  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.
