Elliptical Polarization Uses Laser Beams to Move Atoms in a Vacuum

Researchers at the University of Bonn have developed a method that significantly shortens the time required to be able to transport individual atoms via laser light in a vacuum. To perform quantum experiments, individual atoms need to be transported into exactly the right position.

“We do this using laser beams that serve as conveyor belts of light, so to speak,” said Andrea Alberti, who led the study at the Institute of Applied Physics at the University of Bonn.

Illustration of the experimental apparatus, with the vacuum cell and the objective lens embedded within. Two of the four laser beams are drawn (not to scale). Inset: fluorescence image of two atoms. Courtesy of Stefan Brakhane/University of Bonn.

This conveyer belt of light contains countless pockets, each of which can hold a single atom. These pockets can be moved back and forth at will, allowing an atom to be transported to a specific location in space. To move the atoms in different directions, typically many of these laser conveyer belts are necessary. For this process to take place under controlled conditions, all pockets of the conveyor belt must have the same shape and depth.

“To ensure this homogeneity, the lasers must overlap with micrometer precision,” said Gautam Ramola, lead author of the study.

“It’s kind of like having to aim a laser pointer from the stands of a soccer stadium to hit a bean that’s on the kickoff spot,” Alberti said. “But that’s not all — you actually have to do it blindfolded.”

Quantum experiments need to take place in a near-perfect vacuum, where laser beams are invisible. The team therefore used the atoms themselves to measure the propagation of laser beams.

“To do this, we first changed the laser light in a characteristic way — we also call it elliptical polarization,” Alberti said. When the atoms are illuminated by a laser beam prepared in this way, they react, changing their state in a characteristic way. These changes can be measured with a very high precision.

“Each atom acts like a small sensor that records the intensity of the beam,” Alberti said. “By examining thousands of atoms at different locations, we can determine the location of the beam to within a few thousandths of a millimeter. Such an adjustment would normally take several weeks and you would still have no guarantee that the optimum had been reached. With our process, we only needed about one day to do this.”

The research was published in Physical Review Applied (www.doi.org/10.1103/PhysRevApplied.16.024041).