Adam Sarhage
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Current CMOS technology is reaching its
theoretical limits for operating speed, and the
next generation of supercomputers could
require hundreds of megawatts of power to
operate if based on CMOS technology.
Superconducting logic, which uses zeroresistance superconductors, could provide
the basis for future supercomputers using
1/300th of the power required by a
comparable CMOS supercomputer.
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Comprised of a non-superconducting
material or insulating material sandwiched
between two superconducting materials
Superconducting electronics are able to pass
though without resistance until the critical
current is reached
Superconducting Material
Insulating Material
Schematic Symbol
I-V Curve
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Using the properties of the Josephson
junction, circuits can be built which pass
individual flux quanta (a quantum being the
smallest amount of magnetic flux) through an
IC.
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Traditionally, as is the case with Rapid Single
Flux Quantum (RSFQ) technology, the
transmission lines are biased using DC
current.
This DC current requires a resistor dissipating
power at each point set of Josephson
Junctions on the JTL.
As the technology scales up, a substantial
and prohibitive amount of power is dissipated
in the DC biasing resistors.
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A new technology, Reciprocal Quantum Logic
(RQL) seeks to eliminate the DC biasing
resistors by providing the biasing current
through a transformer coupled AC bias/clock
line.
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Logical bits are stored in the phase of the flux
quanta.
A logical “1” is a produced by a flux
quantum’s phase shift of +2π during the
positive half of the clock cycle followed by a
phase shift of -2π during the immediately
proceeding negative half of the clock cycle.
A logical “0” is produced by not changing the
phase of a flux quantum during the clock
cycle.
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RQL requires a JTL buffer between the output
of a logic gate and the input of another logic
gate.
A finite number of logic gates and buffers can
be placed on a single phase of the clock.
With a 20GHz clock, approximately 4
gates/buffers can be on the same clock
phase.
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Gates/buffers can only be connected to their
own clock phase or the next highest clock
phase. (e.g. A phase 00 buffer can only be
connected to a phase 00 or phase 01 buffer,
not a phase 10 or 11 buffer.)
This makes synthesis using VHDL unique and
challenging.
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Fulton, T. A., R. C. Dynes, and P. W. Anderson. "The Flux
Shuttle-A Josephson Junction Shift Register Employing Single
Flux Quanta." Proceedings of the IEEE 61.1 (1973): 28-35.
Print.
Herr, Quentin P., Anna Y. Herr, Oliver T. Oberg, and
Alexander G. Ioannidis. "Ultra-low-power Superconductor
Logic." Journal of Applied Physics (2011)
Oberg, Oliver T. "SUPERCONDUCTING LOGIC CIRCUITS
OPERATING WITH RECIPROCAL MAGNETIC FLUX QUANTA."
Diss. University of Maryland, College Park, 2011. Web.
<http://drum.lib.umd.edu/bitstream/1903/12338/1/Oberg_
umd_0117E_12789.pdf>.