Non-defective degeneracy in non-Hermitian bipartite system

Abstract: Starting from a Hermitian operator with two distinct eigenvalues, we construct a non-Hermitian bipartite system in Gaussian orthogonal ensemble according to random matrix theory, where we introduce the off-diagonal fluctuations through random eigenkets and realizing the bipartite configuration consisting of two $D\times D$ subsystems (with $D$ the Hilbert space dimension). As required by the global thermalization (chaos), one of the two subsystems is full ranked, while the other is rank deficient. For the latter subsystem, there is a block with non-defective degeneracies containing the non-linear symmetries, as well as the accumulation effect of the linear map in adjacent eigenvectors. The maximally mixed state made by the eigenvectors of this special region exhibit not thermal ensmeble behavior (neither canonical or Gibbs), and exhibit similar character with the corresponding reduced density, which can be verified through the Loschmitch echo and variance of the imaginary spectrum. This non-defective degeneracy region partly meets the Lemma in 10.1103/PhysRevLett.122.220603 and theorem in 10.1103/PhysRevLett.120.150603. The coexistence of strong entanglement and initial state fidelity in this region make it possible to achieve a maximally mixed density which, however, not be a thermal canonical ensemble (with complete insensitivity to the environmental energy or temperature). Outside this region, the collection of eigenstates (reduced density) always exhibit restriction on the corresponding Hilbert space dimension, and thus suppress the thermaliation. There are abundant physics for those densities in Hermitian and non-Hermitian bases, where we investigate seperately in this work.