Rapid single flux quantum
Encyclopedia
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 transistor
s 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:
See also: Quantum flux parametron
, a related digital logic technology.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, rapid single flux quantum (RSFQ) is a digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
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 transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s 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
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
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 quantaMagnetic flux quantumThe magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor. The phenomenon of flux quantization was discovered B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Nabauer, in 1961...
that are produced by Josephson junctions instead of transistors in semiconductor electronics; - the magnetic flux quanta are carried by picosecondPicosecondA picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
-duration voltage pulses that travel on superconducting transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s, instead of static voltage levels in semiconductor electronics. - Consequently the area of the quantized voltage pulses that carry single magnetic flux quantaMagnetic flux quantumThe magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor. The phenomenon of flux quantization was discovered B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Nabauer, in 1961...
is constant. Depending on the parameters of the Josephson junctions, the pulses can be as narrow as 1 picosecondPicosecondA picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
with an amplitude of about 2 mV, or broader (typically 5-10 picosecondPicosecondA picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
s) 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 amplitude 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
Quantum flux parametron
Invented by Eiichi Goto at the University of Tokyo, the Quantum Flux Parametron is an improvement over his earlier parametron based digital logic technology. Unlike its predecessor, QFP uses superconducting Josephson junctions on integrated circuits to improve speed and energy efficiency enormously...
, a related digital logic technology.
Advantages
- Interoperable with CMOS circuitry, microwaveMicrowaveMicrowaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
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 CMOSCMOSComplementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
semiconductors circuits - 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 heliumLiquid heliumHelium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
is necessary for the most complex circuits, although high-temperature superconductors can also be used for RSFQ, but with low or medium complexity to date. Cryogenic cooling is also an advantage since it reduces the working environment's thermal noise. - As RSFQ is a disruptive technologyDisruptive technologyA disruptive technology or disruptive innovation is an innovation that helps create a new market and value network, and eventually goes on to disrupt an existing market and value network , displacing an earlier technology there...
, dedicated educational degrees and specific commercial software are still to be developed.
Applications
- Optical and other high-speed network switching devices
- Digital signal processingDigital signal processingDigital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
, even up to radiofrequency signals - Ultrafast routers
- Software-Defined RadioSoftware-defined radioA software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices...
(SDR) - High speed analog-to-digital converterAnalog-to-digital converterAn analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
s - Petaflop supercomputers
External links
- Superconducting Technology Assessment, study of RSFQ for computing applications, by the NSA (2005).