Extreme measures: Atom interferometry's precision could make it the Swiss Army knife of physics

Science News,  Feb 16, 2008  by Ewen Callaway

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In spring 2010, the military plans to embark on a road trip across the country to test a new way of navigating. Instead of taking a path marked by a dog-eared road atlas, a compass, or even global positioning satellites, the vehicle will follow one mapped by super-cold cesium atoms.

This cross-country trek will be a field test for the Defense Department's Precision Inertial Navigation Systems program to navigate by measuring the Earth's rotation using atoms that behave like waves. The vehicle won't drive blind, but the machine guiding it could make such a feat possible. And someday the new system could also improve the accuracy of gyroscope navigation in airplanes 200-fold, says Air Force Lt. Col. Jay Lowell, who is leading the project.

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The atoms' direction-finding powers come from a technique called atom interferometry. Once a lab curiosity, atom interferometry is now becoming the Swiss Army knife of physics. It has the potential to steer airplanes and submarines, uncover buried caches of oil and diamonds, and perhaps hunt down cave-dwelling terrorists. The technology is also helping scientists probe the very nature of the universe, from detecting theoretical waves of gravity sent out by exploding stars to measuring deviations in the strength of gravity at super-close distances. Physicists are even rallying to put an atom interferometer in orbit to test theories like Einstein's general relativity with unparalleled exactness.

"In the last 10 years, atom interferometry has gone from inventions and demonstrations into precision measurement tools," says physicist Mex Cronin, an expert on the technique at the University of Arizona in Tucson.

THE PATH LESS TRAVELED At its heart, atom interferometry is similar to light interferometry, a 200-year-old technique that itself improved the accuracy of many measurements.

Shine alight through a half-silvered glass plate and half the waves pass through, while half bounce off at an angle. A couple of regular mirrors can reflect the two beams back together. If one wave travels a little bit further than the other, the recombined waves will be slightly out of sync. With visible light, this effect produces a pattern of white-and-black stripes: white stripes correspond to areas where the waves line up and black stripes to where they cancel each other out. Physicists use this effect--called an interference pattern--to calculate differences in the distance each beam travels.

During the late 1800s, two American physicists, Albert Michelson and Edward Morley, used a light interferometer to try to detect the "luminiferous ether," then thought to occupy all space. Just as sound consists of vibrations in air, light was supposedly a vibration of the ether, scientists thought. If so, the apparent direction of Earth's motion through the ether would alter the velocities of light beams taking different paths. However, Michelson and Morley's experiment found no such difference, casting doubt on the ether's existence.

Classical physics explains the interference of light waves just fine. Atom interferometry, however, hinges on the bizarre behavior of atoms predicted by quantum mechanics, the math that describes how matter works at sub-microscopic scales. Just as waves of light can sometimes act like particles called photons, atoms can be coaxed into showing off their inner waves. In this condition, an atom can exist in two or more places at once, called a superposition. "It's just weird and you finally end up saying this is a very weird theory," says physicist David Pritchard, a pioneer in the field who works at the Massachusetts Institute of Technology.

One advantage of using atoms for interferometry is their tiny wavelengths. In the 1920s, French physicist Louis-Victor de Broglie proposed that a particle could behave as a wave, and the wavelength would be determined by the particle's speed and mass; the heavier and faster the particle, the shorter its wavelength. Shortly thereafter, experiments proved de Broglie right. The atoms used in interferometry have wavelengths around a hundredth of a nanometer, while the wavelength of visible light measures from 400 to 700 nanometers. Atom waves split into two paths can be used to detect much smaller differences than light. If the path an atom takes varies by even a thousandth of a nanometer (a picometer) an atom interferometer can spot the difference, Cronin says.

Another benefit of atoms is the breadth of their physical characteristics, which include mass, magnetic sensitivity, and ability to hold an electric charge. And atoms feel the pull of gravity. If light interferometry is an old wooden meter stick, then atom interferometry is a modern tape measure, scale, and voltmeter rolled into one.

Yet these beneficial traits also make atoms tough to observe as waves, Cronin says. Atoms flitting about at room temperature tend to bump into one another. To better detect the waves, physicists cool the atoms to a few millionths of a degree above absolute zero-the temperature at which all atomic motion virtually stops--and manipulate them to fly in the same direction.