atom
Image: B. UBERHOLZ/University of Bonn

The promise of quantum computing is still largely theoretical. To be certain, the unique ways in which atoms behave in the quantum realm, such as becoming entangled with each other, offer scientists entirely new means for carrying out calculations. But physicists must first learn to physically manipulate atoms in this world before they can put them to work. To that end, a team at the University of Bonn recently made a significant advance, showing how to maneuver a fixed number of atoms from one kind of laser trap to another. They report their findings in the October 30 issue of Physical Review Letters.

Dieter Meschede, Victor Gomer and their colleagues swapped neutral cesium atoms from a magneto-optical trap (MOT) to an optical diode trap with a degree of precision never before attained. The MOT holds atoms in a magnetic field by way of radiation pressure from several lasers. The atoms absorb photons from the lasers and reemit them in random directions until they are slowed enough to settle down in the middle of the field. The optical diode trap grasps atoms more gingerly, using the oscillating electric field of a single laser to polarize and attract them to the beam's focal point. In this sort of trap, the atoms are more restful--and ready for manipulation--because the beam can be tuned to a frequency at which the atoms do not absorb photons.

To accomplish the trade, the Bonn team created an especially small MOT--one on the same scale as their optical diode trap. They then aligned the centers of the traps, and isolated a few cesium atoms in a 10-micron-wide clump in the MOT. A sensitive photon detector measured fluorescent emissions from the trapped atoms, revealing their exact number. (In repeated experiments, this number was usually between one and 10.) Next the group switched on the optical diode trap and, within milliseconds, shut down the MOT. All the atoms remained suspended within the second laser trap; with the MOT back on, fluorescent emissions showed that none were missing.

The team now plans to try moving the atoms another few millimeters into a different kind of device, but some scientists are already delighted with the first act. "This is quantum mechanics at its ultimate," says Arthur Ashkin, who pioneered the optical diode trap and is now retired from Bell Laboratories/Lucent Technologies. "I think the community will jump on it."