Entanglement and magic transitions in an all-to-all non-Hermitian spin model

Abstract: The long-time state of a non-Hermitian system is determined by the eigenvalue with the largest imaginary part. In interacting many-body systems this eigenvalue usually cannot be tracked analytically, and the character of the state it selects is unknown. We construct a non-Hermitian spin ensemble of $L$ spins with exactly $k$-local all-to-all interactions, in which this dominant eigenvalue can be tracked analytically from the clean limit into the disordered regime. Disorder produces a competition between an isolated spectral outlier and the edge of a many-body spectral bulk. We study three cases: purely anti-Hermitian disorder, purely Hermitian disorder, and mixed disorder of equal strength. For purely anti-Hermitian and mixed disorder, we show that when the bulk overtakes the outlier in imaginary part, the dominant eigenstate switches from an outlier state with low entanglement and magic (nonstabilizerness) to a bulk state with substantially larger entanglement and magic, with both changing at the same threshold. For $k \gg \sqrt{L}$ the bulk has a sharp spectral edge and the transition thresholds follow in closed form, while for $k \ll \sqrt{L}$ spectral tails broaden the transition into a crossover. Purely Hermitian disorder provides a contrasting case with no outlier-to-bulk switching. Finally, we map the non-Hermitian evolution exactly onto postselected trajectories of a monitored quantum system, connecting this spectral mechanism to measurement-induced transitions.