Superconducting logic refers to a class of

logic circuit A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...

s or

logic gate A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, ...

s that use the unique properties of superconductors, including zero-resistance wires, ultrafast

Josephson junction In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...

switches, and quantization of magnetic flux (fluxoid). Superconducting computing is a form of cryogenic computing, as superconductive electronic circuits require cooling to

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

temperatures for operation, typically below 10

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...

. Often superconducting computing is applied to

quantum computing Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...

, with an important application known as

superconducting quantum computing Superconducting quantum computing is a branch of solid state quantum computing which implements superconducting electronic circuits using superconducting qubits as artificial atoms, or quantum dots. For superconducting qubits, the two logic states ...

. Superconducting digital logic circuits use single flux quanta (SFQ), also known as

magnetic flux quanta The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field multiplied by the loop area , i.e. . Both and can be arbitrary, meaning can be as well. However, if one deals with the superconducti ...

, to encode, process, and transport data. SFQ circuits are made up of active Josephson junctions and passive elements such as inductors, resistors, transformers, and transmission lines. Whereas voltages and capacitors are important in semiconductor logic circuits such as

CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...

, currents and inductors are most important in SFQ logic circuits. Power can be supplied by either

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

, depending on the SFQ logic family.

Fundamental concepts

The primary advantage of superconducting computing is improved power efficiency over conventional

technology. Much of the power consumed, and heat dissipated, by conventional processors comes from moving information between logic elements rather than the actual logic operations. Because superconductors have zero electrical

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

, little energy is required to move bits within the processor. This is expected to result in power consumption savings of a factor of 500 for an exascale computer. For comparison, in 2014 it was estimated that a 1 exa

FLOPS In computing, floating point operations per second (FLOPS, flops or flop/s) is a measure of computer performance, useful in fields of scientific computations that require floating-point calculations. For such cases, it is a more accurate meas ...

computer built in CMOS logic is estimated to consume some 500 megawatts of electrical power.Kogge P (2011)
"The tops in flops"
IEEE Spectrum, vol. 48, pp. 48–54, 2011. Superconducting logic can be an attractive option for ultrafast CPUs, where switching times are measured in picoseconds and operating frequencies approach 770 GHz.Courtland R (2011)
"Superconductor Logic Goes Low-Power"
IEEE spectrum, 22 June 2011Holmes DS, Ripple AL, Manheimer MA (2013)
"Energy-efficient superconducting computing—power budgets and requirements"
IEEE Trans. Appl. Supercond., vol. 23, 1701610, June 2013. However, since transferring information between the processor and the outside world does still dissipate energy, superconducting computing was seen as well-suited for computations-intensive tasks where the data largely stays in the cryogenic environment, rather than

big data Though used sometimes loosely partly because of a lack of formal definition, the interpretation that seems to best describe Big data is the one associated with large body of information that we could not comprehend when used only in smaller am ...

applications where large amounts of information are streamed from outside the processor. As superconducting logic supports standard digital machine architectures and algorithms, the existing knowledge base for CMOS computing will still be useful in constructing superconducting computers. However, given the reduced heat dissipation, it may enable innovations such as three-dimensional stacking of components. However, as they require

inductors An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a ...

, it is harder to reduce their size. As of 2014, devices using

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...

as the superconducting material operating at 4 K were considered state-of-the-art. Important challenges for the field were reliable cryogenic memory, as well as moving from research on individual components to large-scale integration.

Josephson junction count The Josephson junction count is the number of Josephson junctions on a superconducting integrated circuit chip. Josephson junctions are active circuit elements in superconducting circuits. The Josephson junction count is a measure of circuit or de ...

is a measure of superconducting circuit or device complexity, similar to the

transistor count The transistor count is the number of transistors in an electronic device (typically on a single substrate or "chip"). It is the most common measure of integrated circuit complexity (although the majority of transistors in modern microprocessors ...

used for semiconductor integrated circuits.

History

Superconducting computing research has been pursued by the U. S.

National Security Agency The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...

since the mid-1950s. However, progress could not keep up with the increasing performance of standard CMOS technology. As of 2016 there are no commercial superconducting computers, although research and development continues. Research in the mid-1950s to early 1960s focused on the

cryotron The cryotron is a switch that operates using superconductivity. The cryotron works on the principle that magnetic fields destroy superconductivity. This simple device consists of two superconducting wires (e.g. tantalum and niobium) with differen ...

invented by

Dudley Allen Buck (Dr.) Dudley Allen Buck (1927–1959) was an electrical engineer and inventor of components for high-speed computing devices in the 1950s. He is best known for invention of the cryotron, a superconductive computer component that is operated in l ...

, but the liquid-helium temperatures and the slow switching time between superconducting and resistive states caused this research to be abandoned. In 1962

Brian Josephson Brian David Josephson (born 4 January 1940) is a Welsh theoretical physicist and professor emeritus of physics at the University of Cambridge. Best known for his pioneering work on superconductivity and quantum tunnelling, he was awarded the N ...

established the theory behind the

Josephson effect In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...

, and within a few years IBM had fabricated the first Josephson junction. IBM invested heavily in this technology from the mid-1960s to 1983. By the mid-1970s IBM had constructed a

superconducting quantum interference device True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting th ...

using these junctions, mainly working with

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

-based junctions and later switching to lead/niobium junctions. In 1980 the Josephson computer revolution was announced by IBM through the cover page of the May issue of Scientific American. One of the reasons which justified such a large-scale investment lies in that Moore's law - enunciated in 1965 - was expected to slow down and reach a plateau 'soon'. However, on the one hand Moore's law kept its validity, while the costs of improving superconducting devices were basically borne entirely by IBM alone and the latter, however big, could not compete with the whole world of semiconductors which provided nearly limitless resources.N. De Liso, G. Filatrella, D. Gagliardi, C. Napoli (2020)
"Cold numbers: Superconducting supercomputers and presumptive anomaly"
Industrial and Corporate Change, vol. 29, no. 2, pp.485-505, 2020. Thus, the program was shut down in 1983 because the technology was not considered competitive with standard semiconductor technology. The Japanese

Ministry of International Trade and Industry The was a ministry of the Government of Japan from 1949 to 2001. The MITI was one of the most powerful government agencies in Japan and, at the height of its influence, effectively ran much of Japanese industrial policy, funding research and di ...

funded a superconducting research effort from 1981 to 1989 that produced the ETL-JC1, which was a 4-bit machine with 1,000 bits of RAM. In 1983,

Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...

created niobium/ aluminum oxide Josephson junctions that were more reliable and easier to fabricate. In 1985, the

Rapid single flux quantum In electronics, rapid single flux quantum (RSFQ) is a digital electronic device that uses superconducting devices, namely Josephson junctions, to process digital signals. In RSFQ logic, information is stored in the form of magnetic flux quanta ...

logic scheme, which had improved speed and energy efficiency, was developed by researchers at

Moscow State University M. V. Lomonosov Moscow State University (MSU; russian: Московский государственный университет имени М. В. Ломоносова) is a public research university in Moscow, Russia and the most prestigious ...

. These advances led to the United States' Hybrid Technology Multi-Threaded project, started in 1997, which sought to beat conventional semiconductors to the petaflop computing scale. The project was abandoned in 2000, however, and the first conventional petaflop computer was constructed in 2008. After 2000, attention turned to

. The 2011 introduction of reciprocal quantum logic by Quentin Herr of

Northrop Grumman Northrop Grumman Corporation is an American multinational aerospace and defense technology company. With 90,000 employees and an annual revenue in excess of $30 billion, it is one of the world's largest weapons manufacturers and military techn ...

, as well as energy-efficient rapid single flux quantum by Hypres, were seen as major advances. The push for

exascale computing Exascale computing refers to computing systems capable of calculating at least "1018 IEEE 754 Double Precision (64-bit) operations (multiplications and/or additions) per second (exaFLOPS)"; it is a measure of supercomputer performance. Exascale ...

beginning in the mid-2010s, as codified in the

National Strategic Computing Initiative The National Strategic Computing Initiative (NSCI) is a United States initiative calling for the accelerated development of technologies for exascale supercomputers, and funding research into post-semiconductor computing. The initiative was crea ...

, was seen as an opening for superconducting computing research as exascale computers based on CMOS technology would be expected to require impractical amounts of electrical power. The

Intelligence Advanced Research Projects Activity The Intelligence Advanced Research Projects Activity (IARPA) is an organization within the Office of the Director of National Intelligence responsible for leading research to overcome difficult challenges relevant to the United States Intellige ...

, formed in 2006, currently coordinates the U. S. Intelligence Community's research and development efforts in superconducting computing.

Conventional computing techniques

Despite the names of many of these techniques containing the word "quantum", they are not necessarily platforms for

Rapid single flux quantum (RSFQ)

(RSFQ) superconducting logic was developed in the Soviet Union in the 1980s.Likharev KK, Semenov VK (1991)
"RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems"
IEEE Transactions on Applied Superconductivity, Vol. 1, No. 1, March 1991, pp. 3-28. Information is carried by the presence or absence of a single flux quantum (SFQ). The

s are

critically damped Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples incl ...

, typically by addition of an appropriately sized shunt resistor, to make them switch without a hysteresis. Clocking signals are provided to logic gates by separately distributed SFQ voltage pulses. Power is provided by bias currents distributed using resistors that can consume more than 10 times as much static power than the dynamic power used for computation. The simplicity of using resistors to distribute currents can be an advantage in small circuits and RSFQ continues to be used for many applications where energy efficiency is not of critical importance. RSFQ has been used to build specialized circuits for high-throughput and numerically intensive applications, such as communications receivers and digital signal processing. Josephson junctions in RSFQ circuits are biased in parallel. Therefore, the total bias current grows linearly with the Josephson junction count. This currently presents the major limitation on the integration scale of RSFQ circuits, which does not exceed a few tens of thousands of Josephson junctions per circuit.

LR-RSFQ

Reducing the resistor (R) used to distribute currents in traditional RSFQ circuits and adding an inductor (L) in series can reduce the static power dissipation and improve energy efficiency.Yamanashi Y, Nishigai T, and Yoshikawa N (2007)
"Study of LR-loading technique for low-power single flux quantum circuits"
IEEE Trans. Appl. Supercond., vol.17, pp.150–153, June 2007.Ortlepp T, Wetzstein O, Engert S, Kunert J, Toepfer H (2011)
"Reduced Power Consumption in Superconducting Electronics"
IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.770-775, June 2011.

Low Voltage RSFQ (LV-RSFQ)

Reducing the bias voltage in traditional RSFQ circuits can reduce the static power dissipation and improve energy efficiency.Tanaka M, Ito M, Kitayama A, Kouketsu T, Fujimaki A (2012)
"18-GHz, 4.0-aJ/bit Operation of Ultra-Low-Energy Rapid Single-Flux-Quantum Shift Registers"
Jpn. J. Appl. Phys. 51 053102, May 2012.Tanaka M, Kitayama A, Koketsu T, Ito M, Fujimaki A (2013)
"Low-Energy Consumption RSFQ Circuits Driven by Low Voltages"
IEEE Trans. Appl. Supercond., vol. 23, no. 3, pp. 1701104, June 2013.

Energy-Efficient Single Flux Quantum Technology (ERSFQ/eSFQ)

Efficient rapid single flux quantum (ERSFQ) logic was developed to eliminate the static power losses of RSFQ by replacing bias resistors with sets of inductors and current-limiting Josephson junctions.Mukhanov OA (2011)
"Energy-Efficient Single Flux Quantum Technology"
IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.760-769, June 2011.DE Kirichenko, S Sarwana, AF Kirichenko (2011)
"Zero Static Power Dissipation Biasing of RSFQ Circuits"
IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.776-779, June 2011. Efficient single flux quantum (eSFQ) logic is also powered by direct current, but differs from ERSFQ in the size of the bias current limiting inductor and how the limiting Josephson junctions are regulated.Volkmann MH, Sahu A, Fourie CJ, and Mukhanov OA (2013)
"Implementation of energy efficient single flux quantum (eSFQ) digital circuits with sub-aJ/bit operation"
Supercond. Sci. Technol. 26 (2013) 015002.

Reciprocal Quantum Logic (RQL)

Reciprocal Quantum Logic (RQL) was developed to fix some of the problems of RSFQ logic. RQL uses reciprocal pairs of SFQ pulses to encode a logical '1'. Both power and clock are provided by multi-phase

signals. RQL gates do not use resistors to distribute power and thus dissipate negligible static power.Herr QP, Herr AY, Oberg OT, and Ioannidis AG (2011)
"Ultra-low-power superconductor logic"
J. Appl. Phys. vol. 109, pp. 103903-103910, 2011. Major RQL gates include:

AndOr Andor may refer to: * ''Andor'' (TV series), a television series in the ''Star Wars'' universe **Cassian Andor, the titular character * Andor (''Wheel of Time''), a country in Robert Jordan's ''The Wheel of Time'' novels * Andor Technology, a ...

, AnotB, Set/Reset (with nondestructive readout), which together form a universal logic set and provide memory capabilities.Oberg OT (2011)
Superconducting Logic Circuits Operating With Reciprocal Magnetic Flux Quanta
University of Maryland, Department of Physics, PhD dissertation.

Adiabatic Quantum Flux Parametron (AQFP)

Adiabatic Quantum flux parametron (AQFP) logic was developed for energy-efficient operation and is powered by alternating current.Takeuchi N, Ozawa D, Yamanashi Y and Yoshikawa N (2013)
"An adiabatic quantum flux parametron as an ultra-low-power logic device"
Supercond. Sci. Technol. 26 035010.Takeuchi N, Yamanashi Y and Yoshikawa N (2015)
"Energy efficiency of adiabatic superconductor logic"
Supercond. Sci. Technol. 28 015003, Jan. 2015. On January 13, 2021, it was announced that a 2.5 GHz prototype AQFP-based processor called MANA (Monolithic Adiabatic iNtegration Architecture) had achieved an energy efficiency that was 80 times that of traditional semiconductor processors, even accounting for the cooling.

Quantum computing techniques

{{Main, Superconducting quantum computing Superconducting quantum computing is a promising implementation of

quantum information Quantum information is the information of the state of a quantum system. It is the basic entity of study in quantum information theory, and can be manipulated using quantum information processing techniques. Quantum information refers to both th ...

technology that involves nanofabricated

superconducting Superconductivity is a set of physical properties observed in certain materials where Electrical resistance and conductance, electrical resistance vanishes and magnetic field, magnetic flux fields are expelled from the material. Any material e ...

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

s coupled through

Josephson junctions In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...

. As in a superconducting electrode, the phase and the charge are

conjugate variables Conjugate variables are pairs of variables mathematically defined in such a way that they become Fourier transform duals, or more generally are related through Pontryagin duality. The duality relations lead naturally to an uncertainty relation— ...

. There exist three families of superconducting qubits, depending on whether the charge, the phase, or neither of the two are good quantum numbers. These are respectively termed

charge qubit In quantum computing, a charge qubit (also known as Cooper-pair box) is a qubit whose basis states are charge states (i.e. states which represent the presence or absence of excess Cooper pairs in the island). In superconducting quantum computing, ...

flux qubit In quantum computing, more specifically in superconducting quantum computing, flux qubits (also known as persistent current qubits) are micrometer sized loops of superconducting metal that is interrupted by a number of Josephson junctions. These d ...

s, and hybrid qubits.

References

External links

Superconducting Technology Assessment
NSA, 2005 - Promoted RSFQ R&D projects.
ExaScale Computing Study: Technology Challenges in Achieving... Report 2008
"6.2.4 Superconducting Logic" Logic families Integrated circuits Quantum electronics Superconductivity Digital electronics Josephson effect