A deuterium molecule, two atoms of heavy hydrogen, falls apart under a laser pulse. One electron is ripped away, leaving an ion behind. At that moment, they are still linked: the direction of one immediately tells you which way the other went. This link is called quantum entanglement.
In the classic double-slit experiment, a particle produces fringes — interference. But if you can tell which slit it went through, the fringes disappear. Here, the two possible molecular states play the role of the slits. While the electron and ion are entangled, the path information has 'leaked' to the ion, and there are no fringes. When the experimenters selected only one type of ion, the entanglement vanished, and the fringes returned. This is called a quantum eraser: the path information is erased, and the wave nature becomes visible again.
Deuterium is the same heavy hydrogen that makes water 'heavy.' Its nucleus contains an extra neutron.
In Ted Chiang's story 'Story of Your Life,' knowing the future changes the present — here, knowing the particle's path erases its wave-like past.