Illustration: Alan Stonebraker

Figure 1: (Top) In Young’s double-slit experiment light falls onto two slits and is detected on a screen placed behind the slits. An interference pattern is observed, because one cannot determine whether the light passed through the right or left slit. (Bottom) In the experiment proposed by Hu, Collins, and Schneider, x-ray photons eject the single electron from $H2+$. The yellow arrow represents the polarization axis of the x-ray beam and the blue arrow represents its propagation direction. The x-ray absorption primarily takes place when the electron is close to one of the two protons, which therefore act as slits. When the angular distribution of the emitted electrons is measured, the internuclear distance between the two protons can be determined.