W. D. Heiss

The physics of exceptional points

A short resume is given about the nature of exceptional points (EPs) followed by discussions about their ubiquitous occurrence in a great variety of physical problems. EPs feature in classical as well as in quantum mechanical problems. They are associated with symmetry breaking for -symmetric Hamiltonians, where a great number of experiments has been performed, in particular in optics, and to an increasing extent in atomic and molecular physics. EPs are involved in quantum phase transition and quantum chaos; they produce dramatic effects in multichannel scattering, specific time dependence and more. In nuclear physics, they are associated with instabilities and continuum problems. Being spectral singularities they also affect approximation schemes.This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to Quantum physics with non-Hermitian operators.

Exceptional points of non-Hermitian operators

Exceptional points associated with non-Hermitian operators, i.e. operators being non-Hermitian for real parameter values, are investigated. The specific characteristics of the eigenfunctions at the exceptional point are worked out. Within the domain of real parameters the exceptional points are the points where eigenvalues switch from real to complex values. These and other results are exemplified by a classical problem leading to exceptional points of a non-Hermitian matrix.

Repulsion of resonance states and exceptional points

Level repulsion is associated with exceptional points which are square root singularities of the energies as functions of a (complex) interaction parameter. This is also valid for resonance state energies. Using this concept it is argued that level anticrossing (crossing) must imply crossing (anticrossing) of the corresponding widths of the resonance states. Further, it is shown that an encircling of an exceptional point induces a phase change of one wave function but not of the other. An experimental setup is discussed where this phase behavior, which differs from the one encountered at a diabolic point, can be observed.

The chirality of exceptional points

Abstract:Exceptional points are singularities of the spectrum and wave functions of a Hamiltonian which occur as functions of a complex interaction parameter. They are accessible in experiments with dissipative systems. We show that the wave function at an exceptional point is a specific superposition of two configurations. The phase relation between the configurations is equivalent to a chirality which should be detectable in an experiment.

Avoided level crossing and exceptional points

The connection between level repulsions and the singularities associated with the analytically continued energy levels is investigated. The authors also conjecture that there are necessarily specific consequences for the state vectors when the statistical analysis of the energy spectrum indicates quantum chaos. A procedure which allows a qualitative assessment of the positions of the exceptional points is suggested. The importance of their distribution for quantum chaos is discussed within this context.

Phases of wave functions and level repulsion

Abstract:Avoided level crossings are associated with exceptional points which are the singularities of the spectrum and eigenfunctions, when considered as functions of a complex coupling parameter. It is shown that the wave function of one state changes sign but not the other, if the exceptional point is encircled in the complex plane. An experimental setup is suggested where this peculiar phase change could be observed.

Observation of exceptional points in electronic circuits

Two damped coupled oscillators have been used to demonstrate the occurrence of exceptional points in a purely classical system. The implementation was achieved with electronic circuits in the kHz-range. The experimental results perfectly match the mathematical predictions at the exceptional point. A discussion about the universal occurrence of exceptional points—connecting dissipation with spatial orientation—concludes this paper.

Chirality of wavefunctions for three coalescing levels

The analytic structure in the vicinity of three coalescing eigenvalues (EP3) of a matrix problem is investigated. It is argued that the three eigenfunctions—also coalescing at the EP3—invoke a true chiral behaviour in the vicinity of the EP3 and that they can be related to a three-dimensional helix. The orientation of the helix depends on the distribution of the widths of the three levels in the vicinity of the EP3.

Choice of a metric for the non-Hermitian oscillator

The harmonic oscillator Hamiltonian, when augmented by a non-Hermitian PT -symmetric part, can be transformed into a Hermitian Hamiltonian. This is achieved by introducing a metric which, in general, renders other observables such as the usual momentum or position as non-Hermitian operators. The metric depends on one real parameter, the full range of which is investigated. The explicit functional dependence of the metric and each associated Hamiltonian is given. A specific choice of this parameter determines a specific combination of position and momentum as being an observable; this can be in particular either standard position or momentum, but not both simultaneously. Singularities of the metric are explored and their removability is investigated. The physical significance of these findings is discussed.

Collectivity, phase transitions, and exceptional points in open quantum systems

Phase transitions in open quantum systems, which are associated with the formation of collective states of a large width and of trapped states with rather small widths, are related to exceptional points of the Hamiltonian. Exceptional points are the singularities of the spectrum and eigenfunctions, when they are considered as functions of a coupling parameter. In the present paper this parameter is the coupling strength to the continuum. It is shown that the positions of the exceptional points (their accumulation point in the thermodynamical limit) depend on the particular type and energy dependence of the coupling to the continuum in the same way as the transition point of the corresponding phase transition.