ENERGY EFFICIENT SINGLE FLUX QUANTUM TECHNOLOGY IN VLSI

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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 and superconducting phase. superconductive circuits require cooling to cryogenic temperatures for operation, typically within a few degrees celsius of absolute zero.
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 capacitances are important in semiconductor logic circuits such as cmos, currents and inductances are most important in sfq logic circuits. power can be supplied by either direct current or alternating current, depending on the sfq logic family.
THE JOSEPHSON EFFECT:
 Josephson effect is the phenomenon of supercurrent—i.e. a current that flows indefinitely long without any voltage applied—across a device known as a Josephson junction (JJ), which consists of two superconductors coupled by a weak link. The weak link can consist of a thin insulating barrier (known as a superconductor–insulator–superconductor junction, or S-I-S), a short section of non-superconducting metal (S-N-S), or a physical constriction that weakens the superconductivity at the point of contact (S-s-S).
The Josephson effect is an example of a macroscopic quantum phenomenon. It is named after the British physicist Brian David Josephson, who predicted in 1962 the mathematical relationships for the current and voltage across the weak link. The DC Josephson effect had been seen in experiments prior to 1962, but had been attributed to "super-shorts" or breaches in the insulating barrier leading to the direct conduction of electrons between the superconductors. The first paper to claim the discovery of Josephson's effect, and to make the requisite experimental checks, was that of Philip Anderson and John Rowell. These authors were awarded patents on the effects that were never enforced, but never challenged.
Before Josephson's prediction, it was only known that normal (i.e. non-superconducting) electrons can flow through an insulating barrier, by means of quantum tunneling. Josephson was the first to predict the tunneling of superconducting Cooper pairs. For this work, Josephson received the Nobel prize in physics in 1973. Josephson junctions have important applications in quantum-mechanical circuits, such as SQUIDs, superconducting qubits, and RSFQ digital electronics. The NIST standard for one volt is achieved by an array of 19,000 Josephson junctions in series.

RAPID SINGLE FLUX QUANTUM LOGIC (RSFQ):
Rapid single flux quantum (RSFQ) superconducting logic was developed in Russia in the 1980s. 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 as rapidly as possible. Clocking signals are provided to logic gates by separately distributed SFQ 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 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 signal processing.

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.
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.

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 CPUbuilt with energy-efficient superconducting logic may have the potential to be 10-100 times more energy efficient than conventional CMOS logic.
In 2014, it was estimated that a 1 extraFLOP/s computer built in CMOS logic is estimated to consume some 500 megawatts of electrical power. The improved power efficiency of superconducting logic over conventional CMOS might make superconducting logic an enabling technology for exascale computing.



 
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