When diamond is bombarded with nitrogen ions accelerated to a few thousand electron volts, its formerly pristine crystal lattice is imbued with point-like defects. Most prized and intriguing of those defects is the negatively charged nitrogen–vacancy (NV) center, shown in figure 1, which consists of a nitrogen atom adjacent to a vacant lattice site.

The unpaired electrons in the dangling bonds surrounding the vacancy together behave like a spin-1 atom, with a trio of electron-spin quantum states that can be externally manipulated. Shielded from their surroundings by the diamond lattice, NV centers have a long spin coherence time that makes them appealing as building blocks for a quantum computer. And because the spin states shift in energy in response to an external magnetic field, the defects also serve as tiny magnetometers that can pick up magnetic signals in single living cells or picoliter-sized samples. (See Physics Today, May...

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