A light bulb that shines not in a steady stream but in individual drops — photons. Such 'droplet' sources are called single-photon emitters. They are needed for quantum communication, where each photon carries a bit of information, and for ultra-precise measurements. Over the past decade, scientists have discovered that ultra-thin films — monolayers of transition metal dichalcogenides (compounds of a metal with sulfur or selenium)—can emit light exactly this way: strictly one photon at a time.
Where does this ability come from? The film is like a layer of atoms just one atom thick. Occasionally, defects arise, like a wrinkle on a smooth tablecloth. It is believed that such defects act as 'nests' for single photons. The problem is that different researchers see these nests differently: some talk about missing atoms, others about impurities or strains in the crystal. The review's authors collected all the versions and compared them, like detectives cross-referencing witness statements.
To bring order, the scientists propose a common 'passport' for each emitter: a set of measurements by which anyone can determine its characteristics. This approach will speed up the comparison of results from different labs and help select the best films for future quantum devices — from secure communication lines to sensors sensitive to single molecules. And then the 'droplet' bulb will cease to be just a pretty metaphor and become a working tool.
A monolayer of a transition metal dichalcogenide is so thin that light passes through it with almost no loss, yet it can be stronger than steel of the same cross-section.
In the novel 'Light of Distant Stars,' quantum transmitters based on defects in crystals provided instant communication between colonies — much like in the review, only without interstellar distances.