In a first, scientists spot a table-tennis-like force hiding inside a focused laser beam

A spinning table-tennis ball can suddenly bend away from its expected path. This is called the Magnus effect. Surprisingly, now, physicists have found an optical version of the similar effect hiding inside a tightly focused laser beam—except the ball is a single ion, and the sideways shift is only a few hundred nanometers. In a one-of-a-kind experiment, a team of researchers has observed the optical Magnus effect in a single trapped ion for the first time, showing that the point where light interacts most strongly with an atom can sit away from the beam’s apparent center. “We directly observe and spatially map an optical analog of the Magnus effect,” the study authors note. The finding could matter for quantum computers, where lasers are used to manipulate qubits and even tiny changes in their interactions can affect the accuracy of quantum operations. Turning an ion into a light sensor The puzzle starts with an assumption that seems obvious. If a laser is brightest at its center, that should also be where it interacts most strongly with an atom. However, tightly focusing light changes its electromagnetic structure. Components of the field that are normally negligible become important, producing polarization gradients and a longitudinal electric field. The possibility of such a sideways optical force had already been predicted theoretically. In a 2020 study, physicist Robert Spreeuw proposed that light could produce off-axis forces resembling the Magnus effect. But directly mapping the effect at the atomic scale remained difficult. The new experiment used a single calcium-40 ion held almost motionless inside an electromagnetic ion trap. The researchers directed a tightly focused 729-nanometer laser at the ion and moved the beam across it with extremely fine control. Similar single-ion techniques for mapping electromagnetic fields are being explored to understand the tiny disturbances that can affect quantum hardware. The ion effectively became a microscopic probe. By measuring how strongly the laser drove a transition between the ion’s quantum states at different positions, the researchers could work out where the interaction was strongest. “Our ion acts like a tiny sensor that we can use to feel out the structure of the laser light. This makes it possible to measure a shift of just a few hundred nanometers,” Philip Leindecker, lead researcher and a student at ETH Zurich, said. The hidden offset comes into focus The resulting maps revealed that the strongest atom-light interaction was displaced sideways from the beam’s center in a way that depended on the ion’s internal quantum state—a signature of the optical Magnus effect. For two different transitions, the researchers measured displacements of about 240 ± 16 nanometers and 463 ± 20 nanometers, closely matching their theoretical predictions of roughly 232 and 464 nanometers. The team also used phase-sensitive measurements to characterize the transverse polarization gradients created by the tightly focused beam. The effect is closely related to the way a spinning ball experiences a sideways force, but there is an important difference. Here, nothing comparable to a ball is physically curving through space. Instead, the structure of the light itself shifts where the atom-light interaction is strongest. This matters because the same light fields used to manipulate trapped-ion qubits can also couple a qubit’s internal state to its motion. If these spatial shifts and polarization gradients are not accounted for, they can potentially introduce errors during quantum operations. The researchers’ measurements therefore provide a way to understand and account for an effect that was previously difficult to see directly. A problem that could become a tool The discovery is not simply about finding a microscopic version of a familiar sporting effect. It could help researchers design more precise optical control schemes for quantum technologies. A 2023 theoretical study proposed using the Magnus effect to create interactions between trapped-ion qubits, while later work has demonstrated related entangling operations using transverse polarization gradients. This experiment used related transverse polarization gradients, rather than demonstrating the optical Magnus effect itself. Related trapped-ion quantum computing systems already rely on precisely controlled laser-based operations to manipulate qubits. “The forces it generates could be used to couple qubits to one another, enabling more complex computations,” Leindecker added. The current experiment does not demonstrate a quantum computer based on the optical Magnus effect. Instead, it establishes and measures the underlying physics, giving researchers a detailed picture of how tightly focused light interacts with a single ion. Turning the measured effect into a practical quantum-control tool will require researchers to understand and manage the remaining optical imperfections. This is particularly important as researchers work toward more stable quantum operations and larger quantum processors, where even small sources of error can become increasingly important. The study is published in the journal Physical Review Letters. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Rupendra Brahambhatt is an experienced writer, researcher, journalist, and filmmaker. With a B.Sc (Hons.) in Science and PGJMC in Mass Communications, he has been actively working with some of the most innovative brands, news agencies, digital magazines, documentary filmmakers, and nonprofits from different parts of the globe. As an author, he works with a vision to bring forward the right information and encourage a constructive mindset among the masses.