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Bonn Physicists Film Individual Atoms

Video pictures show transport on a \'conveyor belt\'

For the first time anywhere in the world physicists of the University of Bonn have filmed individual atoms as they are being transported across a space of just under a tenth of a millimetre on a kind of conveyor belt consisting of laser beams.

Even in 2000 Professor Dieter Meschede\'s research team had succeeded in slowing down individual caesium atoms for a period of several seconds to such an extent that they hardly moved at all. They were then transferred to a \'conveyor belt\' consisting of laser beams. This enabled the atoms to be moved up to one centimetre towards the desired destination - this is, after all, a hundred million times as far as an atom of caesium is thick. To put it another way, if the atom was a marble, the conveyor belt would extend approximately from Bonn to Rome. Now, for the first time, the researchers have succeeded in filming this transport process. To this end they projected light of a specific colour onto the atom. Caesium can absorb this kind of light, discharging it a little later rather like a flash of lightning. The physicists intercepted this fluorescent light using a highly sensitive camera, being thus able to take pictures of the luminescent atom once per second. When these are played in succession, the sequence shows the caesium atom moving steadily along on its conveyor belt of laser beams.

This conveyor belt is a stationary wave of light made up of many peaks and troughs - perhaps comparable to a piece of corrugated cardboard. The physicists place the atom in one of these troughs and then set the wave in motion. The atom, hemmed in by the sides of the trough, moves along with it. Before it is on the conveyor belt it is visible through the camera as a circular spot of about a hundredth of a millimetre in diameter, whose luminance decreases towards its edges. \'The atom is almost stationary, "almost" being the operative word,\' is how Dominik Schrader, a member of Professor Meschede\'s team, explains it. However, as soon as the caesium slides onto the conveyor belt, its image changes its shape, becoming an elongated lens: now the atom can only vibrate in the \'trough\' parallel to the peaks of the wave, but is no longer vibrating in all directions.

A second film even shows three atoms which are moving leftwards together, until the physicists reverse the direction of the conveyor belt. In general the experiment demonstrates the astonishing ability to move individual atoms to a particular location. This opens up fascinating perspectives and is the sine qua non of what is known as a \'quantum grid\', on which the Bonn team are already working. For this they intend to load two caesium atoms with various kinds of \'information\', then securing these between two tiny mirrors. There they are meant to interact with each other, i.e. exchange information by discharging and receiving fluorescent light. A grid like this would be the first basic step towards a quantum computer.

Background: How can atoms be made stationary?

At room temperature atoms and molecules move roughly at the speed of a jet plane, and at higher temperatures even faster. However, atoms can only be transported by laser beams if they are stationary. Therefore the scientists have to cool them, i.e. to slow down their unco-ordinated movement. They thus bombard the atoms with six laser beams; two each of these intersecting each other in each axis. Although light consists of waves, it does also, on the other hand, possess particle properties. By means of this laser bombardment the physicists can therefore decelerate or accelerate their caesium atom - \'however, the magnitudes involved are roughly as if you wanted to stop a lorry by using a tennis-ball thrower,\' as Bonn physicist Dr. Arno Rauschenbeutel explains it. \'Even so, the caesium atoms are slowed down in a very short time, because every second they collide with many millions of light particles.\' 

Atoms in fact have the property of allowing themselves to be slowed down (or speeded up) particularly well by light of a specific colour - basically as if a lorry reacted more positively to red tennis balls rather than blue ones. Furthermore, light changes its colour when something moves towards the source or away from it - rather like the way the siren of an ambulance has a higher pitch when the vehicle is coming towards you. So if a flying atom is \'pushed\' by a laser beam, it reacts to the bombardment with light to a greater or lesser extent - depending on whether, as a result of the atom\'s own movement, the laser now has a \'more suitable\' colour, i.e. is more effective, or not. If we therefore decide to bombard a flying atom simultaneously with two lasers of suitable colour from the front and the back, the oncoming beam slows it down much more effectively than the additional thrust supplied by the beam from behind. The result is that the atom slows down. The physicists use this Doppler effect to decelerate their caesium atoms until they are almost stationary. An alternative way of putting it would be to say that they cool them down to a temperature which lies a mere 100 millionth of a degree above absolute zero.

For the conveyor belt they then use high-powered infrared laser beams. The atoms stick to these beams like scraps of paper to a plastic ruler which has been rubbed with a cloth and electrically charged. The effect is not very great. However, with these very cold atoms it is sufficient to grasp them with optical tweezers and to move them precisely in space.


Contact person:
Professor Dieter Meschede
Institute of Applied Physics of the University of Bonn
Tel.: ++49-228-733478
E-mail:
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