Editing Rapid single flux quantum (section)

{{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.