A quantum dot the size of a few thousand atoms blinks like a faulty light bulb. It flashes brightly, then almost goes dark. Previously, it was thought that in dark moments it emits not only single photons but also extra pairs — a defect for quantum communication. New measurements on individual dots showed the opposite: for some dots, the dim state has fewer extra photons.
The researchers tracked the emission of a single dot using ultrasensitive detectors. They compared how often the dot emitted two photons at once in the bright and dark states. It turned out that in the dark state, the probability of double emission drops more sharply than the probability of single emission. The parameter indicating purity improved from 0.155 to 0.120—lower is better.
The cause was found in the behavior of excitons — bound pairs of an electron and a hole. In the bright state, some excitons form biexcitons (two pairs at once), which emit two photons simultaneously. In the dark state, hot excitons fall into a trap — a self-trapped state where energy dissipates slowly and without extra photons. Thus, the trap acts as a filter: it lets single photons through but blocks double ones.
This mechanism does not require cooling to ultra-low temperatures or complex tricks. The perovskite crystal lattice itself helps purify the light. If we can learn to keep the dot in such a dark but clean state, quantum communications will become simpler and cheaper.
Perovskite quantum dots can be synthesized in ordinary glassware at room temperature — unlike many other materials for quantum optics.
In the series 'The Expanse', quantum communication works through entangled photons — the cleaner the source of single photons, the more reliable the channel.