Advances in topological photonics and non-Hermitian optics have drastically changed our perception on how interdisciplinary concepts may empower unprecedented applications. Bridging the two areas could uncover the reciprocity between topology and non-Hermiticity in complex systems. So far, such endeavors have focused mainly on linear-optics regime. Here, we establish a nonlinear non-Hermitian topological platform for control of parity-time (PT) symmetry and topological edge states. Experimentally, we demonstrate that optical nonlinearity effectively modulates the gain and loss of a topological interface waveguide in a non-Hermitian Su-Schrieffer-Heeger lattice, leading to switching between PT and non-PT-symmetric regimes accompanied by destruction and restoration of topological zero modes. Theoretically, we examine the fundamental issue of the interplay between two antagonistic effects: the sensitivity close to exceptional points and the robustness of non-Hermitian topological modes. Realizing single-channel control of global PT-symmetry via local nonlinearity may herald new possibilities for light manipulation and unconventional device applications.
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