Superconducting logic

Superconducting logic refers to a class of logic circuits or logic gates that use the unique properties of superconductors, including zero-resistance wires, ultrafast Josephson junction switches, and quantization of magnetic flux (fluxoid). Superconductive electronic circuits require cooling to cryogenic temperatures for operation, typically below 10 kelvins.

Superconducting digital logic circuits use single flux quanta (SFQ), also known as magnetic flux quanta, to encode, process, and transport data. SFQ circuits are made up of active Josephson junctions and passive elements such as inductors, resistors, transformers, and transmission lines. Whereas voltages and capacitors are important in semiconductor logic circuits such as CMOS, currents and inductors are most important in SFQ logic circuits. Power can be supplied by either direct current or alternating current, depending on the SFQ logic family.

Josephson junction count is a measure of superconducting circuit or device complexity, similar to the transistor count used for semiconductor integrated circuits.

Rapid single flux quantum (RSFQ)

Rapid single flux quantum (RSFQ) superconducting logic was developed in Russia in the 1980s.[1] Information is carried by the presence or absence of a single flux quantum (SFQ). The Josephson junctions are critically damped, typically by addition of an appropriately sized shunt resistor, to make them switch without a hysteresis. Clocking signals are provided to logic gates by separately distributed SFQ voltage pulses.

Power is provided by bias currents distributed using resistors that can consume more than 10 times as much static power than the dynamic power used for computation. The simplicity of using resistors to distribute currents can be an advantage in small circuits and RSFQ continues to be used for many applications where energy efficiency is not of critical importance.

RSFQ has been used to build specialized circuits for high-throughput and numerically intensive applications, such as communications receivers and digital signal processing.

Josephson junctions in RSFQ circuits are biased in parallel. Therefore, the total bias current grows linearly with the Josephson junction count. This currently presents the major limitation on the integration scale of RSFQ circuits, which does not exceed a few tens of thousands of Josephson junctions per circuit.

LR-RSFQ

Reducing the resistor (R) used to distribute currents in traditional RSFQ circuits and adding an inductor (L) in series can reduce the static power dissipation and improve energy efficiency.[2][3]

Low Voltage RSFQ (LV-RSFQ)

Reducing the bias voltage in traditional RSFQ circuits can reduce the static power dissipation and improve energy efficiency.[4][5]

Energy-Efficient Single Flux Quantum Technology (ERSFQ/eSFQ)

Efficient rapid single flux quantum (ERSFQ) logic was developed to eliminate the power static power losses of RSFQ by replacing bias resistors with sets of inductors and current-limiting Josephson junctions.[6][7]

Efficient single flux quantum (eSFQ) logic is also powered by direct current, but differs from ERSFQ in the size of the bias current limiting inductor and how the limiting Josephson junctions are regulated.[8]

Reciprocal Quantum Logic (RQL)

Reciprocal Quantum Logic (RQL) was developed to fix some of the problems of RSFQ logic. RQL uses reciprocal pairs of SFQ pulses to encode a logical '1'. Both power and clock are provided by a multi-phase alternating current signals. RQL gates do not use resistors to distribute power and thus dissipate negligible static power.[9]

Major RQL gates include: AndOr, AnotB, Set/Reset (with nondestructive readout), which together form a universal logic set and provide memory capabilities.[10]

Adiabatic Quantum Flux Parametron (AQFP)

Adiabatic Quantum flux parametron (AQFP) logic was developed for energy-efficient operation and is powered by alternating current.[11][12]

Applications

Superconducting logic can be an attractive option for ultrafast CPUs, where switching times are measured in picoseconds and operating frequencies approach 770 GHz. A CPU built with energy-efficient superconducting logic may have the potential to be 10-100 times more energy efficient than conventional CMOS logic.[13][14]

In 2014, it was estimated that a 1 exaFLOP/s computer built in CMOS logic is estimated to consume some 500 megawatts of electrical power.[15] The improved power efficiency of superconducting logic over conventional CMOS might make superconducting logic an enabling technology for exascale computing.

See also

References

  1. ↑ Likharev KK, Semenov VK (1991). "RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems", IEEE Transactions on Applied Superconductivity, Vol. 1, No. 1, March 1991, pp. 3-28.
  2. ↑ Yamanashi Y, Nishigai T, and Yoshikawa N (2007). "Study of LR-loading technique for low-power single flux quantum circuits", IEEE Trans. Appl. Supercond., vol.17, pp.150–153, June 2007.
  3. ↑ Ortlepp T, Wetzstein O, Engert S, Kunert J, Toepfer H (2011). "Reduced Power Consumption in Superconducting Electronics", IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.770-775, June 2011.
  4. ↑ Tanaka M, Ito M, Kitayama A, Kouketsu T, Fujimaki A (2012). "18-GHz, 4.0-aJ/bit Operation of Ultra-Low-Energy Rapid Single-Flux-Quantum Shift Registers", Jpn. J. Appl. Phys. 51 053102, May 2012.
  5. ↑ Tanaka M, Kitayama A, Koketsu T, Ito M, Fujimaki A (2013). "Low-Energy Consumption RSFQ Circuits Driven by Low Voltages", IEEE Trans. Appl. Supercond., vol. 23, no. 3, pp. 1701104, June 2013.
  6. ↑ Mukhanov OA (2011). "Energy-Efficient Single Flux Quantum Technology", IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.760-769, June 2011.
  7. ↑ DE Kirichenko, S Sarwana, AF Kirichenko (2011). "Zero Static Power Dissipation Biasing of RSFQ Circuits", IEEE Transactions on Applied Superconductivity, vol.21, no.3, pp.776-779, June 2011.
  8. ↑ Volkmann MH, Sahu A, Fourie CJ, and Mukhanov OA (2013). "Implementation of energy efficient single flux quantum (eSFQ) digital circuits with sub-aJ/bit operation", Supercond. Sci. Technol. 26 (2013) 015002.
  9. ↑ Herr QP, Herr AY, Oberg OT, and Ioannidis AG (2011). "Ultra-low-power superconductor logic", J. Appl. Phys. vol. 109, pp. 103903-103910, 2011.
  10. ↑ Oberg OT (2011). Superconducting Logic Circuits Operating With Reciprocal Magnetic Flux Quanta, University of Maryland, Department of Physics, PhD dissertation.
  11. ↑ Takeuchi N, Ozawa D, Yamanashi Y and Yoshikawa N (2013). "An adiabatic quantum flux parametron as an ultra-low-power logic device", Supercond. Sci. Technol. 26 035010.
  12. ↑ Takeuchi N, Yamanashi Y and Yoshikawa N (2015). "Energy efficiency of adiabatic superconductor logic", Supercond. Sci. Technol. 28 015003, Jan. 2015.
  13. ↑ Courtland R (2011). "Superconductor Logic Goes Low-Power", IEEE spectrum, 22 June 2011
  14. ↑ Holmes DS, Ripple AL, Manheimer MA (2013). "Energy-efficient superconducting computing—power budgets and requirements", IEEE Trans. Appl. Supercond., vol. 23, 1701610, June 2013.
  15. ↑ Kogge P (2011). "The tops in flops", IEEE Spectrum, vol. 48, pp. 48–54, 2011.

External links

This article is issued from Wikipedia - version of the Friday, November 20, 2015. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.