A tiny defect in a boron nitride crystal behaves like a microscopic indicator bulb. Shine light on it, and it glows, with the shade shifting with temperature: the warmer it gets, the redder the glow. At the same time, if you bathe the defect in microwaves, its brightness flickers in sync with an external magnetic field. The reason lies in the electron’s spin — the quantum equivalent of a spinning top — that flips its rotation under magnetic influence. So this single pinpoint independently reports heat (via color) and magnetic field (via pulsations). The method harks back to the work of Isidor Isaac Rabi on magnetic resonance, while modern lasers, co-invented by Charles Townes, have made it possible to study individual defects.

It turns out these sensors are so small you can mount them on the tip of an optical fiber — yielding a hair-thin probe capable of peering deep inside a live microchip or even a living cell. Unlike similar devices based on carbon (diamond), boron nitride is cheap and easy to process. This technology paves the way for affordable multifunctional instruments that blend spectroscopy and photometry with magnetism analysis.