Rapid Single Flux Quantum (RSFQ) is a digital electronics technology that relies on quantum effects in superconducting materials to switch signals, instead of transistors. However, it 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
• Instead of voltage levels, digital signals are represented by picosecond-duration pulses that travel down superconducting microstrip transmission lines.
• The pulses are single quanta at the lowest energy level allowed by quantum mechanics in the system, and hence cannot change significantly in transit. They do not lose energy, spread out, or interfere.
• The quantum pulses are switched by Josephson junctions instead of transistors
• Unlike normal circuitry, signals cannot be split into multiple outputs without active circuit elements

Advantages

• Interoperable with CMOS circuitry
• Extremely fast operating frequency (up to hundreds of gigahertz)
• Low power consumption
• Existing chip manufacturing technology can be adapted to manufacture RSFQ circuitry
• Good tolerance to manufacturing variations
• RSFQ circuitry is essentially self clocking, making asynchronous designs much more practical.

Disadvantages

• Requires cryogenic cooling; liquid helium may be necessary unless high-temperature superconductors can be maufactured in this way.

Applications

• Optical and other high-speed network switching devices
• Digital signal processing, even up to radiofrequency signals.
• High speed analog to digital converters
• Petaflop supercomputers

References

• An excellent introduction to the basics and links to further information at the Stony Brook University.
• K.K. Likharev and V.K. Semenov, IEEE Trans. Appl. Supercond. 1 (1991), 3.
• 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