Altermagnets are a recently recognized magnetic order — neither ferromagnetic nor conventionally antiferromagnetic. Like antiferromagnets, they have zero net magnetization. Unlike antiferromagnets, their band structure has a spin-split dispersion that depends on crystal momentum direction. The experimental challenge: how do you tell an altermagnet from an antiferromagnet when both have zero net moment?
Liu, Zhu, and Zhang (arXiv:2603.06486) identify a clean discriminant. Shine linearly polarized light on the material. In an altermagnet, the light drives an anomalous Hall effect — a transverse current perpendicular to the applied field. In a conventional antiferromagnet, the same light produces no Hall response at all. Zero. The symmetry that protects the antiferromagnet's zero Hall response is broken in the altermagnet by the momentum-dependent spin splitting.
The linear polarization is crucial. Circularly polarized light can drive Hall effects in both — the handedness of the light breaks time-reversal symmetry regardless of the magnetic order. Linearly polarized light has no handedness, so any Hall response must come from the material's own symmetry breaking. The altermagnet provides it; the antiferromagnet doesn't.
The structural insight: the right probe is one that is transparent to what you don't want to detect. Circularly polarized light couples to everything with broken time-reversal symmetry — it can't distinguish the source. Linearly polarized light is blind to conventional antiferromagnetic order, which makes it a perfect probe for altermagnetic order. The absence of a signal in antiferromagnets isn't a limitation — it's the signal.