Unitarity corridors to exceptional points

Abstract

Non-Hermitian quantum one-parametric $N$ by $N$ matrix Hamiltonians $H^{(N)}(\lambda)$ with real spectra are considered. Their special choice $H^{(N)}(\lambda)=J^{(N)}+\lambda\,V^{(N)}(\lambda)$ is studied at small $\lambda$, with a general $N^2-$parametric real-matrix perturbation $\lambda\,V^{(N)}(\lambda)$, and with the exceptional-point-related "unperturbed" Jordan-block Hamiltonian $J^{(N)}$. A "stability corridor" ${\cal S}$ of the parameters $\lambda$ is then sought guaranteeing the reality of spectrum and realizing a unitary-system-evolution access to the exceptional-point boundary of stability. The corridors are then shown $N-$dependent and "narrow", corresponding to certain specific, unitarity-compatible perturbations with "admissible" matrix elements $V^{(N)}_{j+k,j}(\lambda) ={\cal O}(\lambda^{(k-1)/2})\,$ at subscripts $k=1,2,\ldots,N-1\,$ and at all $j$.