Drop two pebbles into a pond: each sends out ripples, and on the water you'll see crests where waves reinforce each other and troughs where they cancel. Protons behave much the same way: when there are two, they create a double ripple that adds up into a recognizable pattern.
In the new experiment, a beam of hydrogen ions, single protons and pairs bound into molecules, was directed at a cloud of helium. During a collision, an ion could snatch an electron from helium and deflect. The physicists measured the angles at which these 'renewed' particles flew off and saw that for a two-proton molecule, the picture resembles light passing through two narrow slits. The bright and dark stripes are interference — a sign that the molecule acts as a double aperture.
To understand how the molecule captures an electron, the authors 'took' the interference pattern and extracted from it the contribution of a single proton, as if subtracting the ripple from the second pebble out of the overall picture. It turned out that when the captured electron settles into the lower energy level, the single 'slit' from a proton and the double one from the molecule produce stripes of the same width. But if the electron lands on an excited level, the patterns diverge: the molecule starts to 'shine' differently than a lone proton.
This trick, decomposing a complex pattern into simple parts, lets you peek into the mechanics of electron capture without heavy calculations. It's like guessing the shape of a hand from its shadow on the wall: the shadow is double, but if you know how the fingers fold, you can subtract one silhouette and see the other.
A molecule of two protons is the simplest 'double-slit' object in nature: the distance between the protons is only about 0.1 nanometers, yet that's enough for the wave nature of particles to show in full force.