A Single Trapped Calcium Ion Showed a Laser Beam's Grip Is Strongest a Few Hundred Nanometers Off Center. Physicists Are Calling It the Optical Magnus Effect.

A spinning table tennis ball curves because the air on one side moves faster than the air on the other. Physicists in Switzerland have now found that a tightly focused beam of light does something similar to a single atom. The point where the beam pushes hardest on the atom is not the center of the beam, where the light is brightest, but a spot a few hundred nanometers to one side. They call it the optical Magnus effect, and they measured it with an instrument that is about as simple as a quantum experiment gets: one calcium ion, held in place by electric fields, used as a probe.

The work was carried out at the Paul Scherrer Institute and ETH Zurich with collaborators at the University of Amsterdam, led by first author Philip Leindecker of PSI's Center for Photon Science and ETH's physics department. It appeared in Physical Review Letters on August 6 and was highlighted this weekend. "Our ion acts like a tiny sensor that we can use to feel out the structure of the laser light," Leindecker said.

The method was direct. The team trapped a single calcium ion in an electromagnetic field, aimed a focused laser at it, and then moved the ion step by step through the beam, measuring at each position how strongly the light and the ion interacted. The expectation from textbook optics is that the coupling should peak at the beam's axis and fall off symmetrically on either side. Instead the peak was displaced sideways by several hundred nanometers. The offset did not change when the researchers focused the beam more or less tightly. It depended only on the wavelength of the light.

That last detail is what rules out an alignment error or an artifact of the trap, and what earns the effect its name. Light carries angular momentum, and near the focus of a tightly concentrated beam the field takes on a structure that is not symmetric about the axis. The ion feels that asymmetry as a sideways displacement of the interaction, much as a spinning ball feels an asymmetric pressure field and curves. Because the asymmetry is built into the wave itself, its scale is set by the wavelength, which is why tighter focusing does not change it.

For most of optics this is a curiosity. For the people building quantum computers out of trapped ions, it is not. Those machines control each qubit with focused laser beams, and the fidelity of every gate depends on knowing exactly how the light acts on the ion. A coupling that peaks off-axis by a few hundred nanometers, in systems where ions are spaced by a few micrometers, is large enough to introduce errors if it is not accounted for. It is also, potentially, a tool. "The forces it generates could be used to couple qubits to one another, enabling more complex computations," Leindecker said.

The measurement also demonstrates something about the probe. A single trapped ion turns out to be a sensor fine enough to map the internal structure of a light field at a resolution well below the wavelength of the light itself, which is beyond what any conventional detector can do. The same technique could be used to characterize the exotic beams, with orbital angular momentum or engineered polarization, that are increasingly used in quantum sensing and communication.