Superconductive Logic
The demand for high-performance, low-power microprocessors is increasing [1],[2]. Since Moore’s law for transistors is reaching saturation, a potential technology to achieve energy efficient high-performance computing is superconductive technology. Superconductivity is a phenomenon in which conductors exhibit zero DC resistance [3]. Coupling two superconductors with a weak link introduce the active device used in superconductive circuits, the Josephson junction (JJ). New logic families have been proposed by utilizing superconductivity and JJs. In these logic families, the logic values are presented as the presence or absence of a single flux quantum (SFQ) pulse [4].
SFQ logic is five orders of magnitude more energy efficient than CMOS since the data (SFQ pulses) are 1 mV high and 2 ps wide. SFQ circuits have been experimentally demonstrated to operate at frequencies up to 770 GHz. SFQ circuits rely on superconductive loops (inductors) to store and transfer the SFQ pulses. Combining different superconductive loops provides SFQ with different logic circuits. The design of an SFQ processor is fundamentally different from CMOS. Since the information is voltage pulses, each logic is sequential (synchronous), a bias current needs to be applied to every JJ in the design, and conditions that did not exist in CMOS. Problems like flux trapping and static power dissipation from bias resistors are all problems that need to be considered and solved in SFQ circuits. Hence, design methodologies and tools must be developed to overcome these issues.
In our group, we explore new superconductive circuits and develop design methodologies and tools. We designed logic circuits and methodologies using a novel JJ called 2-JJ. The logic circuits did not rely on inductors to store and release the information. Hence, scalability is improved compared to prior SFQ logic, such as Rapid SFQ (RSFQ). Using 2-JJ in circuit design improved latency and energy compared to RSFQ while achieving the same parameter margin as RSFQ [5].
[1] J. Ren, G. Tang, F. Wang, et al., “Superconducting Single Flux Quantum (SFQ) Technology for Power-Efficiency Computing,” CCF Transactions on High Performance Computing, pp. 1–29, 2022
[2] K. K. Likharev, “Progress and Prospects of Superconductor Electronics,” Superconductor Science and Technology, vol. 3, pp. 325–337, Jul. 1990
[3] J. Bardeen, L. N. Cooper, and J. R. Schrieffer, “Theory of Superconductivity,” Physical Review Journals Archive, vol. 108, pp. 1175–1204, Dec. 1957
[4] K.K. Likharev and V.K. Semenov, “RSFQ Logic/Memory Family: A New Josephson-Junction Technology for Sub-Terahertz-Clock-Frequency Digital Systems,” in IEEE Transactions on Applied Superconductivity, vol. 1, pp. 3-28, Mar 1991
[5] I. Salameh, E. G. Friedman and S. Kvatinsky, “Superconductive Logic Using 2ϕ—Josephson Junctions With Half Flux Quantum Pulses,” in IEEE Transactions on Circuits and Systems II: Express Briefs, vol. 69, pp. 2533-2537, May 2022
