Rapid Single Flux Quantum

In electronics, rapid single flux quantum (RSFQ) is a digital electronics technology that relies on quantum effects in superconducting devices, namely Josephson junctions, to process digital signals. Josephson junctions are the active elements for RSFQ electronics, like transistors are the active elements for semiconductor electronics. However, RSFQ is not a quantum computing technology in the traditional sense. Even so, RSFQ is very different from the traditional CMOS transistor technology used in every day computers:

• it is based on superconductors, so a cryogenic environment is required;
• the digital information is carried by magnetic flux quanta that are produced by Josephson junctions instead of transistors in semiconductor electronics;
• the magnetic flux quanta are carried by picosecond-duration voltage pulses that travel on superconducting transmission lines, instead of static voltage levels in semiconductor electronics.
• Consequently the area of the quantized voltage pulses that carry single magnetic flux quanta is constant. Depending on the parameters of the Josephson junctions, the pulses can be as narrow as 1 picosecond with an amplitude of about 2 mV, or broader (typically 5-10 picoseconds) with a lower amplitude;
• since pulses usually propagate on superconducting lines, their dispersion is limited and usually negligible if no spectral component of the pulse is above the frequency of the energy gap of the superconductor;
• in 2010, the typical values of the maximum pulse amplitude, usually called the IcRn product, is of the order of 0.5 to 1 mV. Rn is the normal resistance of the Josephson junction that generates the voltage pulses, while Ic is its critical current.
• In the case of pulses of 5 picoseconds, it is typically possible to clock the circuits at frequencies of the order of 100 GHz (one pulse every 10 picoseconds).

See also: Quantum flux parametron, a related digital logic technology.