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arXiv:1812.09319 · Physical Review A

Heisenberg uncertainty relations for the non-Hermitian resonance-state solutions to the Schrödinger equation

Gastón García‐Calderón, Jorge Villavicencio

Published Feb 7, 2019• 7 citations
arXivHTMLPDF

Abstract

Resonance (quasinormal) states correspond to non-Hermitian solutions to the Schr\"odinger equation obeying outgoing boundary conditions which lead to complex energy eigenvalues and momenta. Following the normalization rule for resonance states obtained from the residue at a complex pole of the outgoing Green's function to the problem, we propose a definition of expectation value for these states and use it to investigate the extent of validity of the Heisenberg uncertainty relations for potentials that vanish after a distance. We derive analytical expressions for the expectation values involving the momentum and the position for a given resonance state and find in model calculations that the Heisenberg uncertainty relations are satisfied for a broad range of potential parameters. A comparison of our approach with that based on the regularization method by Zel'dovich yields very similar results except for resonance energies very close to the energy threshold. Our work shows that the validity of the Heisenberg uncertainty relations may be extended to the non-Hermitian resonance-state solutions to the Schr\"odinger equation.

DOI
10.1103/physreva.99.022108

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•Jan 1, 2013•2 citations
Resonance (quasinormal) states correspond to non-Hermitian solutions to the Schr"odinger equation obeying outgoing boundary conditions which lead to complex energy eigenvalues and momenta. Following the normalization rule for resonance states obtained from the residue at a complex pole of the outgoing Green's function to the problem, we propose a definition of expectation value for these states and use it to investigate the extent of validity of the Heisenberg uncertainty relations for potentials that vanish after a distance. We derive analytical expressions for the expectation values involving the momentum and the position for a given resonance state and find in model calculations that the Heisenberg uncertainty relations are satisfied for a broad range of potential parameters. A comparison of our approach with that based on the regularization method by Zel'dovich yields very similar results except for resonance energies very close to the energy threshold. Our work shows that the validity of the Heisenberg uncertainty relations may be extended to the non-Hermitian resonance-state solutions to the Schr"odinger equation.
The Hermitian properties of the Hamiltonian are related not only to the operator itself but also to the functions on which it acts. Hermitian Hamiltonians operate on functions in the L 2 Hilbert space which correspond to boundary conditions which vanish at infinity. In this chapter, in order to move into the non-Hermitian domain, we will impose on the solutions to the time-independent Schrödinger equation (TISE) different boundary conditions which lead to solutions which can be associated with different types of the complex poles of the scattering matrix. These solutions will contain information which was not available within the scope of functions in L 2 . By imposing outgoing boundary conditions on the eigenfunctions of the timeindependent Hamiltonian complex eigenvalues, E res = ε − (i/2)Г, are obtained. These complex energies are associated with decaying resonance states which were discussed in the previous two chapters. The bound states (if they exist) appear as real eigenvalues since they result from exactly such outgoing boundary conditions which appear under the threshold energy. When incoming boundary conditions are imposed two kind of solution are obtained. One type of solution is the complex conjugates of the decay resonance solutions mentioned above. In scattering theory text books (see Taylor for example) the physical resonance solutions are associated with the poles of the scattering matrix which are embedded in the lower half of the complex energy plane. However, in nuclear physics the complex poles embedded in the upper half of the complex energy plane, so-called virtual states , are denoted as capture resonances .
The absolute value squared of the probability amplitude corresponding to the overlap of an initial state with a continuum wave solution to the Schrödinger equation of the problem, has the physical interpretation provided by the Born rule. Here, it is shown that for an open quantum system, the above probability may be written in an exact analytical fashion as an expansion in terms of the non‐Hermitian resonance (quasinormal) states and complex poles to the problem which provides an underlying non‐Hermitian character of the Born rule.
As discussed in the previous chapter, the poles of the S-matrix are identified with discrete eigenvalues of the time-independent Schrödinger equation, where the asymptotes of the corresponding eigenfunctions are either purely outgoing waves or purely incoming waves. More specifically, the bound and decay resonance poles are obtained by imposing the outgoing boundary conditions on the solutions of the time-independent Schrödinger equation, while the anti-bound and virtual states (sometimes referred to as capture resonances) are associated with the solutions obtained under the requirement of the incoming boundary conditions. Except for the bound states, all other poles of the S-matrix are associated with exponentially divergent wavefunctions which by definition do not belong to the Hilbert space of conventional Hermitian quantum mechanics. This fact represents a major difficulty for the development of a non-Hermitian quantum mechanical formalism. Consequently, one may wonder, for example, how to properly define an inner product in non-Hermitian quantum mechanics (NHQM) if the wavefunctions diverge asymptotically. We recall in this context that the concept of an inner product constitutes a fundamental building block of standard (Hermitian) quantum mechanics (QM), by means of which one defines the quantum mechanical expectation values of physically observable quantities over the quantum states under consideration. An inner product for NHQM is necessary in order to accommodate the tools of conventional QM. Furthermore, one might anticipate that an appropriate NHQM inner product would also facilitate practical numerical calculation of the S-matrix poles for those cases where the eigenvalues of the Hamiltonian do not possess an analytical closed form expression (unlike the cases studied in the previous chapter).
Resonances are very interesting phenomena, which appear in a wide range of classical and quantum systems.The mathematical description of quantum-mechanical resonances is connected with complex eigenvalues of the Hamiltonian of a given system.This fact introduces various difficulties, especially since resonance eigenfunctions are not normalisable.This work aims at developing techniques for the treatment of quantum-mechanical resonances with variational methods.For this purpose particular similarity transformations of the Hamiltonian, such as the Zel'dovich and complex-scaling transformations, are employed.A precise mathematical description of the transformation operators and extended function spaces is given.In addition a generalisation of the Rayleigh-Ritz variational principle is derived, which can be applied to non-Hermitian eigenvalue problems.Several basis-selection criteria are proposed for adapting the stochastic variational method to the non-Hermitian case.Their efficiencies are demonstrated for a one-dimensional problem with a potential considered before in the literature.It is found that the stochastic variational method can practically be extended to quantum-mechanical resonances and complex eigenvalues.It usually outperforms alternative methods for treating non-Hermitian Hamiltonians. KurzzusammenfassungSowohl in klassischen als auch in Quantensystemen sind Resonanzen sehr interessante Phänomene.Ihre mathematische Beschreibung ist eng mit komplexen Eigenwerten des Hamilton-Operators eines Systems verknüpft, was zu einigen Schwierigkeiten in der Handhabung führt, da die zugehörigen Eigenfunktionen nicht normierbar sind.Das Ziel dieser Arbeit ist es zu beschreiben, wie Variationsmethoden zur Behandlung von quantenmechanischen Resonanzen verwendet werden können.Dafür werden Ähnlichkeitstransformationen wie die Zel'dovich-und die Complex-Scaling-Transformation angewandt, und es wird eine mathematisch präzise Beschreibung der involvierten Operatoren und erweiterten Funktionenräume gegeben.Außerdem wird eine Erweiterung des Rayleigh-Ritz-Prinzips auf nicht-hermitische Eigenwertprobleme vorgestellt, und die stochastische Variationsmethode auf den nicht-hermitischen Fall erweitert.Dafür stellen wir verschiedene mögliche Auswahlkriterien für Basisfunktionen vor und testen unsere Methoden an einem eindimensionalen Problem, das auch schon in der Literatur untersucht worden ist.Unsere Varianten der stochastischen Variationsmethode können die Resonanzpositionen erfolgreich und effizienter als andere Methoden zur Behandlung von nicht-hermitischen Problemen berechnen.