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Broken rotational symmetry on the Fermi surface of a high-T$_\mathrm{c}$ superconductor

Broken fourfold rotational (C$_4$) symmetry is observed in the experimental properties of several classes of unconventional superconductors. It has been proposed that this symmetry breaking is important for superconducting pairing in these materials, but in the high superconducting transition temperature (high-T$_{\mathrm{c}}$) cuprates this broken symmetry has never been observed on the Fermi surface. We have measured a pronounced anisotropy in the angle dependence of the interlayer magnetoresistance of the underdoped high-T$_{\mathrm{c}}$) superconductor YBa$_2$Cu$_3$O$_{6.58}$, directly revealing broken C$_4$ symmetry on the Fermi surface. Moreover, we demonstrate that this Fermi surface has C$_2$ symmetry of the type produced by a uniaxial or anisotropic density-wave phase. This establishes the central role of C$_4$ symmetry breaking in the Fermi surface reconstruction of YBa$_2$Cu$_3$O$_{6+δ}$, and suggests a striking degree of universality among unconventional superconductors.