AAAS: Take a Walk on the Quantum Side (If you're a fan of results that make your brain hurt, read on.)

[David Farber <dave () farber net>]

Begin forwarded message:

From: bobr () bobrosenberg phoenix az us
Date: July 12, 2009 3:36:28 PM EDT
To: "Dave Farber" <dave () farber net>
Subject: AAAS: Take a Walk on the Quantum Side (If you're a fan of  
results that make your brain hurt, read on.)

Hi Dave

AAAS succinctly explains what Brownian motion and quantum computers  
have in common.

Anybody have an aspirin?  My brain hurts.   ;-)

Cheers,
Bob

Take a Walk on the Quantum Side
Physicists demonstrate quantum version of famed random walk--a concept  
key to
explaining the diffusion of liquids
   http://sciencenow.sciencemag.org/cgi/content/full/2009/710/2?etoc


--
Bob Rosenberg
P.O. Box 33023
Phoenix, AZ  85067-3023
Mobile:  602-206-2856
LandLine:  602-274-3012
bob () bobrosenberg phoenix az us



Take a Walk on the Quantum Side

By Adrian Cho
ScienceNOW Daily News
10 July 2009
If you're a fan of results that make your brain hurt, read on.

Compounding one challenging concept with another, a team of atomic  
physicists has
put a new twist on the classic "random walk"--an idealized wandering  
that is key for
explaining the diffusion of one liquid in another and myriad other  
real-world
phenomena. This time, researchers have set a single atom ambling  
according to the
rules of quantum mechanics and found that it covers more distance than  
it would
without them. The advance, reported today in Science, could have uses  
in budding
quantum information technologies, the researchers say. But the real  
accomplishment
seems to be the marrying of two classic physics concepts.

In less politically correct times, physics professors explained the  
random walk as
follows. Suppose a drunk stands under a lamppost, staggering to the  
right or left
with equal probability. After some number of steps, N, he is likely to  
have taken a
few more steps to the left than to the right, or vice versa. In fact,  
after N steps,
on average the drunk will have moved a distance proportional to the  
square root of N
from the post. That may seem like a pointless thing to know, but such  
a random walk
neatly describes the motion of a molecule in a sample of liquid or  
electrons
rattling around in a metal.

Now, Michal Karski, Artur Widera, and colleagues at the University of  
Bonn in
Germany have thrown the weirdness of quantum mechanics into the mix. A  
drunk or any
other "classical" object must move either to the left or to the right.  
But according
to quantum mechanics, a tiny particle such as an atom can actually  
move in opposite
directions at the same time and end up in a so-called superposition  
state in which
it's in two places at once. There is a catch, though: Whenever  
somebody measures the
particle's position, the delicate quantum state will collapse so that  
the particle
is found in one place or the other.

Taking advantage of all this, the researchers have made a single  
cesium atom take a
"quantum walk" along a chain of spots of laser light formed by two  
opposing laser
beams. Starting with the atom in one spot, they tickle it with radio  
waves to make
it spin in opposite directions--up and down--at once. They then fiddle  
with the
polarizations of the laser beams in a way that pulls the "up" part of  
the quantum
state to the right and the "down" part to the left. That puts the atom  
in a
hard-to-imagine state in which it sits in one spot spinning up while  
at the same
time it sits in the next spot spinning down.

The researchers then repeat the process again and again so that the  
atom ends up in
a quantum state in which it occupies many light spots at once. When  
the researchers
measure the atom's position, the state collapses to just one spot. But  
by performing
the experiment many times, they can sketch out that state. And they  
find that, after
N steps in the process, an average atom has moved a distance  
proportional to N from
its original spot--farther than it would get classically.

"It is difficult to imagine a cleaner, more textbooklike demonstration  
of the idea
of a quantum walk," says Poul Jessen, an experimental physicist at the  
University of
Arizona in Tucson. The advance could be more than academic, adds  
Reinhard Werner, a
theorist at the University of Hannover in Germany. In principle, he  
says, by putting
several atoms into the chain of light spots and letting them interact,  
it might be
possible to construct a kind of quantum computer that could crack  
problems that a
conventional computer cannot. A full-blown computer is a long way off,  
Werner says,
but "this is a first step toward more complicated things."






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