Researchers at the Paul Scherrer Institute PSI and ETH Zurich have, for the first time, directly observed the optical Magnus effect, a phenomenon mirroring the way spin alters the trajectory of a ball in classical mechanics. The team directed a tightly focused laser beam at a single ion, revealing that the point of maximum interaction shifted sideways, a finding with implications for controlling qubits in quantum computing.

“The forces it generates could be used to couple qubits to one another, enabling more complex computations,” explains first author Philip Leindecker from PSI and ETH Zurich. The researchers report their findings in Physical Review Letters, detailing how accounting for this effect is crucial for precise qubit control.

Optical Magnus Effect Demonstrated with Trapped Calcium Ions

This unexpected displacement, analogous to the spin imparted on a table tennis ball, has implications for the precision control of qubits in emerging quantum computers. The team utilized an ion trap, employing electromagnetic fields to hold the electrically charged calcium atom nearly motionless, allowing for precise measurements of the light’s influence, ETH Zurich says. “Our ion acts like a tiny sensor that we can use to feel out the structure of the laser light,” explains first author Philip Leindecker from the PSI Center for Photon Science and the Department of Physics at ETH Zurich.

The experiment successfully measured this shift with remarkable precision, detecting changes as small as a few hundred nanometres. Importantly, the magnitude of this shift proved dependent solely on the wavelength of the light, not the laser’s focusing intensity. Because laser light is used to manipulate qubits, accounting for the optical Magnus effect is crucial; unaddressed, the effect could introduce errors into quantum computations.

Our ion acts like a tiny sensor that we can use to feel out the structure of the laser light.
Philip Leindecker, PSI Center for Photon Science and the Department of Physics at ETH Zurich

Laser Focus Shift Impacts Qubit Control

The precision required for manipulating qubits hinges on a detailed understanding of laser-ion interactions, and recent work reveals a subtle but significant shift in that dynamic. This unexpected displacement arises from the altered electromagnetic field created when a laser beam is tightly focused. Rather than interacting strongest at the beam’s center, the ion experiences maximum force slightly to the side, a result that could introduce errors if not accounted for in qubit control systems.

Laser light is routinely used to selectively change the state of qubits, and this effect could interfere with that precise manipulation. The findings, detailed in Physical Review Letters, underscore the need to incorporate the optical Magnus effect into models of qubit control.

The forces it generates could be used to couple qubits to one another, enabling more complex computations.

This wavelength dependency distinguishes the observed effect and has implications for quantum computing, where laser light manipulates qubits. The University of Amsterdam had previously predicted the optical Magnus effect theoretically, but this work provides the first experimental confirmation and detailed characterization.