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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