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Physical Review A

Quantal localization and the uncertainty principle

J G Leopold, D. Richards

Published Sep 1, 1988• 18 citations
Publisher

Abstract

We give a dynamical explanation for the localization of the wave function for the one-dimensional hydrogen atom, with the Coulomb singularity, in a high-frequency electric field, which leads to a necessary condition for classical dynamics to be valid. Numerical tests confirm the accuracy of the condition. Our analysis is relevant to the comparison between the classical and quantal dynamics of the kicked rotor and standard map.

DOI
10.1103/physreva.38.2660

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AIP conference proceedings•Jan 1, 1990•2 citations
We give a dynamical explanation for the localization of the wave function for the one-dimensional hydrogen atom, with the Coulomb singularity, in a high-frequency electric field, which leads to a necessary condition for classical dynamics to be valid. Numerical tests confirm the accuracy of the condition. Our analysis is relevant to the comparison between the classical and quantal dynamics of the kicked rotor and standard map.
The authors examine the effect of the Coulomb singularity on the classical and semiclassical dynamics of the one-dimensional model for atomic hydrogen, sometimes called the surface-state-electron model, in a periodic electric field and determine a necessary condition, in terms of the field strength, frequency and the initial quantum number, for the violation of the uncertainty principle. They show that when this violation occurs the quantal transition probabilities are significantly less than the equivalent classical probabilities. This criterion is tested numerically and shown to provide a good estimate of the boundary to one of the regions in which classical dynamics fails. The authors also show that when the uncertainty principle is not violated the fluctuations of the quantal about the classical probability spectra can be related to structure in classical phase space. These fluctuations are due to different classical and quantal transport through cantori.
Analytical expressions for dipole matrix elements between high atomic states of nonhydrogenic and perturbed hydrogenic atoms are derived. The theoretical investigation is fulfilled for discrete and continuum atomic states and for the one-dimensional atom. It is shown that the quantal transition probabilities between perturbed atomic states depend on the nature of perturbation, e.g. matrix elements for the hydrogenic atom in an electromagnetic field differ from those of the nonhydrogenic atom. The results obtained generalize and increase the accuracy of some known expressions for the nonhydrogenic atom and extend them for the perturbed hydrogenic atom. Further, the analysis of classical dynamics and the derivation of mapping equations of motion for the one-dimensional atom in a harmonic field is fulfilled. The relationship between the energy change of the classical atom in a harmonic field and expressions for dipole matrix elements is revealed and analysed.
New experimental results are presented for short-pulse microwave ionization of highly excited hydrogen atoms. A comparison of these results with quantum numerical computations and analytical predictions provides for the first time experimentally grounded evidence of the localization phenomenon that leads to the suppression of the quantum version of the chaotic diffusion in action space occurring in the classical limit. The onset of chaotic motion in externally driven classical systems is a key problem in nonlinear dynamics that is now well understood in its essential aspects. In many physically interesting cases, as the perturbation strength increases beyond some critical value, the system starts absorbing energy in a diffusive way. The question whether “diffusive” excitation processes can take place also in externally driven quantum systems is then a very interesting one for the physics of atoms and molecules in external electromagnetic fields. This is a deep question involving the nature and the validity of the quasiclassical approximation when the underlying classical dynamics is chaotic. The question of “quantum diffusion” was first addressed in a simple model system, the kicked rotator. It was chosen because the features of classical chaos in it were relatively clean and well understood. Besides that, the numerical simulation of its quantum dynamics could be easily accomplished. The major indication of this model was that quantum mechanics suppresses the classical chaotic diffusion in action via a destructive interference effect similar to that responsible for the Anderson localization well known in condensed matter physics.
This thesis investigates quantum transport in the energy space of two paradigm systems of quantum chaos theory. These are highly excited hydrogen atoms subject to a microwave field, and kicked atoms which mimic the delta-kicked rotor model. Both of these systems show a complex dynamical evolution arising from the interaction with an external time-periodic driving force. In particular two quantum phenomena, which have no counterpart on the classical level, are studied: the suppression of classical diffusion, known as dynamical localisation, and quantum resonances as a regime of enhanced transport for the delta-kicked rotor. The first part of the thesis provides new support for the quantitative analogy between energy transport in strongly driven highly excited atoms and particle transport in Anderson-localised solids. A comprehensive numerical analysis of the atomic ionisation rates shows that they obey a universal power-law distribution, in agreement with Anderson localisation theory. This is demonstrated for a one-dimensional model as well as for the real three-dimensional atom. We also discuss the implications of the universal decay-rate distributions for the asymptotic time-decay of the survival probability of the atoms. The second part of the thesis clarifies the effect of decoherence, induced by spontaneous emission, on the quantum resonances which have been observed in a recent experiment with delta-kicked atoms. Scaling laws are derived, based on a quasi-classical approximation of the quantum evolution. These laws describe the shape of the resonance peaks in the mean energy of an experimental ensemble of kicked atoms. Our analytical results match perfectly numerical computations and explain the initially surprising experimental observations. Furthermore, they open the door to the study of the competing effects of decoherence and chaos on the stability of the time evolution of kicked atoms. This stability may be characterised by the overlap of two identical initial states which are subject to different time evolutions. This overlap, called fidelity, is investigated in an experimentally accessible situation.
We provide a brief review of the response of an excited hydrogen atom to a microwave field, and a detailed discussion of the behaviour of a one‐dimensional model in a high frequency field. In particular we show that although classical dynamics is incorrect for the parameters of experimental interest an examination of the classical behaviour is helpful in understanding the quantal behaviour. We show further that, for these parameters, the quantal probabilities fluctuate dramatically with the variation of most system parameters and attribute this non‐classical behaviour to the dominant influence of relatively few, about ten, quasi‐resonant states.