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Physicists Measure Tiny Force That Limits How Far Machines Can Shrink

September 24, 2005
University of Arizona
University of Arizona physicists have directly measured how close speeding atoms can come to a surface before atom wavelengths change. Their research has huge implications for nanotechnology and atom optics.

Alex 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.
Credit: Image courtesy of University of Arizona

University of Arizona physicists have directly measured howclose speeding atoms can come to a surface before the atoms'wavelengths change.

Theirs is a first, fundamental measurementthat confirms the idea that the wave of a fast-moving atom shortens andlengthens depending on its distance from a surface, an idea firstproposed by pioneering quantum physicists in the late 1920s.

Themeasurement tells nanotechnologists how small they can make extremelytiny devices before a microscopic force between atoms and surfaces,called van der Waals interaction, becomes a concern. The result isimportant both for nanotechnology, where the goal is to make devices assmall 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 precisesensors and study quantum mechanics.

UA optical sciences doctoralcandidate John D. Perreault and UA assistant professor of physicsAlexander D. Cronin report the experiment in the Sept. 23 PhysicalReview Letters. The paper is online at

Perreaultand Cronin used a sophisticated device called an atom interferometer inmaking the measurement. Cronin brought the 12-foot-long device to UAfrom MIT three years ago. The atom interferometer was assembled over 15years with more than $2 million in research grants from the NationalScience Foundation, the UA and the Research Corp. Now in Cronin'slaboratory on the third floor of the UA's Physics and AtmosphericSciences Building, the machine is one of only a half-dozen suchinstruments operating in the United States and Europe. It splits andrecombines atom waves so that scientists can observe the position ofthe wave crests.

"Our research provides the first directexperimental evidence that a surface 25 nanometers away (25 billionthsof a meter) causes a shift in the atom wave crests," Perreault said."It shows that the van der Waals interaction may be a small scaleforce, but it's a big deal for atoms."

Perreault and Cronin foundthat atoms closer than 25 nanometers to a surface are very stronglyattracted to the surface because of the van der Waals interaction-- sostrongly 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 everydayexperience -- it's the force a person feels from Earth's gravity. Aroller coaster rider might feel 3 to 4 g's for brief moments during aride. Fighter pilots can experience accelerations of up to 8 g forbrief periods during tactical maneuvers, but can black out if subjectedto 4 to 6 g's for more than a few seconds.

"We might say thatwhen an atom is between 10 and 20 nanometers from a surface, it getssucked toward the surface with a force a million times its weight,"Cronin said. "And when it gets closer, it gets pulled even harder."

Themomentary acceleration of the atom as it passes by the surface isexpressed in a famous equation which relates the speed of an atom toits wavelength, Cronin said. When atoms are accelerated and closer tothe surface, their wavelengths become shorter. When farther from asurface, atoms return to their original wavelength. Perreault andCronin used the atom interferometer to measure the wavelength shift.

Nanotechnologyresearch aims to build much smaller transistors and motors, forexample, than currently exist. Atom optics research aims to exploit thewave behavior of atoms in ways that will make more precise gyroscopesfor navigation, gravity gradiometers for subterranean mapping and otherfield sensors.

"I think the impact of our work stems from theintersection of the fields of atom optics and nanotechnology,"Perreault said. "It answers the question of how far you can miniaturizean atom optics device - for example, a device that guides atoms on achip to form a very tiny interferometer - before this nano-interactiondisrupts operations."

The idea that atoms behave as waves as wellas particles goes back to 1924. They're called "de Broglie waves" forearly 20th-century French quantum physicist Prince Louis-Victor deBrogli, who first proposed the concept of atom waves. Scientists havegrappled with the dual wave-particle nature of atoms for decades and,in the 1990s, they began chilling atoms to near absolute zero andstudying the wave properties of atoms in detail.

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