New method to make laser beams visible in vacuum

The method developed simplifies ultra-precise adjustment for quantum optics experiments.


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While traveling through the vacuum, laser beams are invisible unless shot directly into your eye. But, now, physicists make laser beams visible in a vacuum by developing a method that makes it easier to perform the ultra-precise laser alignment required to manipulate individual atoms.

During the interaction of atoms, atoms often exhibit unusual behavior due to their quantum behavior. The effect can be used to develop quantum computers, solving specific problems that conventional computers struggle with.

However, such experiments need maneuvering individual atoms into exactly the right position. This task is performed by using laser beams that serve as conveyor belts of light.

Such a conveyor belt of light contains countless pockets, each of which can hold a single atom. It is possible to move those pockets back and forth- allowing an atom to be transported to a specific location in space.

Illustration of the experimental apparatus
Illustration of the experimental apparatus – , with in the center 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. © Stefan Brakhane / University of Bonn

Gautam Ramola, the study’s lead author, said, “If you want to move the atoms in different directions, you usually need many of these conveyor belts. When more atoms are transported to the same location, they can interact with each other. 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.”

Dr. Andrea Alberti, who led the study at the Institute of Applied Physics at the University of Bonn, said, “This task is less trivial than it sounds. For one thing, it requires great accuracy. 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.”

“But that’s not all—you have to do it blindfolded. This is because quantum experiments occur in an almost perfect vacuum, where the laser beams are invisible.”

In this study, physicists used atoms to measure the propagation of laser beams. They did this by changing the laser light in a specific way called elliptical polarization. Illumination of atoms by laser beam changes their state in a significant way. It is possible to measure those changes with high precision.

Alberti said, “Each atom acts like a small sensor that records the intensity of the beam. By examining thousands of atoms at different locations, we can determine the location of the beam to within a few thousandths of a millimeter.”

“In this way, the researchers succeeded, for example, in adjusting four laser beams so that they intersected at exactly the desired position. 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.”

Journal Reference:
  1. Gautam Ramola et al., Ramsey Imaging of Optical Traps, Physical Review Applied (2021). DOI: 10.1103/PhysRevApplied.16.024041


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