Physicists at the Commerce Department’s National Institute of Standards and Technology have opened a new field of physics with experiments demonstrating for the first time that atoms can display some unusual properties previously seen only with high intensity laser light waves. They report their findings in the March 18 issue of Nature.
Using sodium atoms cooled to very near absolute zero, the NIST team demonstrated that three atom waves can be mixed to produce a fourth wave, in exactly the same manner as optical laser beams can be combined to form a new laser light beam. These experiments, conducted in a vacuum, show that under very specific conditions, matter waves can mimic the way high-intensity laser light waves behave in certain materials.
"We are at the threshold of a new area of research: non-linear atom optics," says William D. Phillips, leader of the NIST Laser Cooling and Trapping Group. Also working on the atom wave mixing experiments are Steven Rolston, Kristian Helmerson, Paul Julienne, Jesse Simsarian, Edward Hagley and Jesse Wen, all of NIST; Lu Deng of Georgia Southern University; and Marek Trippenbach and Yehuda Band of Ben-Gurion University, Israel. The work was funded in part by the Office of Naval Research and the National Aeronautics and Space Administration.
Scientists expect that this new field of non-linear atom optics will parallel the development of non-linear optics, which emerged as scientists discovered many of the strange, unique and unexpected abilities of laser light following the demonstration of the first laser in 1960.
What is non-linear atom optics? After the invention of the laser, scientists began to note the unusual properties of laser light. The high intensity of the laser beam when passed through a material allows it to create new colors of light from other colors. This and similar phenomena were dubbed non-linear optics. In non-linear atom optics, scientists replace the light waves with atomic matter waves, but no material is needed because unlike light waves the atom waves interact directly with each other.
This first non-linear atom optics experiment demonstrates that atoms can be coaxed into waves analogous to laser-like light and combined to create a new wave, just as laser beams can be combined to form a laser beam of another color. To accomplish this, the NIST physicists first created a dense cloud of very cold atoms in which all the atoms fell into their lowest possible energy states and became indistinguishable from one another. This exotic state of matter is known as a Bose-Einstein condensate. It was first predicted by Albert Einstein more than 70 years ago and was first achieved in a gas by physicists at NIST and the University of Colorado in 1995.
Next the scientists pulsed beams of laser light with predetermined directions and frequencies onto the Bose-Einstein condensate, thereby splitting it into three distinct, intense matter waves. Each matter wave had a unique velocity and direction. The scientists applied the same rules governing light to determine the necessary velocities and directions to mix the three matter waves so they would form a fourth. As anticipated, the interaction of these matter waves did in fact produce a fourth matter wave with just the properties the scientists had predicted.
"Calculations by theorists Paul Julienne, Marek Trippenbach and Yehuda Band showed it should be easy to do," Phillips, a Nobel laureate, explains. "The theory pushed us in a new direction of investigation. We found a nice agreement between their theory and our experiments."
"Up until the early 1960s, people thought all properties of light could be explained by the classical theories covering electromagnetic fields," Phillips explains. "Then researchers began to understand that light could exhibit strange quantum behavior. Non-linear optics has been key in the development of quantum optics, which explores such behavior. We can now look forward to the analogous development of quantum atom optics," Phillips says. Another possible application is the amplification of matter waves, making a beam of atoms more intense by creating additional atoms that are exact copies of those in the original beam.
For more information and to see pictures of matter wave mixing, see http://physics.nist.gov/atomoptics on the World Wide Web.
As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST promotes economic growth by working with industry to develop and apply technology, measurements and standards through four partnerships: the Measurement and Standards Laboratories, the Advanced Technology Program, the Manufacturing Extension Partnership and the Baldrige National Quality Program.