Physicists discover a hidden “curveball” in quantum light

Now, an international team working at the Paul Scherrer Institute PSI has observed a related effect at the atomic scale. For the first time, researchers have experimentally demonstrated the optical Magnus effect by focusing laser light on a single ion and measuring how the light interacts with it.

Instead of causing an atom to follow a curved path, the effect shifts the location where the laser interacts most strongly with the ion. That interaction point moves slightly sideways, a finding that could matter for quantum computers that use laser light to control qubits with extreme precision. The results were published in Physical Review Letters.

A Laser's Strongest Interaction Is Slightly Off Center

At first glance, it seems reasonable to expect that an ion would interact most strongly with a laser exactly at the beam's brightest point. But when laser light is focused very tightly, the structure of its electromagnetic field becomes more complicated.

Because of that altered field structure, the strongest interaction does not occur exactly at the center of the beam. Instead, it appears slightly to one side. This small sideways displacement is the optical equivalent of the Magnus effect that makes a spinning table tennis ball curve through the air.

That tiny shift could become important in quantum computing. Lasers are often used to change the states of qubits with very high precision. If the optical Magnus effect is ignored, it could interfere with that control and contribute to errors.

The same effect may also be useful. "The forces it generates could be used to couple qubits to one another, enabling more complex computations," explains first author Philip Leindecker from the PSI Center for Photon Science and the Department of Physics at ETH Zurich.

Using a Single Ion to Map Laser Light

To detect the effect, the researchers used a single calcium ion as an extremely sensitive probe. The electrically charged atom was held nearly motionless in an ion trap, which uses electromagnetic fields to keep the ion fixed in place.

Trapped ions are also widely used in quantum computing. They can function as qubits, with their quantum states manipulated using carefully controlled laser pulses.

In the experiment, the team moved the calcium 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," Leindecker explains. "This makes it possible to measure a shift of just a few hundred nanometers."

The measurements uncovered another surprising feature. The size of the sideways shift depends only on the wavelength of the light and not on how tightly the laser beam is focused.

Researchers at the University of Amsterdam had predicted the optical Magnus effect theoretically several years ago. By using a trapped calcium ion as a microscopic probe, the team has now observed the effect experimentally for the first time and measured its behavior in greater detail.