A magnetic field is invisible, but you can make it 'glow'. To do that, take a tiny diamond with defects — places where a carbon atom is replaced by nitrogen and a neighboring lattice site is vacant. These defects, like sensitive compass needles, respond to the slightest changes in the magnetic field and emit light of varying brightness.

In the new study, a diamond plate was pressed against a crystal of the superconductor Hg-1223, which can operate at relatively high temperatures, and the whole assembly was squeezed under a pressure of 4 gigapascals, as if you stacked forty cars on a fingernail. Under pressure, superconductivity strengthens, and the material starts to expel the magnetic field from its interior: this is known as the Meissner effect. A camera recorded the diamond's glow, and a computer reconstructed the magnitude and direction of the field at each point in space from the brightness and color of the spots.

From these data, the scientists built a map of field expulsion at different temperatures and compared it with a model describing the behavior of type-II superconductors. The match allowed them to calculate the critical current density — the maximum current the material can carry without losing superconductivity. This is a key characteristic for any application, from magnets in MRI machines to power transmission cables.

This method requires no electrical contacts and works under high pressure, where conventional measurements are difficult. Now it's possible to study superconductors under conditions approaching those found in planetary interiors and to search for materials that retain their remarkable properties at room temperature.