https://www.scientificamerican.com/article/what-are-the-limits-of-manipulating-nature/

Matt Trusheim flips a switch in the darkened laboratory, and an intense green laser illuminates a tiny diamond locked in place beneath a microscope objective. On a computer screen an image appears, a fuzzy green cloud studded with brighter green dots. The glowing dots are color centers in the diamond—tiny defects where two carbon atoms have been replaced by a single atom of tin, shifting the light passing through from one shade of green to another.

Later, that diamond will be chilled to the temperature of liquid helium. By controlling the crystal structure of the diamond on an atom-by-atom level, bringing it to within a few degrees of absolute zero and applying a magnetic field, researchers at the Quantum Photonics Laboratory run by physicist Dirk Englund at the Massachusetts Institute of Technology think they can select the quantum-mechanical properties of photons and electrons with such precision that they can transmit unbreakable secret codes.