Superconducting logic

Superconducting logic is a class of logic circuits built on the basis of superconductors and Josephson junctions , and using the effect of quantization of the magnetic flux ^[1] . The lack of electrical resistance allows you to create logic circuits with high speed, and recent developments have high energy efficiency ^[2] . Superconducting logic is an option for creating processors with a high switching frequency of individual logic elements - up to hundreds of GHz ^[2] .

The superiority of energy-efficient superconducting logic over traditional CMOS is one of the options for creating exaflop computing technologies. According to estimates for June 2011, an exaflop class computer built on CMOS logic should consume about 0.5 gigawatts of energy, while a computer based on energy-efficient superconducting logic could have 10-100 times less power consumption ^[2] .

Content

Principle of Operation

Superconducting logic uses the property of certain metals (niobium, lead) to become superconductors when they are cooled to a temperature several degrees above absolute zero . In the ring of a superconductor, an electric current will circulate endlessly due to the lack of resistance. The current in the ring creates a magnetic flux, and the magnitude of this flux is always equal to an integer number of quanta of the magnetic flux, i.e., quantization of the magnetic flux is observed.

The Josephson effect is used to change the number of quanta of magnetic flux in a superconducting circuit. This effect consists in the passage of a superconducting current through a thin layer of a dielectric (for example, aluminum oxide), which separates the two superconductors. The superconductivity of the Josephson junction depends on the magnitude of the flowing current. If the current exceeds a value called the critical current, superconductivity disappears, a voltage drop occurs on the contact, and the contact itself begins to emit electromagnetic waves. By passing an external current through a Josephson junction, it is possible both to create magnetic flux quanta in a superconducting circuit and to remove them from it.

The first attempts to create a workable technology on Josephson contacts were made in the USA by IBM (1969-1983) and in Japan (1981-1990) ^[3] . However, the practical implementation of the development did not follow, since the achieved frequencies of the order of 1 GHz only slightly exceeded the speed of the usual CMOS logic.

Types of Superconducting Logic

Fast Single Quantum Logic

Rapid single-quantum logic (BOKL, Engl. Rapid Single Flux Quantum, RSFQ ) was developed in the early 80s by physicists Konstantin Likharev, Vasily Semenov and Oleg Mukhanov ^[1] . This technology has long been fundamental in creating superconducting logic. Despite the name, this technology has nothing to do with quantum computers. It uses a magnetic flux quantum to represent the information bit, moving along the Josephson transmission line in the form of a short voltage pulse.

BOKL is used in high-speed telecommunication devices, digital signal processing circuits, high-speed ADCs and DACs . In 2002, based on this technology, an experimental 8-bit FLUX-1 processor with a clock frequency of 20 GHz was created ^[4] . However, this type of logic has a number of drawbacks that do not allow reaching the level of integration possible in modern CMOS microprocessors. Such disadvantages are the accumulation of jitter as the number of circuit elements increases and significant power consumption ^[3] . For the operation of BOCL elements, a constant bias current is passed through them. The network of resistors used for current distribution can consume tens of times more energy than is necessary to perform logical operations ^[2] ^[5] .

Mutual quantum logic

Reciprocal Quantum Logic (RQL ) is a new type of superconducting logic that solves some problems of fast single-quantum logic ^[6] ^[7] . The developer is Northrop Grumman Corporation . In mutual quantum logic, a mutual pair of quanta of the magnetic flux (positive and negative) is used to represent a bit of information ^[5] .

The logical elements of mutual quantum logic operate on a pulsed principle, do not require bias resistors, which reduces power consumption by tens of times compared with previous generations of superconducting logic. The set of logical elements includes: two schematically combined elements “AND” and “OR” with common inputs (implementing the logical functions “AND” and “OR”), the element “AI- (NOT-B)” (realizing the transmission of a pulse from the input And in the absence of a pulse at input B), and the “Set / Reset” element (performing the functions of a memory element) ^[5] ^[6] .

Notes

↑ ¹ ² Zinoviev D. Chilling alternative. The ups and downs of fast single-quantum logic (neopr.) . Ixbt (1999).
↑ ¹ ² ³ ⁴ Courtland R. Superconductor Logic Goes Low-Power (neopr.) . IEEE spectrum (2011).
↑ ¹ ² High Speed Integrated Circuit Technology, Towards 100 GHz Logic (Selected Topics in Electronics & Systems) / Mark JW Rodwell. - World Scientific Publishing Co. Pte. Ltd., 2001 .-- S. 285. - 372 p. - ISBN 981-02-4638-2 .
↑ Superconducting Technology Assessment (Neopr.) . NSA (2005).
↑ ¹ ² ³ Oberg, Oliver Timothy. Superconducting Logic Circuits Operating With Reciprocal Magnetic Flux Quanta (Neopr.) . University of Maryland (2011).
↑ ¹ ² Quentin P. Herr, Anna Y. Herr, Oliver T. Oberg, Alexander G. Ioannidis. Ultra-Low-Power Superconducting Logic (Neopr.) . Northrop Grumman Systems Corp. (2011).
↑ Superconductivity will save electricity in computer centers (neopr.) . Lenta.ru (2011).

Literature

ExaScale Computing Study: Technology Challenges in Achieving Exascale Systems. Report 2008 , “6.2.4 Superconducting Logic”
Superconducting Technology Assessment , a study of the applicability of RSFQ to create supercomputers from the NSA , 2005. (Eng.)
KK Likharev and VK Semenov, RSFQ logic / memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems // IEEE Trans. Appl. Supercond. 1 (1991), 3. doi: 10.1109 / 77.80745

Links

Josephson Junction Digital Circuits - Challenges and Opportunities / FED Report, February 1998
Superconductor ICs: the 100-GHz second generation // IEEE Spectrum, 2000
Lecture 1: Superconducting electronics , Lecture 2: Fast single-quantum logic // I. Voitovich, V. Korsunsky, Nanoelectronic element base of computer science. Qualitatively new directions, INTUIT, 01/22/2014, ISBN 978-5-9556-0167-0
http://arxiv.org/pdf/1103.4269.pdf

[ixbt-1] ¹ ² Zinoviev D. Chilling alternative. The ups and downs of fast single-quantum logic (neopr.) . Ixbt (1999).

[ieee-2] ¹ ² ³ ⁴ Courtland R. Superconductor Logic Goes Low-Power (neopr.) . IEEE spectrum (2011).

[rodwell-3] ¹ ² High Speed Integrated Circuit Technology, Towards 100 GHz Logic (Selected Topics in Electronics & Systems) / Mark JW Rodwell. - World Scientific Publishing Co. Pte. Ltd., 2001 .-- S. 285. - 372 p. - ISBN 981-02-4638-2 .

[nsa_assessment-4] Superconducting Technology Assessment (Neopr.) . NSA (2005).

[Maryland-5] ¹ ² ³ Oberg, Oliver Timothy. Superconducting Logic Circuits Operating With Reciprocal Magnetic Flux Quanta (Neopr.) . University of Maryland (2011).

[Northrop-6] ¹ ² Quentin P. Herr, Anna Y. Herr, Oliver T. Oberg, Alexander G. Ioannidis. Ultra-Low-Power Superconducting Logic (Neopr.) . Northrop Grumman Systems Corp. (2011).

[lenta-7] Superconductivity will save electricity in computer centers (neopr.) . Lenta.ru (2011).