Once in a while when a diamond forms, a nitrogen atom steals the place of a carbon atom next to an empty carbon site in the crystal lattice. Researchers call such impurities nitrogen–vacancy (NV) centers and often deliberately add them to a diamond lattice for various applications, including as a component in quantum information technology or as a microscopic biological sensor. (See the article by Lilian Childress, Ronald Walsworth, and Mikhail Lukin, Physics Today, October 2014, page 38.)
The sensing ability arises from the centers’ optical behavior. An NV center’s electronic ground state is a spin triplet, and the energy difference between the sublevel with spin quantum number 0 and the degenerate −1 and +1 sublevels is temperature dependent. The splitting can be measured using optically detected magnetic resonance spectroscopy. Shining a green laser on an NV center will raise one of its two localized electrons to the...
