IEEE Spectrum December, 2008 - 22

PICTURING MEMRISTANCE: Hp Labs senior fellow r. stanley williams [left] and research physicist Duncan stewart [right] explain the fourth
fundamental circuit element. williams worked with nearly 100 scientists and engineers to find the memristor. photo: paul Sakuma/ap photo

pipe in one direction, it expands (becoming less resistive). But send the water in the
opposite direction and the pipe shrinks
(becoming more resistive). Further, the
memristor remembers its diameter when
water last went through. Turn off the flow
and the diameter of the pipe "freezes" until
the water is turned back on.
That freezing property suits memristors brilliantly for computer memory.
The ability to indefinitely store resistance
values means that a memristor can be used
as a nonvolatile memory. That might not
sound like very much, but go ahead and
pop the battery out of your laptop, right
now-no saving, no quitting, nothing.
You'd lose your work, of course. But if
your laptop were built using a memory
based on memristors, when you popped
the battery back in, your screen would
return to life with everything exactly
as you left it: no lengthy reboot, no halfdozen auto-recovered files.
But the memristor's potential goes far
beyond instant-on computers to embrace
one of the grandest technology challenges:
mimicking the functions of a brain. Within
a decade, memristors could let us emulate, instead of merely simulate, networks
30

NA * iEEE SpEctrum * dEcEmbEr 2008

of neurons and synapses. Many research
groups have been working toward a brain
in silico: IBM's Blue Brain project, Howard
Hughes Medical Institute's Janelia Farm,
and Harvard's Center for Brain Science are
just three. However, even a mouse brain
simulation in real time involves solving
an astronomical number of coupled partial differential equations. A digital computer capable of coping with this staggering workload would need to be the size of
a small city, and powering it would require
several dedicated nuclear power plants.
Memristors can be made extremely
small, and they function like synapses.
Using them, we will be able to build analog electronic circuits that could fit in a
shoebox and function according to the
same physical principles as a brain.
A hybrid circuit-containing many
connected memristors and transistors-
could help us research actual brain
function and disorders. Such a circuit
might even lead to machines that can
recognize patterns the way humans can,
in those critical ways computers can't-
for example, picking a particular face
out of a crowd even if it has changed significantly since our last memory of it.

t

he story of the memristor is
truly one for the history books. When
Leon Chua, now an IEEE Fellow, wrote
his seminal paper predicting the memristor,
he was a newly minted and rapidly rising
professor at UC Berkeley. Chua had been
fighting for years against what he considered
the arbitrary restriction of electronic circuit
theory to linear systems. He was convinced
that nonlinear electronics had much more
potential than the linear circuits that dominate electronics technology to this day.
Chua discovered a missing link in the
pairwise mathematical equations that
relate the four circuit quantities-charge,
current, voltage, and magnetic f lux-
to one another. These can be related in
six ways. Two are connected through
the basic physical laws of electricity and
magnetism, and three are related by the
known circuit elements: resistors connect voltage and current, inductors connect f lux and current, and capacitors
connect voltage and charge. But one
equation is missing from this group:
the relationship between charge moving through a circuit and the magnetic
flux surrounded by that circuit-or more
subtly, a mathematical doppelgänger
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Table of Contents for the Digital Edition of IEEE Spectrum December, 2008

IEEE Spectrum December, 2008 - Cover1
IEEE Spectrum December, 2008 - Cover2
IEEE Spectrum December, 2008 - 1
IEEE Spectrum December, 2008 - 2
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IEEE Spectrum December, 2008 - 45
IEEE Spectrum December, 2008 - Cover3
IEEE Spectrum December, 2008 - Cover4
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