nobuyuki Yoshikawa, Shane A. Cybart

Published May 23, 2023• 0 citations
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.2305.14184

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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.
We report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.
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 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.
Complementary metal-oxide-semiconductor (CMOS) circuit technology has sustainedthe exponential growth of computing power for more than 20 years. However, it hasbeen estimated that the power consumption of a next-generation supercomputer willexceed the power supplied from a single power plant. This indicates the need to dramaticallyimprove the energy efficiency of logic devices to achieve future high-endcomputers.As an extremely energy-efficient logic, we proposed and have been investigatingadiabatic quantum-flux-parametron (AQFP) devices. Due to adiabatic switching operations,its bit energy (energy dissipation per bit) can go below Ic 0, where Ic is acritical current of a Josephson junction and 0 is a single-flux-quantum (SFQ). Ic 0corresponds to the energy dissipated at every 2 phase transition in conventional nonadiabaticsuperconductor logics.We have two objectives in this study. One is to reveal physical energy bounds oncomputation through investigation of the energy efficiency of AQFP logic. The otheris to develop fundamental circuits necessary for energy-efficient computing systemsusing AQFP logic.First, we numerically and experimentally demonstrated the sub-Ic 0 bit-energyoperations of AQFP gates to show that AQFP gates operate in adiabatic modes. Weinvestigated the dependence of energy dissipation and operation margins on circuitparameters. After optimizing the parameters, we calculated the bit energy and the biterror rate at 4.2 K to verify the sub-Ic 0 bit-energy operations at a finite temperature.Additionally, we measured the bit energy of an AQFP gate using a superconductorresonator-based method.Secondly, we numerically demonstrated the sub-kBT bit-energy operations, wherekB is the Boltzmann constant and T is temperature, to show that AQFP gates can operatebeyond thermodynamic energy bounds. We investigated the dependence of energydissipation on operation frequencies and damping conditions of Josephson junctions.By using underdamped junctions, we confirmed the sub-kBT bit-energy operationsat a finite temperature thorough circuit simulation. This result indicates that thereis no minimum energy dissipation required for the operations of AQFP gates unlessthe entropy of the system decreases, which corresponds to the Landauer’s argument on the minimum energy dissipation for computation. Also, we discussed quantummechanicalenergy bounds and experimentally demonstrated AQFP gates with underdampedjunctions.Additionally, we investigated reversible computing using AQFP logic with underdampedjunctions, in order to deal with an important but unresolved question: “Isreversible computing achievable by using practical devices?” We built a purely reversiblegate by using AQFP gates, which we designated as the reversible quantumflux-parametron (RQFP) gate. We calculated its energy dissipation and confirmed thatthere is no minimum energy dissipation for reversible logic operations using RQFPgates. To the best of our knowledge, these are the first calculation results that show nominimum energy dissipation in reversible computing that use practical circuit structures.Moreover, our experimental results have demonstrated the logical and physicalreversibility of the RQFP gate. We believe that this is the first demonstration thatshows both logical and physical reversibility using practical devices.Finally, we proposed a novel energy-efficient latch for AQFP logic, which we designatedas the quantum-flux-latch (QFL). Latches are necessary circuitries in largecomputing systems but had been missing in AQFP logic. We discussed its energyefficiency through simulation and experimentally demonstrated correct operations ofthe QFL and a 1-bit shift register using QFLs. Also, we proposed high-speed test circuitsusing QFLs and experimentally demonstrated logic operations of AQFP circuitsat high speed.We strongly believe that our results showed the extremely high energy efficiency ofAQFP logic. These results will move the AQFP logic to practical usage as extremelyenergy-efficient integrated circuits.
This book concerns a Josephson device for supercomputers which has extremely low heat dissipation (about 106 times less than semiconductor devices and 103 times less than voltage-based Josephson devices). In the previous book on Quantum Flux Parametrons (QFPs), DC Flux Parametron, the basic device operation are described. This book deals in much greater depth on the problems which are faced by the QFP. The device characteristics are worked out in detail showing clearly the analysis methods used. A new logic gate using the QFP is described with respect to its basic scheme, operation, and ways for forming logic circuits. The problems faced by the basic QFP are much reduced in the new logic gate. As the QFP operates near the Heisenberg and Boltzmann limits for computing devices, we also show the relationship between speed and stability. The book contains the latest analytical results on QFPs.The material presented in the book can be understood with very little mathematical training or knowledge about superconducting physics. It is also self-contained and does not require reading of other material. Most of the device characteristics can be reproduced from the equations given using simple programs. A circuit simulator is not needed except for high speeds when transient behavior becomes important.
Adiabatic quantum-flux-parametron (AQFP) logic is a promising technology for future energy-efficient, high- performance information processing systems because it has significantly low power consumption due to the adiabatic switching of Josephson junctions. We are developing a high-performance field-programmable gate array (FPGA) using superconducting AQFP circuits to reduce its energy consumption. The all-AQFP FPGA consists of logic blocks, switch blocks, connection blocks, and memory using AQFP circuits. In particular, the memory is composed of AQFP buffer chains, enabling high-density and low-power memory. The switch blocks can perform data routing as well as several logic functions, including majority,and, andorfunctions, which increases the design flexibility of the all-AQFP FPGA. We fabricated a one-unit all-AQFP FPGA and demonstrated its reconfigurable operation at low speed. It was found that much lower power consumption can be achieved in the new FPGA than in the other superconducting FPGAs.