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Cronin (left) and John Perreault make an adjustment on the atom
interferometer that they used in making the first direct measurement
of atom wavelength shift caused by the van der Waals interaction.
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Physicists Measure Tiny Force That Limits How Far Machines
Can Shrink
By Lori Stiles
September 22, 2005
University of Arizona physicists have directly measured
how close speeding atoms can come to a surface before the atoms'
wavelengths change.
Theirs is a first, fundamental measurement
that confirms the idea that the wave of a fast-moving atom shortens and
lengthens depending on its distance from a surface, an idea first proposed
by pioneering quantum physicists in the late 1920s.
The
measurement tells nanotechnologists how small they can make extremely tiny
devices before a microscopic force between atoms and surfaces, called van
der Waals interaction, becomes a concern. The result is important both for
nanotechnology, where the goal is to make devices as small as a few tens
of billionths of a meter, and for atom optics, where the goal is to use
the wave nature of atoms to make more precise sensors and study quantum
mechanics.
UA optical sciences doctoral candidate John D. Perreault
and UA assistant professor of physics Alexander D. Cronin report the
experiment in the Sept. 23 Physical Review Letters. The paper is online at
http://xxx.lanl.gov/PS_cache/physics/pdf/0505/0505160.pdf
Perreault
and Cronin used a sophisticated device called an atom interferometer in
making the measurement. Cronin brought the 12-foot-long device to UA from
MIT three years ago. The atom interferometer was assembled over 15 years
with more than $2 million in research grants from the National Science
Foundation, the UA and the Research Corp. Now in Cronin's laboratory on
the third floor of the UA's Physics and Atmospheric Sciences Building, the
machine is one of only a half-dozen such instruments operating in the
United States and Europe. It splits and recombines atom waves so that
scientists can observe the position of the wave crests.
This simple
sketch shows the placement of diffraction gratings - represented by
the vertical dashed lines - that split and recombine atom waves. The
gratings are about a meter apart. |
"Our research provides the first direct experimental
evidence that a surface 25 nanometers away (25 billionths of a meter)
causes a shift in the atom wave crests," Perreault said. "It shows that
the van der Waals interaction may be a small scale force, but it's a big
deal for atoms."
Perreault and Cronin found that atoms closer than
25 nanometers to a surface are very strongly attracted to the surface
because of the van der Waals interaction-- so strongly that the atoms are
accelerated with the force of a million g's.
A "g" is a term for
acceleration due to gravity. One g is an everyday experience -- it's the
force a person feels from Earth's gravity. A roller coaster rider might
feel 3 to 4 g's for brief moments during a ride. Fighter pilots can
experience accelerations of up to 8 g for brief periods during tactical
maneuvers, but can black out if subjected to 4 to 6 g's for more than a
few seconds.
"We might say that when an atom is between 10 and 20
nanometers from a surface, it gets sucked toward the surface with a force
a million times its weight," Cronin said. "And when it gets closer, it
gets pulled even harder."
Cronin (left), Perreault and the atom interferometer.
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The momentary acceleration of the atom as it passes by
the surface is expressed in a famous equation which relates the speed of
an atom to its wavelength, Cronin said. When atoms are accelerated and
closer to the surface, their wavelengths become shorter. When farther from
a surface, atoms return to their original wavelength. Perreault and Cronin
used the atom interferometer to measure the wavelength
shift.
Nanotechnology research aims to build much smaller
transistors and motors, for example, than currently exist. Atom optics
research aims to exploit the wave behavior of atoms in ways that will make
more precise gyroscopes for navigation, gravity gradiometers for
subterranean mapping and other field sensors.
"I think the impact
of our work stems from the intersection of the fields of atom optics and
nanotechnology," Perreault said. "It answers the question of how far you
can miniaturize an atom optics device - for example, a device that guides
atoms on a chip to form a very tiny interferometer - before this
nano-interaction disrupts operations."
The idea that atoms behave
as waves as well as particles goes back to 1924. They're called "de
Broglie waves" for early 20th-century French quantum physicist Prince
Louis-Victor de Brogli, who first proposed the concept of atom waves.
Scientists have grappled with the dual wave-particle nature of atoms for
decades and, in the 1990s, they began chilling atoms to near absolute zero
and studying the wave properties of atoms in detail.
 
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