Atoms in a Bose-Einstein condensate can be manipulated with light to form a highly directional atom laser, physicists at the Commerce Department’s National Institute of Standards and Technology report in the March 12 issue of Science.
The NIST atom laser represents a significant step forward from the first atom laser demonstrated in 1997 at the Massachusetts Institute of Technology in that its atoms stream forward in a chosen direction as a very narrow beam. The direction of the earlier MIT atom laser beam was determined by gravity and had a big spread due to the tendency of the atoms to repel each other.
"The atom laser is as different from an ordinary atom beam as an optical laser is from a flashlight. It now gives you for atom beams what you have had with laser light," says William D. Phillips, leader of the Laser Cooling and Trapping Group in the NIST Physics Laboratory.
Steven Rolston, Kristian Helmerson, Edward Hagley and Jesse Wen, all of NIST; Lu Deng of Georgia Southern University; and Mikio Kozuma, now at the University of Tokyo also worked on developing the atom laser. Their work was funded in part by the Office of Naval Research and the National Aeronautics and Space Administration.
The NIST atom laser was made from a gaseous Bose-Einstein condensate, an exotic form of matter first achieved in Boulder, Colo., in 1995 by NIST physicist Eric Cornell and University of Colorado physicist Carl Wieman. In the atom laser experiment at NIST in Gaithersburg, Md., scientists trapped sodium atoms in a magnetic field and cooled them to a millionth of a degree above absolute zero at which point they began to Bose condense.
They further cooled the gas to about 50 billionths of a degree above absolute zero, so that nearly all the atoms became part of the condensate. In a Bose-Einstein condensate, a state that Albert Einstein predicted more than 70 years ago, all atoms behave as a single entity in which individual atoms are indistinguishable from one another.
Although practical uses of the atom laser could be years away, scientists are excited about the NIST invention and its potential. "As when the optical laser was invented 40 years ago, the potential applications of the atom laser may not yet be apparent," says Phillips, a Nobel laureate for his work on cooling and trapping atoms with laser light.
Nevertheless, scientists anticipate being able to create holographic images producing any picture or pattern desired on a flat surface. This eventually may lead to improvements in lithography, the manufacturing technique for making exquisitely small features on computer chips. The atom laser may lead to improvements in instruments that currently use an atom beam, such as novel gyroscopes and atom interferometers used in research. Such instruments may one day be used in navigation or in prospecting for oil.
To make their atom laser, NIST scientists aim two optical lasers at the supercold Bose-Einstein condensate, one from the left side and one from the right. The atoms absorb photons from one laser beam and emit photons into the other laser beam. This process transfers momentum to the atoms and gives them a kick in the direction of one of the laser beams.
In order to select the direction of the atom laser beam, NIST scientists tune the optical lasers to slightly different frequencies. The atoms preferentially absorb photons from the higher frequency laser and emit them into the lower frequency one. Therefore, they move in a single direction, toward one laser and away from the other.
Although the atoms gain momentum from the laser beams, they are still held in the trap by the magnetic field. In order to free the atoms, which are like tiny magnets all pointing in the same direction, NIST scientists have to change the atoms’ orientation so they no longer feel the attraction of the magnetic field used to confine them. Reorienting the atoms requires energy, which scientists provide by increasing the difference between the frequencies of the optical lasers.
By pulsing the lasers very quickly, the scientists are able to overlap the small clumps of atoms that get kicked out of the trap with each pulse, effectively making a continuous beam of atoms. By varying the intensity of the laser light, the scientists are able to create atomic laser beams of varying intensities at the chosen direction and speed.
The NIST work represents a significant step toward making a truly continuous atom laser. Since the NIST atom laser removes atoms from a Bose-Einstein condensate containing a finite number of atoms, it eventually runs out of atoms. For a truly continuous atom laser, scientists would have to find a way to replenish the atoms in the Bose-Einstein condensate while removing the atoms that make up the atom laser beam.
The NIST atom laser is very well collimated, that is the atoms streaming out of the Bose-Einstein condensate remain as a very narrow beam, much as light in a laser pointer spreads very little even across a large auditorium. The atom laser is about 60 millionths of a meter wide, about the diameter of a human hair, and travels at about 6 centimeters per second.
NIST scientists look forward to better, more intense atom lasers becoming important scientific and maybe even practical tools.
For more information and to see images of the atom laser, go to the NIST Physics Laboratory’s news page on the World Wide Web at http://physics.nist.gov/atomoptics.
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.