Hot gas settles on a substrate, building up a transparent film layer by layer. But instead of a perfect mirror, the surface sometimes gets covered with tiny bumps, like frost patterns on glass in severe cold. Usually, people try to remove them because they get in the way of making smooth components. Physicists from Oxford and their colleagues did the opposite: they looked inside the bumps.
It turned out that the side walls of these pyramids capture nitrogen a thousand times more actively than a flat surface. Nitrogen in diamond is not an impurity to be expelled but a raw material for creating nitrogen-vacancy centers: paired defects where a nitrogen atom and a vacancy sit together and glow under a laser. Such centers act as highly sensitive magnetic needles. In each bump, the centers align not randomly but along four directions tied to its facets. This is a rare stroke of luck: on a flat substrate, they are usually oriented chaotically.
Growth is not perfect: inside the bumps, stacking faults and edge dislocations are visible — lines where atomic layers are shifted. But these very imperfections seem to help nitrogen incorporate into the lattice and form glowing pairs. The bumps, once considered junk, turned out to be natural laboratories for assembling quantum defects. Now the task is to learn how to control their appearance and geometry to grow diamond chips with ordered quantum dots.
A nitrogen-vacancy center in diamond can be compared to a lone atom suspended in a rigid cage: it senses a magnetic field a hundred times weaker than what an ordinary compass can detect, and it glows to report the measurement.
In Neal Stephenson's novel "The Diamond Age," diamonds become the foundation of an entire civilization — from construction to computers; working with bumps brings closer the dream of quantum circuits grown right inside a crystal.