The trick involves whittling down a synthetic diamond until it’s just 200 nanometers thick, etching optical cavities into its surface (to increase the brightness of light emissions) and using a special nanoscale implanter to shoot silicon ions into those cavities. You can only inject a limited amount of silicon if you do nothing else, but the team managed to create more silicon-filled defects by blasting the diamond with electron beams (which creates more cavities) and heating the diamond until the holes moved around and bonded with silicon. The more you repeat this process, the more defects you get.
The technology isn’t ready for prime time. The resulting defects aren’t quite in their ideal locations (they’re about 50 nanometers off), so they don’t emit enough light to maintain the quality researchers would like. This is much better than previous approaches, though, and hints that real, diamond-based quantum computers are within reach.