An ultra-thin crystal made of tungsten and sulfur atoms resembles a perfect parking lot: every spot is taken, everything is neat. But sometimes an atom is missing — leaving a defect pit. If an exciton (a pair consisting of an electron and a 'hole' that carries light energy) falls into such a pit, it can get stuck and emit light differently than in a pristine area.

Until now, this trapping was only inferred from indirect signs: the signal from excitons at defects was too weak. Now, scientists took a crystal with a large number of such pits and shone ultra-short laser pulses on it. It turned out that excitons fall into the pits in 300 femtoseconds, as if a car found a free parking spot in a billionth of a second. Moreover, excitons at defects have shorter lifetimes than in the pristine crystal, so some of them manage to 'jump' back. Furthermore, if the laser has lower energy, excitons at defects can receive an extra kick and escape, like a car driving out of a pit onto a smooth road.

Such 'pits' in two-dimensional crystals are already used in prototypes of quantum computers and ultra-sensitive sensors. Understanding how excitons enter and exit these pits will help make such devices faster and more reliable.