A particle can be in two places at once – even if it is made of antimatter. Researchers have just performed an antimatter twist on a classic experiment used to show one of the foundational tenets of quantum mechanics: that all particles are also waves.

In the most basic version of the double-slit experiment , first performed in 1801, a beam of light illuminates a plate with two parallel slits cut into it. The light that passes through the slits hits a screen, creating stripes of light and darkness …