electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, rapid single flux quantum (RSFQ) is a digital electronic device that uses

superconducting Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

devices, namely

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 mec ...

s, to process digital signals. In RSFQ logic, information is stored in the form of magnetic flux quanta and transferred in the form of Single Flux Quantum (SFQ) voltage pulses. RSFQ is one family of superconducting or SFQ logic. Others include Reciprocal Quantum Logic (RQL), ERSFQ – energy-efficient RSFQ version that does not use bias resistors, etc. Josephson junctions are the active elements for RSFQ electronics, just as

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s are the active elements for semiconductor electronics. RSFQ is a classical digital, not quantum computing, technology. RSFQ is very different from the CMOS

technology used in conventional computers: *

Superconducting Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

devices require cryogenic temperatures. * picosecond-duration SFQ voltage pulses produced by

s are used to encode, process, and transport digital information instead of the voltage levels produced by transistors in semiconductor electronics. * SFQ voltage pulses travel on superconducting

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

s which have very small, and usually negligible, dispersion if no spectral component if the pulse is above the frequency of the

energy gap In solid-state physics, an energy gap is an energy range in a solid where no electron states exist, i.e. an energy range where the density of states vanishes. Especially in condensed-matter physics, an energy gap is often known more abstractly as ...

of the superconductor. * In the case of SFQ pulses of 1 ps, it is possible to clock the circuits at frequencies of the order of 100 GHz (one pulse every 10 picoseconds). An SFQ pulse is produced when magnetic flux through a superconducting loop containing a Josephson junction changes by one flux quantum, Φ₀ as a result of the junction switching. SFQ pulses have a quantized area ʃ''V''(''t'')''dt'' = Φ₀ ≈ = 2.07 mV⋅ps = 2.07 mA⋅pH due to magnetic flux quantization, a fundamental property of superconductors. Depending on the parameters of the Josephson junctions, the pulses can be as narrow as 1 ps with an amplitude of about 2 mV, or broader (e.g., 5–10 ps) with correspondingly lower amplitude. The typical value of the pulse amplitude is approximately 2''I''_c''R''_n, where ''I''_c''R''_n is the product of the junction critical current, ''I''_c, and the junction damping resistor, ''R''_n. For Nb-based junction technology ''I''_c''R''_n is on the order of 1 mV.

Advantages

* Interoperable with CMOS circuitry,

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

and infrared technology * Extremely fast operating frequency: from a few tens of

gigahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...

up to hundreds of

* Low

power consumption Electric energy consumption is the form of energy consumption that uses electrical energy. Electric energy consumption is the actual energy demand made on existing electricity supply for transportation, residential, industrial, commercial, and ot ...

: about 100,000 times lower than CMOS semiconductors circuits, without accounting for refrigeration * Existing chip manufacturing technology can be adapted to manufacture RSFQ circuitry * Good tolerance to manufacturing variations * RSFQ circuitry is essentially self clocking, making asynchronous designs much more practical.

Disadvantages

* Requires cryogenic cooling. Traditionally this has been achieved using cryogenic liquids such as liquid nitrogen and

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...

. More recently, closed-cycle cryocoolers, e.g.,

pulse tube refrigerator The pulse tube refrigerator (PTR) or pulse tube cryocooler is a developing technology that emerged largely in the early 1980s with a series of other innovations in the broader field of thermoacoustics. In contrast with other cryocoolers (e.g. Sti ...

s have gained considerable popularity as they eliminate cryogenic liquids which are both costly and require periodic refilling. Cryogenic cooling is also an advantage since it reduces the working environment's

thermal noise A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

. * The cooling requirements can be relaxed through the use of

high-temperature superconductor High-temperature superconductors (abbreviated high-c or HTS) are defined as materials that behave as superconductors at temperatures above , the boiling point of liquid nitrogen. The adjective "high temperature" is only in respect to previou ...

s. However, only very-low-complexity RFSQ circuits have been achieved to date using high-''T''_c superconductors. It is believed that SFQ-based digital technologies become impractical at temperatures above ~ 20 K – 25 K because of the exponentially increasing bit error rates (thermally-induced junction switching) cause by decreasing of the parameter ''E''_J/''k''_B''T'' with increasing temperature ''T'', where ''E''_J = ''I''_cΦ₀/2π is the Josephson energy. * Static power dissipation that is typically 10–100 times larger than the dynamic power required to perform logic operations was one of the drawbacks. However, the static power dissipation was eliminated in ERSFQ version of RSFQ by using superconducting inductors and Josephson junctions instead of bias resistors, the source of the static power dissipation. * As RSFQ is a disruptive technology, dedicated educational degrees and specific commercial software are still to be developed.

Applications

* Optical and other high-speed network switching devices * Digital signal processing, up to X-band signals and beyond * Ultrafast routers * Software-defined radio (SDR) * High speed

analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...

s * High performance cryogenic computersBunyk, Paul, Mikhail Dorojevets, K. Likharev, and Dmitry Zinoviev. "RSFQ subsystem for HTMT petaFLOPS computing." Stony Brook HTMT Technical Report 3 (1997). * Control circuitry for superconducting qubits and quantum circuits

References

{{Reflist

Readings

Superconducting Technology Assessment
study of RSFQ for computing applications, by the

NSA 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 ...

(2005).

External links

An introduction to the basics and links to further information
at the State University of New York at Stony Brook. * K.K. Likharev and V.K. 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 * A. H. Worsham, J. X. Przybysz, J. Kang, and D. L. Miller, "''A single flux quantum cross-bar switch and demultiplexer,''" IEEE Trans. on Appl. Supercond., vol. 5, pp. 2996–2999, June 1995.
Feasibility Study of RSFQ-based Self-Routing Nonblocking Digital Switches (1996)
* ttps://ieeexplore.ieee.org/document/403302 A Clock Distribution Scheme for Large RSFQ Circuits (1995)
Josephson Junction Digital Circuits – Challenges and Opportunities (Feldman 1998)

Superconductor ICs: the 100-GHz second generation
// IEEE Spectrum, 2000 Digital electronics Quantum electronics Superconductivity Josephson effect