In quantum physics breakthrough, strange computer is on and off
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Fri, 10 Mar 2006 14:48:59 GMT
In quantum physics breakthrough, strange computer is on and off
http://www.kansascity.com/mld/kansascity/news/nation/13946424.htm
BY JEREMY MANIER
Chicago Tribune
CHICAGO - In the bizarre realm of quantum mechanics - the physics
theory that stumped even Albert Einstein - tiny things like electrons
and packets of light often seem to be in two places at once, in total
violation of common sense.
Now a University of Illinois physics team has taken that principle and
built something harder to fathom: a quantum-based computer that can be
awake and asleep at the same time, and spits out answers even if its
program is never triggered.
It's plenty strange, but some experts say such real-world spinoffs of
eerie quantum effects are growing so common that it's our understanding
of "strange" that needs to change.
"This is the way nature is," said Charles Bennett, an IBM researcher
who dreamed up some of the new uses of quantum physics. "We should be
learning how to get used to that."
Quantum mechanics is the theory physicists use to understand events at
the atomic level, which works far differently than the large-scale
world that people inhabit. The theory states that it's impossible to
gain complete knowledge about any subatomic particle, and its location
and other traits often exist only as probabilities.
That maddening, fuzzy quality is fueling creative ideas about how to
put quantum effects to work.
The University of Illinois experiment, published Thursday in the
journal Nature, could help refine the young field of quantum computing.
In theory, computers based on quantum effects could race through
calculations that would take an ordinary computer billions of years to
complete. Applications of such computers could include precise
simulations of how proteins work in the human body.
Recent research also has raised the prospect of unbreakable quantum
codes, a commercial opportunity that some companies already are vying
to exploit. Bennett and others have pioneered a form of "quantum
teleportation" that can replicate the characteristics of light
particles more than a mile away - though nobody expects to be able to
beam people around.
The not-quite-technical term many physicists use for such effects is
"quantum weirdness." Although quantum theory has proved one of the most
successful and accurate ideas in science since Max Planck laid its
foundations a century ago, most great physicists have pronounced the
theory nearly impossible to reconcile with common sense. Einstein could
not accept the theory's glorification of probability, complaining, "God
does not play dice with the universe."
University of Illinois physics professor Paul Kwiat, co-author of the
new quantum computing study, said one of his favorite quotes on the
subject is by Nobel laureate Murray Gell-Mann, who once said: "We know
how to use (quantum mechanics) and how to apply it to problems; and so
we have learned to live with the fact that nobody can understand it."
Kwiat said he and his team don't need to understand what quantum theory
ultimately means for philosophical notions of reality. But they do know
that quantum effects allowed them to dream up one really weird
computer.
Like a frantic one-man band, a quantum computer gets its unique power
by trying to do many things at the same time.
Such devices are far different from the digital computers everyone
uses, which can process just one "bit" of electronic information at a
time, in a stately procession of 0s and 1s.
The uncanny gift of quantum mechanics is that it could permit computers
that calculate many possibilities simultaneously, because their bits
can be 0 and 1 at the same time. Such computers exploit the properties
of light packets called photons or other particles that seem to exist
in more than one physical state at once.
As the number of quantum bits increases, the computer can consider many
more combinations of data at a single stroke. That could open the door
to a "quantum genie," Kwiat said, offering answers to otherwise
impossible problems.
One use of such power may be to find all the possible factors of very
large numbers - which unfortunately is just what code-breakers would
need to crack the tightest modern security codes. A typical code used
in banking transactions would take 100 million personal computers a
thousand years to decipher. A sufficiently powerful quantum computer
might be able to do that job in minutes.
On the bright side, quantum effects might also make it possible to make
tamper-proof codes that no computer - quantum or otherwise - could ever
break.
University of Illinois graduate student Onur Hosten, who co-wrote the
Nature paper with Kwiat, said he has always been fascinated by ways of
seeing quantum weirdness in the ordinary world.
"I spend a lot of time thinking about how far we can push these
effects," Hosten said.
So Hosten wanted to see if he could get accurate information from a
quantum computer even if its program never runs. It turned out he
could.
That's not as crazy as it sounds, if you try to use what Bennett calls
"quantum intuition."
Hosten and Kwiat knew the same physical laws that allow quantum bits to
be 0 and 1 at the same time ought to let them make a quantum computer
that is both running and not running at once. That twinning effect is
called superposition, and it's at the heart of theories about the world
of the very small.
The quantum computer they built is set up to run a simple program using
one photon of light at a time. The light goes through a series of
lenses and mirrors that give an "answer" by directing the photon to one
of many light detectors.
But what if the setup allowed the photon in some cases to be reflected
away from the computer before it arrived there? In those cases you
would not expect to be able to get any information from the computer.
Yet Hosten and Kwiat were able to learn something about the photon's
potential interaction with the computer even if final measurements
showed the photon never took that route.
That's because according to quantum mechanics, the photon actually
exists in two conditions at once - one in which it went through the
computer, and another in which it was bounced away.
The two alternate paths even affect one another, and the interaction
can influence which light detector is triggered. That, in turn,
provides information about what the computer would have found, even if
measurements show the program never actually ran. Specifically, it can
exclude one possible answer.
"What gives us the answer is the possibility of the computer running,"
Hosten said.
Based on that idea, Hosten imagined an even more complicated setup,
using still more optical devices to shoot the photon through a
figure-eight circuit buzzing with possible outcomes. Such a device, he
showed, could narrow down the possibilities to just one answer, even if
the photon never actually went through the program.
Such implications of quantum theory may always seem too strange to
handle. But Bennett said he hopes that today's youth one day will take
quantum weirdness for granted, in a way that Einstein's generation
never could.
"They didn't have as many decades to get used to the idea," Bennett
said.
.
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