Strange behavior in focused laser light could change how quantum computers control qubits A laser’s strongest interaction with a trapped ion can shift sideways, revealing a quantum effect with implications for qubit control. Paul Scherrer Institute Writer: Christian Heid - Physicists have directly observed the optical Magnus effect for the first time by mapping how a tightly focused laser interacts with a single trapped calcium ion. - The strongest atom-light interaction shifted sideways by several hundred nanometers instead of occurring exactly at the laser beam’s center. - The effect could create unwanted errors in laser-controlled qubits, but the same forces may also provide a new way to connect qubits during quantum computations. A spinning table tennis ball can veer sharply across a table even when its forward motion seems to point elsewhere. The same basic phenomenon helps bend football shots and curve baseballs. Physicists call it the Magnus effect, a sideways force produced when a rotating object moves through a surrounding medium. Now an international team has observed an optical counterpart at the scale of a single trapped ion. Instead of watching an atom curve through space, the researchers found that a tightly focused laser interacts most strongly with the ion slightly away from the beam's center. The first direct measurement of this optical Magnus effect appears in Physical Review Letters. Philip Leindecker of the Paul Scherrer Institute's Center for Photon Science and ETH Zurich led the work with collaborators from PSI, ETH Zurich, the University of Amsterdam and other institutions. The finding matters because tightly focused lasers are increasingly used to manipulate individual quantum bits, or qubits. A shift of only a few hundred nanometers can change how precisely those laser beams control trapped particles. The same effect, however, could also provide a useful force for linking qubits during