A potter tries to measure the thickness of a jug's wall, but every time the gauge also reports the curvature of the vessel. That's roughly the situation physicists face when studying quantum materials: two key geometric characteristics — the quantum metric (a measure of the 'distance' between quantum states) and the Berry curvature (a measure of their 'twist')—almost always go hand in hand. Separating them experimentally is extremely difficult.
A new study offers an elegant way out. The authors turned to d-wave altermagnets — crystals with a special magnetic order where the spins of neighboring atoms are arranged in a checkerboard pattern. In such materials, as calculations show, the Berry curvature vanishes completely, as if the potter finally picked up a perfectly straight cylinder. Only the quantum metric remains, and it can be 'probed' with light.
The idea is this: linearly polarized light with a frequency far from resonance is directed at the crystal. The light field gives the electrons a slight 'shake', shifting the energy bands. In ordinary magnets, this shift is caused by both the metric and the Berry curvature. But in a d-wave altermagnet, the curvature is zero, and the entire effect is purely metric. Moreover, due to the crystal's anisotropy (electrons hop differently between the $d_{xz}$ and $d_{yz}$ orbitals), the metric turns out to be different for the two types of electron valleys. The light's polarization 'chooses' which valley to shift more strongly, creating an energy difference, valley polarization, that can be measured.
By measuring the energy shift with spin-resolved ARPES or ultrafast optical pump-probe spectroscopy, one can directly compute the quantum metric. Thus, d-wave altermagnets become the first 'clean' platform where the metric can be separated from the Berry curvature, controlled by light polarization, and read out via valley polarization, just as the potter finally measures the wall thickness without being distracted by the vessel's curves.
The quantum metric determines the minimum possible width of energy bands in insulators, and in superconductors it is linked to the maximum transition temperature.
In Neal Stephenson's novel 'Anathem', quantum states are manipulated through the geometry of higher-dimensional spaces — somewhat akin to controlling valleys with light in altermagnets.