Laser beams create caesium 'strings', potential memory media

13 Jul 2006

Researchers at Bonn University in Germany have used laser beams to create a string of caesium atoms. Such a structure could act as a memory for quantum devices.

A team from Bonn University in Germany has used two sets of laser beams to create a string of evenly separated caesium atoms. Such structures could act as a scalable memory for quantum information devices.

To make the strings, the researchers first trapped and cooled caesium atoms to sub-millikelvin temperatures in a vacuum chamber. Then they transferred the atoms into a horizontal standing-wave optical dipole trap formed by two counterpropagating laser beams.

The atoms headed towards the antinodes of the dipole trap, which were about 0.5 µm apart. As a result, the atoms became aligned in a string but they did not necessarily fill every antinode.

"Unfortunately it cannot be predicted which trough precisely the atoms will land in," said Arno Rauschenbeutel of Bonn University. "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."

In order to distribute the atoms more evenly, the researchers employed optical tweezers in the form of a vertical standing-wave dipole trap made by retro-reflecting a focused laser beam.

Rauschenbeutel and colleagues used the tweezers to remove an atom from the horizontal trap. Meanwhile they moved the other trapped atoms from left to right by varying the relative phase of the horizontal trapping beams until a suitable antinode site crossed the path of the vertical trap. Then they replaced the original atom in the desired trough by varying the standing wave pattern of the tweezers.

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

In this way, the team made a string of seven atoms each separated by 15 µm. The technique was less successful for atoms that were originally less than 10 µm apart, as the vertical dipole trap had a radius of 10 µm.

The researchers say that the strings of equally spaced atoms could serve as a scalable, neutral-atom quantum register for storing and manipulating quantum information.

"For quantum calculations, all the atoms on the conveyor belt need to have the same distance from each other," said Rauschenbeutel. "Only then can we get them to interact in a controlled way in what is called a quantum gate."

Now the team hopes to build a quantum gate by writing quantum information onto two caesium atoms and placing them between two mirrors. The plan is that the atoms will exchange information by emitting and absorbing fluorescent light.

The researchers reported their work in Nature.