A diamond speck smaller than a hundredth of a millimeter glows red. Inside it is a crystal defect: two neighboring carbon atoms, where one is missing and a nitrogen atom sits nearby. Such a pair captures light and re-emits it, like a tiny beacon that doesn't fade at room temperature.
This speck was brought near a pathway on the chip's surface. The pathway is designed so that light travels along it in only one direction, like a one-way street. Light from the diamond beacon hit the pathway and illuminated it. By changing the point of contact, scientists observed exactly where the light runs and how widely it spreads.
This technique allows peering into light fields that are invisible to the eye, with precision down to the size of the speck itself. It's a step toward chips where single light sources and their pathways are integrated together, like houses and streets in a single neighborhood.
The nitrogen-vacancy defect in diamond is so stable that it's used as a tiny thermometer: by measuring changes in its glow, you can determine the temperature inside a living cell.