Laser beams are normally invisible when they pass through a vacuum, but physicists at the University of Bonn, Germany, have found a way to make them reveal themselves. This feat, which they accomplished using a technique called Ramsey imaging, should make it easier to align lasers with the precision needed to trap and manipulate individual atoms – a crucial step for atom-based quantum computing and other quantum technologies.
Optical traps use highly focused (often criss-crossed) laser beams to generate one or more dips, or “pockets”, in potential energy where individual particles can be held in place. Experimenters can move these pockets back and forth at will, thereby transporting the particles to specific locations in space.
As the number of particles in the same location increases, they start to interact with each other. “To control this process, all the pockets must have the same shape and depth,” explains Gautam Ramola, a PhD student at Bonn and the lead author of a study on the new technique. For that to happen, he adds, the trapping laser beams must overlap with micrometre precision.
Highly homogenous optical traps are especially important for atom-based technologies such as optical lattice clocks, trapped atom interferometers, quantum computing and quantum simulators. However, because these technologies operate under vacuum to preserve the atoms’ delicate quantum states, few other particles are present to scatter or reflect the laser light and thus reveal information about the beams’ intensity profile.
Ramsey phase tracking
Ramola, team leader Andrea Alberti and colleagues overcame this problem by using the atoms themselves to detect how the beams propagated. This technique, dubbed Ramsey phase tracking, works by probing the atoms’ hyperfine splitting – that is, the shift in an atom’s energy levels that occurs due to interactions between the magnetic moment of its nucleus and the orbital motion of its electrons. The Ramsey signal measures how this hyperfine splitting changes in the presence of elliptically polarized laser beams.
“Each atom effectively acts as a small sensor that records the intensity of the beam,” Alberti explains. “By examining thousands of atoms at different locations, we can determine the location of the beam to within a thousandth of a millimetre.”
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The technique, which the researchers describe in Physical Review Applied, allowed the team to adjust four laser beams so that they interacted at exactly the position required. “Such a manoeuvre would normally take several weeks using conventional techniques, with no real guarantee of the final result,” Alberti says. “We only needed about one day to achieve this.”
In their work, the team made measurements on optical traps for caesium atoms, but the technique will also work with other alkali atoms, as well as atoms from certain other groups in the periodic table, such as the magnetic lanthanides. It could also be applied to a range of optical trap geometries, including “flat” and “hollow” traps.
