Inside a diamond anvil cell, a device where two diamonds squeeze a sample to hundreds of gigapascals, pressure is distributed unevenly. Some regions are compressed more, others less. This affects experimental results: the material may behave differently than under perfectly uniform compression. Until recently, mapping pressure in the chamber was only possible at isolated points, for example by the luminescence of ruby. Now Japanese physicists have scattered tiny diamond grains with nitrogen-vacancy centers (a defect in diamond: nitrogen next to a vacancy) throughout the chamber and turned them into barometers.

Each nanodiamond, 50 nanometers in size, is a miniature sensor. Under green laser and microwave radiation, it emits red light whose spectral shape depends on how strongly and uniformly the crystal is compressed. In one configuration, the nanodiamonds lay directly on the diamond surface, and the spectra turned out asymmetric: the uniaxial compression model gave an axial pressure of 22.1 GPa and a transverse pressure of only 12.5 GPa. The anisotropy (pressure ratio) reached 1.77. In another configuration, the nanodiamonds were covered with a layer of salt, and the spectra became symmetric: the hydrostatic model showed a pressure of 17–20 GPa, consistent with the ruby calibration (20.0 GPa). The anisotropy dropped to 1.2–1.5.

This approach has for the first time provided a complete map of non-hydrostaticity in the chamber with micron resolution. Previously, nanodiamonds were used mainly as sensors for magnetic fields or temperature, and pressure was measured at individual points. Now it is possible to see how compression changes from the center to the edges: in the configuration with a single salt layer, the axial pressure varied from 14 to 30 GPa, and the pressure ratio fell from 1.6–1.8 in the center to 1.4 at the edges. This is a direct indication of how the packing of the pressure-transmitting medium (a substance that distributes compression evenly) affects hydrostaticity (equal pressure in all directions).

The method requires no complex optics and is compatible with wide-field microscopy, making it fast and convenient for routine experiments. In the future, it could simultaneously measure pressure, temperature, and magnetic field in one chamber, as well as test hydrostaticity at higher pressures, up to 100 GPa. This is important for the search for new superconductors and the study of phase transitions, where local uniformity of compression plays a key role.