picosecond-duration SFQ voltage pulses produced by Josephson junctions 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 lines 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 ps, it is possible to clock the circuits at frequencies of the order of 100 GHz.
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 ʃVdt = 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 with correspondingly lower amplitude. The typical value of the pulse amplitude is approximately 2IcRn, where IcRn is the product of the junction critical current, Ic, and the junction damping resistor, Rn. For Nb-based junction technology IcRn is on the order of 1 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
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 refrigerators 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 superconductors. However, only very-low-complexity RFSQ circuits have been achieved to date using high-Tc superconductors. It is believed that SFQ-based digital technologies become impractical at temperatures above ~ 20 K – 25 K because of the exponentially increasingbit error rates cause by decreasing of the parameter EJ/kBT with increasing temperature T, where EJ = IcΦ0/2π 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
