Rapid single flux quantum
 | This article may be too technical for most readers to understand. (June 2020) |
In
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 transistors 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:
- cryogenictemperatures.
- Josephson junctionsare 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 (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 ʃV(t)dt = Φ0 ≈ 2.07×10−15 Wb = 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 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
- Low power consumption: about 100,000 times lower than CMOSsemiconductors 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 asynchronousdesigns much more practical.
Disadvantages
- Requires thermal noise.
- The cooling requirements can be relaxed through the use of 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 converters
- High performance cryogenic computers[1][2]
- Control circuitry for superconducting qubits and quantum circuits
See also
- Superconducting logic includes newer logic families with better energy efficiencythan RSFQ.
- Quantum flux parametron, a related digital logic technology.
References
- )
- ^ Bunyk, Paul, Mikhail Dorojevets, K. Likharev, and Dmitry Zinoviev. "RSFQ subsystem for HTMT petaFLOPS computing." Stony Brook HTMT Technical Report 3 (1997).
Readings
- Superconducting Technology Assessment, study of RSFQ for computing applications, by the NSA(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)
- Design Issues in Ultra-Fast Ultra-Low-Power Superconductor Batcher-Banyan Switching Fabric Based on RSFQ Logic/Memory Family (1997)
- A Clock Distribution Scheme for Large RSFQ Circuits (1995)
- Josephson Junction Digital Circuits – Challenges and Opportunities (Feldman 1998) Archived 23 September 2015 at the Wayback Machine
- Superconductor ICs: the 100-GHz second generation // IEEE Spectrum, 2000