Physicists Observe a Hidden 'Curveball' in Laser Light for the First Time

A spinning table tennis ball curves through the air because of the Magnus effect, the same physics that bends the flight of a soccer ball. Now, an international team working at the Paul Scherrer Institute (PSI) in Switzerland has observed a related effect at the atomic scale, focusing laser light on a single ion and measuring how the light interacts with it. The results were published in the journal Physical Review Letters.

Instead of causing an atom to move along a curved path, the "optical Magnus effect" shifts the point where a laser interacts most strongly with an ion slightly sideways from the center of the beam. When laser light is focused very tightly, the structure of its electromagnetic field becomes more complex, and the strongest interaction ends up displaced to one side rather than sitting exactly at the beam's brightest point.

To detect the effect, the researchers used a single calcium ion, held nearly motionless in an ion trap, as an extremely sensitive probe. They moved the ion through different parts of a tightly focused laser beam and measured how strongly it interacted with the light at each position. "Our ion acts like a tiny sensor that we can use to feel out the structure of the laser light," said first author Philip Leindecker of the PSI Center for Photon Science and ETH Zurich's Department of Physics. "This makes it possible to measure a shift of just a few hundred nanometers." The team also found that the size of the sideways shift depends only on the wavelength of the light, not on how tightly the beam is focused.

The effect, predicted theoretically years ago by researchers at the University of Amsterdam, could matter for quantum computers that use lasers to control qubits with extreme precision — an unaccounted-for shift could introduce errors, but Leindecker said the forces it generates "could be used to couple qubits to one another, enabling more complex computations."