A Tractor Beam from Matter Waves
It was a surprise a few years ago that a steady light beam or sound wave hitting a small object could exert a pulling force on it. Now a team of theorists has analyzed such a “tractor beam” made from another kind of wave—the quantum-mechanical matter wave of a beam of particles such as electrons. Andrey Novitsky of the Technical University of Denmark and his colleagues found the conditions under which a matter wave would produce a pulling force.
A tractor beam works when the waves receive a boost in forward momentum as they interact with the object. This beam acceleration results in the object receiving an opposite recoil momentum that pushes it back toward the tractor beam’s source. Often the beam is a so-called Bessel beam, whose rays spread out like a cone. Novitsky and his colleagues analyzed the effects of parameters such as the cone’s apex angle, the beam energy, and the details of the electromagnetic interaction force between the object and the beam.
The team found that there is a wide range of parameters for which a matter wave forms a tractor beam, but the beam-object interaction matters—a Coulomb field cannot lead to a pulling force, whereas a Yukawa field can. The matter-wave case is surprisingly similar to classical tractor beams, the authors note, despite the fact that matter waves have a probabilistic interpretation and are not a physical wave in the classical sense.
This research is published in Physical Review Letters.
David Ehrenstein is the Focus editor for Physics.