Physicists make laser beams visible in vacuum

Credit: CC0 Public Domain
The University of Bonn has developed a new method that simplifies the ultra-precise adjustment of quantum optics experiments.

Only a beam of light can be seen if it hits matter particles, scattering or reflecting back. However, in a vacuum it is invisible. Researchers at the University of Bonn now have a method to visualize laser beams even in these conditions. This makes it possible to achieve the precise laser alignment needed to manipulate individual atoms. The method has been published in the journal Physical Review Applied.

Individual atoms can exhibit strange behavior when they interact with one another. This is called quantum behavior. This can be used to create quantum computers that can solve problems that traditional computers cannot. However, individual atoms must be moved into the correct position for such experiments. Dr. Andrea Alberti, who was the lead of the study at University of Bonn's Institute of Applied Physics, explained that laser beams are used to move individual atoms.

This conveyor belt of light has many pockets that can each hold one atom. The pockets can be moved around at will to allow an atom to travel to a particular place in space. These conveyor belts are necessary to transport the atoms in multiple directions. If more atoms are being transported to the same place, they may interact with one another. This process must take place in controlled conditions. All pockets on the conveyor belt should have the same shape as the others. Gautam Ramola (lead author of the study) says that lasers should overlap with micrometer precision to ensure homogeneity.

A bean in a soccer field

This task is not as easy as it seems. It requires precision, for one. Alberti clarifies that it's similar to aiming a laser pointer at a soccer field to hit a bean on the kickoff spot. But that's not all. You also have to do this blindfolded. Quantum experiments are conducted in an almost perfect vacuum where laser beams cannot be seen.

Bonn researchers used the atoms to measure the propagation laser beams. Alberti says, "To do so, we first changed laser light in a distinctive waywe also call this elliptical Polarization." The atoms react in a specific way to a laser beam that has been prepared. These changes can be measured at a high level of precision.

Alberti says that each atom acts as a tiny sensor to record the intensity of the beam. "We can pinpoint the location of the beam by examining thousands upon thousands of atoms located at different locations. We can do this within a few thousandths millimeters."

The researchers were able to adjust four laser beams so they intersected exactly where they wanted. Alberti states that such an adjustment would normally take many weeks and would not guarantee that the optimal position was reached. "With our process, it took us only about one day to accomplish this."

Further information on Sorting machine for Atoms

Additional information: Gautam Rmola et. al., Ramsey Imaging of Optical Traps. Physical Review Applied (2021). Gautam Ramola et al, Ramsey Imaging of Optical Traps,(2021). DOI: 10.1103/PhysRevApplied.16.024041