An imaginary gauge transformation is at the core of the non-Hermitian skin effect. Here, we show that such a transformation can be performed in momentum space as well, which reveals that certain gain- and loss-modulated systems in their parity-time (PT) symmetric phases are equivalent to Hermitian systems with real potentials. Our analysis in momentum space also distinguishes two types of exceptional points (EPs) in the same system. Besides the conventional type that leads to a PT transition upon the continuous increase of gain and loss, we find real-valued energy bands connected at a Dirac EP in hybrid dimensions, consisting of a spatial dimension and a synthetic dimension for the gain and loss strength.
An imaginary gauge transformation is at the core of the non-Hermitian skin effect. Here, we show that such a transformation can be performed in momentum space as well, which reveals that certain gain- and loss-modulated systems in their parity-time (PT) symmetric phases are equivalent to Hermitian systems with real potentials. Our analysis in momentum space also distinguishes two types of exceptional points (EPs) in the same system. Besides the conventional type that leads to a PT transition upon the continuous increase of gain and loss, we find real-valued energy bands connected at a Dirac EP in hybrid dimensions, consisting of a spatial dimension and a synthetic dimension for the gain and loss strength.
Two types of non-Hermiticity found in physical systems, i.e., non-reciprocal couplings and complex potentials, have gained considerable interest recently. The former leads to the non-Hermitian skin effect, while the latter gives rise to a range of non-Hermitian symmetries such as parity-time (PT) and particle-hole symmetries. Although these two non-Hermitian forms have been incorporated in the same models, so far they have been treated as distinct families of non-Hermiticity. In this work, we establish an isospectral mapping between them using the Lanczos transformation. More specifically, we show that a generalized Hatano-Nelson model H with non-reciprocal and spatially varying couplings can be mapped to a tight-binding model HL with real symmetric couplings and an imaginary on-site potential. This result holds even when the original H already has an imaginary potential and exceptional points. Our work provides a refreshed understanding of the connection between different forms of non-Hermiticity. It also offers an approach to obtaining an intuitive physical understanding of exceptional points in HL that are elusive in H. Finally, our findings also warrant a versatile approach to constructing non-Hermitian systems with a complex potential and a real spectrum, without PT symmetry.
A PT-Symmetric XXZ Chain with an External Constraint Field: Exceptional Points, Finite-Size Scaling, and Falsifiable Predictions This work introduces a non-Hermitian quantum-spin model that combines three established areas of modern theoretical physics: The XXZ Heisenberg spin-½ chain; PT-symmetric gain/loss dynamics; An externally coupled constraint field capable of modifying the location of exceptional points. The resulting Hamiltonian provides a mathematically explicit framework for studying how externally driven constraints interact with open quantum systems exhibiting parity-time symmetry. Rather than proposing a speculative unified theory, the objective of this work is to formulate a concrete, experimentally testable model that produces quantitative predictions capable of confirmation or falsification through laboratory measurement. Scientific Background Parity-time (PT) symmetric systems occupy an important position within contemporary non-Hermitian physics. Although such systems are not Hermitian, they can nevertheless exhibit purely real energy spectra within specific parameter regimes. As gain-loss asymmetry increases, the system eventually reaches a critical transition known as an Exceptional Point (EP). At an Exceptional Point: eigenvalues coalesce; eigenvectors coalesce; sensitivity to perturbations increases dramatically; system behavior changes qualitatively. Exceptional points have become a major area of research in: photonics; optics; condensed matter physics; open quantum systems; topological materials; waveguide lattices; sensing technologies. The present work extends this framework by introducing an explicit constraint-field coupling and examining its impact on exceptional-point behavior. Model Overview The total Hamiltonian consists of three components: Hermitian XXZ Backbone The first contribution is the XXZ spin-chain Hamiltonian. This term represents: coherent spin transport; exchange interactions; anisotropic coupling between neighboring spins. In the absence of gain/loss and external constraints, the model reduces to the conventional XXZ chain studied throughout quantum many-body physics. PT-Symmetric Gain/Loss Sector The second contribution introduces alternating gain and loss across the lattice. This term produces: non-Hermitian dynamics; PT-symmetric behavior; exceptional-point transitions. The gain/loss amplitude controls the transition between: PT-unbroken phases; PT-broken phases. External Constraint Field The third contribution is a spatially structured driving field. This field is modeled as a superposition of harmonically varying modes possessing: amplitude; frequency; phase; spatial localization. The field couples directly to the spin degrees of freedom and modifies the effective dynamics of the system. Exact Two-Site Solution To establish a mathematically transparent foundation, the paper solves the two-site case exactly. The resulting spectrum yields a closed-form exceptional-point condition: γEP=J\gamma_{EP}=J where: JJ is the exchange coupling; γ\gamma is the gain/loss strength. This analytical result separates three physically distinct regimes: PT-Unbroken Phase γ<J\gamma < J Properties: real eigenvalues; coherent transport; stable dynamics. Exceptional Point γ=J\gamma = J Properties: eigenvalue coalescence; eigenvector coalescence; maximal sensitivity. PT-Broken Phase γ>J\gamma > J Properties: complex eigenvalues; dissipative dominance; broken PT symmetry. Central Prediction The principal result of the paper is the prediction that the exceptional point becomes tunable when the external constraint field is activated. Specifically: γEP(λ)=J1+λ⟨G⟩J\gamma_{EP}(\lambda) = J\sqrt{ 1+ \frac{\lambda\langle G\rangle}{J} } where: λ\lambda is the coupling strength; ⟨G⟩\langle G\rangle is the mean constraint-field contribution. For weak coupling: ΔγEP≈λ⟨G⟩2\Delta\gamma_{EP} \approx \frac{\lambda\langle G\rangle}{2} The prediction is therefore simple: increasing constraint coupling should systematically shift the exceptional point. Falsifiability A central design goal of this work is scientific falsifiability. The model can be rejected by experiment. The theory is considered falsified if: no exceptional point is observed; the exceptional point remains fixed when the constraint field is varied; the observed shift fails to scale with the predicted dependence; an alternative model explains the data more successfully. These conditions are intentionally defined prior to experimentation. Finite-Size Scaling Program While the exact solution is obtained for the two-site system, the broader objective is to determine whether the phenomenon survives in larger many-body systems. The proposed finite-size scaling program investigates: N=4,6,8,10,12N = 4,6,8,10,12 site chains through exact diagonalization. The goal is to determine whether the exceptional-point shift converges toward a well-defined thermodynamic limit. A successful scaling analysis would provide evidence that the mechanism is not merely a finite-size artifact. Feedback Dynamics The paper further proposes a feedback mechanism in which the constraint field is partially sourced by the state of the spin chain itself. This introduces: nonlinear behavior; effective gain/loss renormalization; hysteresis effects; potentially emergent collective behavior. A predicted observable is a measurable hysteresis loop in the exceptional-point response. Experimental Realization A practical realization is proposed using PT-symmetric photonic waveguide arrays. In such systems: waveguides represent lattice sites; optical coupling represents exchange interactions; alternating amplification and attenuation implement gain/loss; an externally driven resonator grid provides the constraint field. The experimentally measurable quantity is the eigenvalue splitting: ΔE=4J2−γ2\Delta E = 4\sqrt{J^2-\gamma^2} which collapses at the exceptional point. By measuring how the collapse location changes with constraint strength, the core prediction can be directly tested. Research Roadmap The project is organized into successive stages: Stage 1 Exact analytical treatment of the two-site model. Stage 2 Finite-size numerical diagonalization. Stage 3 Photonic-lattice simulation. Stage 4 Laboratory implementation. Stage 5 Independent experimental replication. Contribution This work should be understood as a contribution to: non-Hermitian quantum physics; PT-symmetric systems; exceptional-point dynamics; open quantum systems. The paper introduces: an explicitly defined Hamiltonian; a solvable toy model; a finite-size scaling program; a laboratory realization pathway; a quantitative prediction subject to experimental verification. Its scientific value rests not on the novelty of the concept alone, but on the existence of a clear pathway by which the prediction may be tested, reproduced, supported, revised, or rejected. Keywords: PT Symmetry, Exceptional Points, XXZ Spin Chain, Non-Hermitian Physics, Open Quantum Systems, Quantum Many-Body Systems, Finite-Size Scaling, Photonic Lattices, Gain-Loss Dynamics, Condensed Matter Physics, Mathematical Physics, Quantum Information, Experimental Quantum Optics, Falsifiable Prediction, Project Prometheus.
Abstract We in this paper study the hermiticity of Hamiltonian and energy spectrum for the SU (1, 1) systems. The Hermitian Hamiltonian can possess imaginary eigenvalues if the potential is unbounded below like the Gamow state in nuclear physics. We provide a rigorous quantization for the Hermitian Hamiltonian of inverted harmonic oscillator with the imaginary eigenvalues, which are derived in algebraic method with imaginary frequency boson operators. Dual sets of mutually orthogonal eigenstates are required corresponding respectively to the complex conjugate eigenvalues. Arbitrary order eigenfunctions are generated from the normalized ground state wave functions, which are spatially non-localized. The Hamiltonian including a non-Hermitian interaction term can be converted by similarity transformation to the Hermitian one with an effective potential of reduced slope, which can be manipulated by the interaction constant. The transformation operator is Hermitian different from the unitary transformation in ordinary quantum mechanics. The effective potential vanishes at a critical value of coupling strength called the exceptional point (EP), where all eigenstates are degenerate with zero eigenvalue. The spectrum becomes real beyond the EP just opposite to the usual transition direction. The classical counterpart of the quantum Hamiltonian with non-Hermitian interaction is a complex function of canonical variables. It becomes by the canonical transformation of variables a real function. The quantum–classical correspondence exists precisely for the system with imaginary eigenvalues.
The investigation of pseudo-Hermitian systems has garnered widespread attention due to their distinctive property of purely real eigenvalues splitting into complex conjugate pairs. This transition profoundly modifies the system's behavior, specifically giving rise to remarkable dynamical effects. Here we explore the novel characteristics of phase transitions and dynamic currents in a pseudo-Hermitian system of coupled sites with tunable gain and loss. With tunable gain and loss, we verify that the system uniquely supports two pseudo-Hermitian configurations, which exhibit distinct symmetries. The phase transitions from the PT-symmetric phase to the spontaneous broken PT-symmetric phase, along with the associated high-order exceptional points (EPs), are unambiguously demonstrated in both types of symmetry configurations. We measure the dynamic current within the four coupled sites, which allows us to probe the EPs and phase transitions between distinct phases. Our results offer insights into the phase transitions and dynamics of pseudo-Hermitian systems, with potential applications in the design and manipulation of novel quantum phenomena across various fields including photonics and other related quantum systems.
In this work, taking the most general non-Hermitian Hamiltonian without parity-time symmetry and with non-Hermitian off-diagonal elements into account, we study the exceptional points of the model system. We find that there exists an exceptional ellipse in theory, compared with the exceptional ring studied in non-Hermitian symmetry Hamiltonian, in which the parameters in the model Hamiltonian take particular values. We also report on the dynamics of the model Hamiltonian and symmetry Hamiltonian. We find that they display different dynamical behaviors under different parameters. Moreover, there exists an unstable behavior of the system. The unstable behavior has a surprising property by which the gain always dominates at long times no matter how small the gain is. This phenomenon enables the compensation for losses in dissipative systems and opens a wide range of applications in quantum optics, plasmonics, and optoelectronics, where the loss is inevitable but can be compensated by the gain.
Parity-time (PT) symmetric systems are classical, gain-loss systems whose dynamics are governed by non-Hermitian Hamiltonians with exceptional-point (EP) degeneracies. The eigenvalues of a PT-symmetric Hamiltonian change from real to complex conjugates at a critical value of gain-loss strength that is called the PT breaking threshold. Here, we obtain the PT threshold for a one-dimensional, finite Kitaev chain---a prototype for a p-wave superconductor---in the presence of a single pair of gain and loss potentials as a function of the superconducting order parameter, on-site potential, and the distance between the gain and loss sites. In addition to a robust, nonlocal threshold, we find a rich phase diagram for the threshold that can be qualitatively understood in terms of the band structure of the Hermitian Kitaev model. In particular, for an even chain with zero on-site potential, we find a re-entrant PT-symmetric phase bounded by second-order EP contours. Our numerical results are supplemented by analytical calculations for small system sizes.