Interference of Non-Hermiticity with Hermiticity at Exceptional Points

Miloslav Znojil ()
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Miloslav Znojil: Department of Physics, Faculty of Science, University of Hradec Králové, Rokitanského 62, 500 03 Hradec Králové, Czech Republic

Mathematics, 2022, vol. 10, issue 20, 1-23

Abstract: The recent growth in popularity of the non-Hermitian quantum Hamiltonians H ( λ ) with real spectra is strongly motivated by the phenomenologically innovative possibility of an access to the non-Hermitian degeneracies called exceptional points (EPs). What is actually presented in the present paper is a perturbation-theory-based demonstration of a fine-tuned nature of this access. This result is complemented by a toy-model-based analysis of the related details of quantum dynamics in the almost degenerate regime with λ ≈ λ ( E P ) . In similar studies, naturally, one of the decisive obstacles is the highly nontrivial form of the underlying mathematics. Here, many of these obstacles are circumvented via several drastic simplifications of our toy models—i.a., our N by N matrices H ( λ ) = H ( N ) ( λ ) are assumed real, tridiagonal and PT -symmetric, and our H ( N ) ( λ ) is assumed to be split into its Hermitian and non-Hermitian components staying in interaction. This is shown to lead to several remarkable spectral features of the model. Up to N = 8 , their description is even shown tractable non-numerically. In particular, it is shown that under generic perturbation, the “unfolding” removal of the spontaneous breakdown of PT -symmetry proceeds via intervals of λ with complex energy spectra.

Keywords: non-Hermitian quantum mechanics of the closed and open systems; non-Hermitian and Hermitian components of the Hamiltonian; control of access to the exceptional point degeneracies and to the related quantum phase transitions; perturbation theory tractability of the matrix toy models exhibiting PT symmetry (search for similar items in EconPapers)
JEL-codes: C (search for similar items in EconPapers)
Date: 2022
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