Picture a layered cake made of flakes of hexagonal boron nitride, a material that resembles graphite but is built from boron and nitrogen atoms. You place one flake on top of another, and where they overlap, after annealing in oxygen, bright single-photon sources appear. There are up to fifteen times more of them than in ordinary areas, and their quality does not get worse.
Previously, such sources were produced by irradiation or plasma, which required sophisticated equipment and often yielded unstable defects. Now all you need is to align two flakes under a microscope and heat them at 1000 °C in a flow of oxygen. The top layer acts as a lid: oxygen penetrates to a depth of about 192 nanometers, but under the overlap the defects survive better.
The scientists verified that the sources emit photons strictly one at a time, do not blink, and do not fade over five minutes. Brightness reached 12 million counts per second, while the emission line remained narrow, from 1.24 to 1.78 nanometers. This simple trick makes it possible to create arrays of single-photon sources for quantum communication and photonic chips without vacuum or expensive equipment. The layered cake, in which the filling of defects is preserved under the top layer, turned out to be a workable recipe.
The overlap region of two hBN flakes just tens of nanometers thick acts as a natural 'cap' that protects the defects from burning out during oxygen annealing. An estimate of the oxygen diffusion length gave 192 nm, which exceeds the thickness of the flakes, yet the defects under the overlap still survive better.