A new method developed at the University of Bonn simplifies ultra-precise
adjustment for quantum optics experiments.
A beam of light can only be seen when it hits matter particles and is
scattered or reflected by them. In a vacuum, however, it is invisible.
Physicists at the University of Bonn have now developed a method that allows
laser beams to be visualized even under these conditions. The method makes
it easier to perform the ultra-precise laser alignment required to
manipulate individual atoms. The researchers have now presented their method
in the journal Physical Review Applied.
When individual atoms interact with each other, they often exhibit unusual
behavior due to their quantum behavior. These effects can, for instance, be
used to construct so-called quantum computers, which can solve certain
problems that conventional computers struggle with. For such experiments,
however, it is necessary to maneuver individual atoms into exactly the right
position. "We do this using laser beams that serve as conveyor belts of
light, so to speak," explains Dr. Andrea Alberti, who led the study at the
Institute of Applied Physics at the University of Bonn.
Such a conveyor 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. 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. In order 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," explains Gautam Ramola, the study's lead author.
A bean in a soccer stadium
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," Alberti
clarifies. "But that's not all—you actually have to do it blindfolded." This
is because quantum experiments take place in an almost perfect vacuum, where
the laser beams are invisible.
The researchers in Bonn 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
explains. 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 continues. "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," Alberti says. "With our
process, we only needed about one day to do this."
Reference:
Gautam Ramola et al, Ramsey Imaging of Optical Traps, Physical Review
Applied (2021).
DOI: 10.1103/PhysRevApplied.16.024041
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Physics