Say good-bye to
the either-or binary digit. Quantum computing is
riding a new wave of supercool subatomic bits
that can be both 1 and 0 at once.
Moore's law is a
death march. In a decade or two, the silicon
chip will be kaput. What then?
The year is 2015.
Computers are fast - really fast. But there's a
supercharged black box that puts the whole
microchip drag race to shame. No one now knows
what it'll be called, but this much is certain:
The letter Q will be right up front.
Q stands
for quantum, and it just may replace e
and i as the tech prefix of choice. Don't
hold out for a qMac anytime soon, but even in
its embryonic state, the quantum computer is
already turning heads. The technology is based
on two facts of life at the submolecular level.
First, quantum particles such as electrons can
exist in multiple states at once. Second,
particles in a group can become so intertwined
that the actions of one affect all others at the
same instant, allowing engineers to build
circuits out of individual atoms.
Legendary
physicist Richard Feynman noted in the early
1980s that, put together, these two traits would
open the door to computational power
inconceivable on conventional computers. A
working QC only a few hundred bits in size could
outstrip Moore's humdrum doubling by
astronomical amounts for certain applications.
But what's not widely known is how many
competitors are vying to be the first to produce
a new warp-speed machine that leaves behind the
pathetic benchmarks of classical 20th-century
silicon.
Some, like Bruce
Kane at the University of Maryland, are working
to push silicon tech down to submicroscopic
scale. Others, like Bell Labs veteran Phil
Platzman, want to replace solid-state
electronics with a new breed of supercooled
liquid computer.
Today, the bit is
king. A conventional computer is just a series
of hundreds of millions of switches whose on and
off positions represent the values 1 and 0. And
every operation a computer performs, whether
calculating your waiter's tip or simulating the
explosion of a nuclear warhead, comes down to a
series of actions that flick those switches from
0 to 1 and back again.
Familiar stuff.
But in a QC, the bit is upgraded to a quantum
bit, or qubit, that doesn't need to choose
between 1 and 0. It can be both at once. As a
result, a memory array of n qubits can
represent every number between 1 and 2n
simultaneously.
A QC's capacity
doubles with each additional qubit. It may be
humbling that the world's largest QC is
currently only 7 qubits in size, and can barely
process single-digit numbers. But a QC of 333
qubits would be able to perform operations
instantaneously on every number between 1 and a
googol (10100), a value considerably
larger than the number of atoms in the universe.
To carry out addition or multiplication on every
positive integer between 0 and 10100
would take one of today's supercomputers several
quadrillion years as it marched through one
number at a time. But a QC would perform the
calculation all at once, and it'd be done.
The basic
technology behind today's 7-qubit prototype at
Los Alamos National Laboratory may be familiar
to anyone who's ever had an MRI scan. Nuclear
magnetic resonance (NMR) works at the subatomic
level, where particles are small enough that
they answer to the fuzzy laws of quantum
mechanics, and bits turn into qubits. The
nucleus of an atom is an electrically charged
spinning ball, causing it to act like a bar
magnet. Each nucleus has magnetic north and
south poles that wobble and rotate together,
like a buoy in a stormy electromagnetic sea that
responds to the waves lapping at its sides. Hit
the buoy's resonant frequency and you can flip
it over like a kayak, and right it again, too.
Within the molecule, each nucleus can be linked
to its neighboring nuclei through the quantum
behavior called entanglement - an all-for-one,
one-for-all state in which one qubit's actions
affect all others it touches. So a chain of
atoms can be rigged up with the conditional
logic - such as AND, OR, XOR - to make a
computer.
Computation in an
NMR computer is done by beaming in pulses of
radio waves tuned to the particular resonant
frequency of each nucleus in molecules of a
liquid solution, such as chloroform and crotonic
acid, and detecting the resonant frequencies
emanating from the resulting nuclear alignment.
Each nucleus' resonant frequency changes
depending on whether its neighbors are in their
1 or 0 states, so radio pulses can be used as
sheepdogs to herd the qubits through an
algorithm's flowcharts. Unlike a conventional
computer, each branch of a flowchart represents
not an either-or choice, but rather a
bifurcation: One state of the computer answers
"yes" and follows the instructions from there,
while another simultaneous state answers "no"
and does likewise. For the right kind of
calculations, such as the factoring of large
numbers, these superimposed splits build up an
exponential speed advantage over classical
bit-based logic. (The biggest challenge for the
programmer is choosing which of many
simultaneous answers to convert back to 1s and
0s for output.)
Ultimately,
however, NMR computing is a dead end. Each qubit
and logic gate needs its own signature resonant
frequency, and there are other problems
involving initialization and readout of the
system. "NMR will stop in the next few years in
terms of number of qubits," says Los Alamos
mathematician Emanuel Knill, who codesigned the
7-qubit machine. "It's like the development of
classical computers - at some point we had to
switch from vacuum tubes to something else."
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Mark K.
Anderson (mark@markkanderson.com)
has written for Science, Harper's,
and Wired News.