An X-ray photon hits the detector like a pebble dropping into still water. The water is a thin film of aluminum and manganese, cooled to near absolute zero. The impact sends a heat wave rippling across the film, and by measuring its height, you can tell how much energy the photon carried.

This is how transition-edge spectroscopy works — a technique already used to peer at distant galaxies and hunt for dark matter. But until now, such detectors for X-rays were made from two layers of different metals, which is complicated and expensive. The authors of the paper used an alloy of aluminum and manganese instead: it's easier to deposit onto a substrate, and the superconducting transition temperature is easy to tune.

Tests in a cryostat with a superconducting quantum interference device amplifier showed that the detector can distinguish two X-ray quanta if their energies differ by at least 12.1 electronvolts. At an energy of 17.48 kiloelectronvolts, that's a relative precision of 0.07 percent. No detector made from this alloy has ever achieved such performance before.

A precise X-ray spectrometer is like a very keen ear that hears not the volume but the pitch of each individual note. With it, you can determine what elements make up a substance without destroying the sample. Such instruments will be useful in materials science and astronomy alike, when you need to figure out what a distant star is shining with.