Detection of an unbroken phase of a non-Hermitian system via a Hermitian factorization surface

Abstract

In the traditional quantum theory, one-dimensional quantum spin models possess a factorization surface where the ground states are fully separable having vanishing bipartite as well as multipartite entanglement. We report that in the non-Hermitian counterpart of these models, these factorization surfaces either can predict the exceptional points where the unbroken-to-broken transition occurs or can guarantee the reality of the spectrum, thereby proposing a procedure to reveal the unbroken phase. We first analytically demonstrate it for the nearest-neighbour rotation-time RT-symmetric XY model with uniform and alternating transverse magnetic fields, referred to as the iATXY model. Exact diagonalization techniques are then employed to establish this fact for the RT-symmetric XYZ model with short- and long-range interactions as well as for the long-ranged iATXY model. Moreover, we show that although the factorization surface prescribes the unbroken phase of the non-Hermitian model, the bipartite nearest-neighbour entanglement at the exceptional point is nonvanishing.