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Physicists Sort Atoms with Laser 'Tweezers'

Photonics.com
Jul 2006
BONN, Germany, July 14, 2006 -- Using laser "tweezers", physicists have succeeded in sorting up to seven atoms and putting them in a line. The researchers said their method -- which could be used as a memory device that has quantum information stored in strings of equally spaced atoms -- clears a major hurdle on the path to building a quantum computer.

In the experiment, which was documented on film, the research team headed by Arno Rauschenbeutel, PhD, and professor Dieter Meschede of the University of Bonn, decelerated several caesium atoms (used in atomic clocks) for a period of several seconds so that they were hardly moving, then loaded them onto a "conveyor belt" consisting of lasers. This conveyor belt is made up of a standing light wave composed of many peaks and troughs and resembles a piece of corrugated iron.

ArnoRauschenbeutel.jpg
University of Bonn laser physicist Arno Rauschenbeutel is using atom sorting to help build a quantum computer. (Photo: Franc Luerweg/University of Bonn)

Unless they are forcibly slowed, atoms can move incredibly fast. For example, at room temperature, atoms making up gases in the air move at a speed of about 1100 mph.

"Unfortunately it cannot be predicted which trough precisely the atoms will land in," Rauschenbeutel said. "It's rather like pouring several eggs from a big dish into an egg carton -- which section each egg rolls into is a matter of chance."

However, anyone wishing to calculate with atoms must be able to place them exactly. "All the atoms on the conveyor belt have to have the same distance from each other," Rauschenbeutel said of the challenge. "Only then can we get them to interact in a controlled way in what is called a quantum gate." By lining up gate operations like these, it would already be possible to carry out simple quantum calculations, he said.

The physicists subsequently "sorted" the atoms in their experiment on the conveyor belt. They did this by first taking a photo to record their positions. They next set the conveyor belt moving -- and with it the caesium atoms trapped in the troughs. In this way they transported the wrongly placed atoms to their laser tweezers -- this is basically nothing more than another conveyor belt consisting of laser beams which is oriented orthogonally to the first conveyor belt.

"When we set the tweezers' light wave in motion, we can lift the wrongly placed atoms off the conveyor belt," Rauschenbeutel said. "Then we move the conveyor belt to the desired position and simply pop the atom back in."

The film shows how well this works: the tweezers select two atoms consecutively from the belt and put them back on again in such a way that they are exactly the same distance from each other and from a third atom. "Sorting seven atoms in this way takes us about two seconds," Rauschenbeutel said.

The next aim of the physicists is to construct a quantum gate. For this purpose they want to "write" quantum information onto two caesium atoms and then place them between two tiny mirrors. The intention is that they should interact there with each other, i.e. exchange information by emitting and absorbing fluorescent light. If this is successful, it will be the next milestone for the researchers on their way to the quantum computer.

The physicists report their research in the July 13 issue of the journal Nature. For more information, visit: www.uni-bonn.de



GLOSSARY
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
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