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Today we have two families of high-transition temperature (Tc) superconductors, based respectively on compounds in which copper and iron atoms occupy a layered square lattice. An open question is how the quantum mechanics of electrons moving cooperatively on such lattices leads to high-Tc superconductivity. Both families display antiferromagnetism as their chemical compositions are varied (see the figure). It is the interplay between the magnetic and electronic properties that is thought to be controlled by intricate quantum entanglement among the electrons, and to be at the origin of the superconducting properties. The antiferromagnetism is strongest at compositions at which Tc is either zero or small. As the composition is varied and the antiferromagnetism decreases, a critical composition is reached at which the antiferromagnetism vanishes at zero temperature—an example of a quantum phase transition. On page 1554 of this issue, Hashimoto et al. (1) report observations of an especially well-characterized example of such a quantum critical point in a high-Tc superconductor, crystals of BaFe2(As1-xPx)2 with minimal chemical disorder. A novel feature of their experiments is that the signature of a magnetic critical point is observed in an electrical property: The antiferromagnetic quantum critical point leads to a change in the ability of the electrons to carry a super-current. The results demonstrate the close connection between antiferromagnetism and high-Tc superconductivity.