Entanglement complexification transition driven by a single non-Hermitian impurity

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Abstract
While non-Hermitian bulk systems and their sensitivity to boundary conditions have been extensively studied, how a non-Hermitian boundary affects the entanglement structure of Hermitian critical systems remains largely unexplored. Here we present a fully analytical framework by exactly solving a Hermitian gapless chain with a single non-Hermitian impurity acting as a non-Hermitian boundary. When the entanglement cut is placed at the impurity, we uncover a sharp \emph{entanglement complexification transition}: the logarithmic entanglement entropy retains its scaling form, but the effective central charge evolves from real to complex values, accompanied by a spectral collapse of the correlation matrix. We demonstrate that the real regime follows analytic continuation from a unitary defect conformal field theory (CFT), whereas the complex regime lies entirely beyond this framework. For the latter, we derive an analytical formula in perfect agreement with numerics. Our results reveal that a single non-Hermitian impurity can drive a Hermitian critical system into a nonunitary defect-CFT phase, establishing a rare analytically solvable platform for boundary non-Hermiticity.
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