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Journal of Physics Condensed Matter

Localization of two-level systems driven by AC-DC fields

Hao Wang, Xian-Geng Zhao

Published Feb 13, 1995• 13 citations
Publisher

Abstract

The quantum behaviour of electrons in a two-level system under the influence of an AC-DC field is investigated. The closed-form solution of quasienergy is obtained, from which the exact level crossing is found to appear at a fan structure in the section (E, E 0 ) through the parameter space (E, E 0 , omega ), where E is the magnitude of the AC field, E 0 is the magnitude of the DC field, and omega is the frequency of the AC field. It is demonstrated that the emergence of the exact crossing of quasienergies is coincident with the onset of coherent destruction of tunnelling.

DOI
10.1088/0953-8984/7/7/002

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The quantum behaviour of electrons in a two-level system under the influence of an AC-DC field is investigated. The closed-form solution of quasienergy is obtained, from which the exact level crossing is found to appear at a fan structure in the section (E, E 0 ) through the parameter space (E, E 0 , omega ), where E is the magnitude of the AC field, E 0 is the magnitude of the DC field, and omega is the frequency of the AC field. It is demonstrated that the emergence of the exact crossing of quasienergies is coincident with the onset of coherent destruction of tunnelling.
The dynamic effect of particles in a periodically driven two-level system is studied with the help of SU(2) symmetry. General closed-form solutions for the quasienergy and the Floquet states are obtained, from which it is found that the avoided quasienergy crossing is confined in the range (−ω/2, ω/2), where ω is the frequency of the driving fields. Of particular interest are high-frequency driving fields, of which it is demonstrated that the exact quasienergy crossings occur at certain values of amplitude and frequency of driving fields, and this phenomenon of dynamic localization should be determined by experiments.
The time-dependent average population in a two-level system interacting with an Ohmic boson bath and driven by a dc-ac field is obtained. The study is performed within the noninteracting-blip approximation and for the high-frequency driving case. The integro-differential master equation and the exact formal solution for the average transition probability are obtained. The dissipative quantum decay and the transition temperature from the coherent to the incoherent motion are investigated. The effect of the external electric field on the the dissipative two-level system is found to destroy its coherence.
We investigate the tunneling dynamics of a single atom in a planar four-well potential driven by a high-frequency ac field. The quasienergy spectrum exhibits anticrossing and crossing, which are related to selective coherent destruction of tunneling (CDT) with several selectable directions. By using the CDTs of different directions, the switchlike effect is shown for the six tunneling pathways among the four wells. Applying the present results, we suggest a scheme for designing a single-atom quantum motor with the driving field as a quantum starter.
The phenomenon of tunneling is investigated for a symmetric double-well potential perturbed by a monochromatic driving force. The analysis is based on a numerical treatment of the quantum map that propagates the system over one period of the external force, and of the spectrum of its eigenphases (quasienergies). The variety in the quasienergy spectrum, such as exact and avoided crossings, leads to novel forms of coherent tunneling. In particular, for specific parameter values of the driving force, we find almost complete localization of the wave packet in one of the wells.
We study the dynamic behavior of an electron in a two-level system driven by dc-ac electric fields. Analytical solutions for the dipole moment and its emission spectrum are obtained under the approximation of the high-frequency driving case, from which the physical property of the system is found to be controlled by the field parameters. A number of time-periodic and quasiperiodic phenomena as well as their signature in the emission spectrum are revealed. By making the proper choice of the field parameters, it is possible to selectively eliminate some components in the spectrum. The analytical findings are confirmed numerically.