An X-ray beam strikes a piece of superconductor, and the material warms up ever so slightly. That heating, by billionths of a degree, is the only trace the light leaves behind. The new detector catches exactly that, like a sensitive thermometer pressed against cold metal.

Inside the sensor is a superconducting film poised right at the edge of losing its superconductivity. The slightest heat alters its magnetic properties, and that change is amplified without extra noise. The design resembles a tuning fork: a gentle pluck produces a clean, sustained tone that is easy to measure.

The prototype already works across a wide range of temperatures and withstands repeated cycles. Calculations show that, when optimally assembled, the sensor can resolve energies to within 100 millielectronvolts. For soft X-rays, that is like telling a whisper from a sigh in a noisy room.

Such a device will be useful in spectroscopy — the science of how substances absorb and emit light. Chemists will see bonds between atoms, and astronomers will see the composition of distant nebulae. The new detector's precision approaches that of the best crystal spectrometers, yet it is far more compact and easier to handle.