Editing Rapid single flux quantum

{{short description|Type of digital electronic device}}
{{Use dmy dates|date=December 2023}}
{{technical|date=June 2020}}
In [[electronics]], '''rapid single flux quantum''' ('''RSFQ''') is a [[Digital data|digital]] electronic device that uses [[superconducting]] devices, namely [[Josephson junction]]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 logic|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]]s are the active elements for semiconductor electronics. RSFQ is a classical digital, not [[quantum computing]], technology.

RSFQ is very different from the [[CMOS]] [[transistor]] technology used in conventional computers:
* [[superconductor|Superconducting]] devices require [[cryogenic]] temperatures.
* [[picosecond]]-duration SFQ voltage pulses produced by [[Josephson junction]]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]]s which have very small, and usually negligible, dispersion if no spectral component of the pulse is above the frequency of the [[energy gap]] of the superconductor.
* In the case of SFQ pulses of 1&nbsp;ps, it is possible to clock the circuits at frequencies of the order of 100&nbsp;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, <span dir="ltr" lang="el">Φ<sub>0</sub></span> as a result of the junction switching. SFQ pulses have a quantized area ʃ''V''(''t'')''dt'' = <span dir="ltr" lang="el">Φ<sub>0</sub></span> ≈ {{val|2.07|e=-15|u=Wb}} = 2.07&nbsp;mV⋅ps = 2.07&nbsp;mA⋅pH due to [[magnetic flux quantum|magnetic flux quantization]], a fundamental property of superconductors. Depending on the parameters of the Josephson junctions, the pulses can be as narrow as 1&nbsp;[[picosecond|ps]] with an amplitude of about 2&nbsp;mV, or broader (e.g., 5–10&nbsp;ps) with correspondingly lower amplitude. The typical value of the pulse amplitude is approximately 2''I''<sub>c</sub>''R''<sub>n</sub>, where ''I''<sub>c</sub>''R''<sub>n</sub> is the product of the junction critical current, ''I''<sub>c</sub>, and the junction damping resistor, ''R''<sub>n</sub>. For Nb-based junction technology ''I''<sub>c</sub>''R''<sub>n</sub> is on the order of 1&nbsp;mV.

== Advantages ==
* Interoperable with CMOS circuitry, [[microwave]] and infrared technology
* Extremely fast operating frequency: from a few tens of [[gigahertz]] up to hundreds of [[gigahertz]]
* Low [[power consumption]]: 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 circuit|asynchronous]] designs much more practical.

== Disadvantages ==

* Requires [[cryogenic]] cooling.  Traditionally this has been achieved using cryogenic liquids such as [[liquid nitrogen]] and [[liquid helium]].  More recently, closed-cycle cryocoolers, e.g.,  [[pulse tube refrigerator]]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]].
* The cooling requirements can be relaxed through the use of [[high-temperature superconductor]]s. However, only very-low-complexity RFSQ circuits have been achieved to date using high-''T''<sub>c</sub> superconductors. It is believed that SFQ-based digital technologies become impractical at temperatures above ~ 20&nbsp;K – 25&nbsp;K because of the exponentially increasing bit error rates (thermally-induced junction switching) cause by decreasing of the parameter ''E''<sub>J</sub>/''k''<sub>B</sub>''T'' with increasing temperature ''T'', where ''E''<sub>J</sub> = ''I''<sub>c</sub>Φ<sub>0</sub>/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.

== 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]]s
* High performance cryogenic computers<ref>{{cite journal|last=Yerosheva|first= Lilia Vitalyevna |author2=Peter M. Kogge |date=April 2001|title=High-Level Prototyping for the HTMT Petaflop Machine (2001)|publisher=Department of Computer Science and EngineeringNotre Dame, Indiana|citeseerx= 10.1.1.23.4753 }}</ref><ref>Bunyk, Paul, Mikhail Dorojevets, K. Likharev, and Dmitry Zinoviev. "RSFQ subsystem for HTMT petaFLOPS computing." Stony Brook HTMT Technical Report 3 (1997).</ref>
* Control circuitry for superconducting qubits and quantum circuits

== See also ==
* [[Superconducting logic]] includes newer logic families with better [[Efficient energy use|energy efficiency]] than RSFQ.
* [[Quantum flux parametron]], a related digital logic technology.

== References ==
{{Reflist}}

== Further reading ==
* [http://www.nitrd.gov/pubs/nsa/sta.pdf Superconducting Technology Assessment], study of RSFQ for computing applications, by the [[NSA]] (2005).
* {{Cite book |title=Single Flux Quantum Integrated Circuit Design |last1=Krylov |first1=Gleb |publisher=Springer |year=2024 |isbn=978-3-031-47474-3 |type=Second Edition|location=Cham |last2=Jabbari |first2=Tahereh |last3=Friedman |first3=Eby G. |doi=10.1007/978-3-031-47475-0 |oclc=1430662174}}

==External links==
*[https://web.archive.org/web/20040505001934/http://pavel.physics.sunysb.edu/RSFQ/Research/WhatIs/rsfqwte1.html 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.&nbsp;2996–2999, June 1995.
* [http://citeseer.ist.psu.edu/zinoviev96feasibility.html Feasibility Study of RSFQ-based Self-Routing Nonblocking Digital Switches (1996)]
*[http://citeseer.ist.psu.edu/zinoviev97design.html Design Issues in Ultra-Fast Ultra-Low-Power Superconductor Batcher-Banyan Switching Fabric Based on RSFQ Logic/Memory Family (1997)]
* [https://ieeexplore.ieee.org/document/403302 A Clock Distribution Scheme for Large RSFQ Circuits (1995)]
* [http://www.ece.rochester.edu/projects/sde/publications/etc98/FEDReport.pdf Josephson Junction Digital Circuits – Challenges and Opportunities (Feldman 1998)] {{Webarchive|url=https://web.archive.org/web/20150923232916/http://www.ece.rochester.edu/projects/sde/publications/etc98/FEDReport.pdf |date=23 September 2015 }}
* [https://spectrum.ieee.org/superconductor-ics-the-100ghz-second-generation  Superconductor ICs: the 100-GHz second generation] // IEEE Spectrum, 2000

[[Category:Digital electronics]]
[[Category:Quantum electronics]]
[[Category:Superconductivity]]
[[Category:Josephson effect]]