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Journal of the Optical Society of America B

Infinite multilevel system coupled to a quantum-field mode

Markus Lindberg, Erkki Kyrola

Published Apr 1, 1992• 14 citations
Publisher

Abstract

We investigate a multilevel system coupled to a quantum-field mode, neglecting the damping. The model is shown to be exactly solvable in the case of equal couplings and detunings. The eigenvalues and eigensolutions are calculated. We compute the time evolution of the level populations. The behavior is compared with that of the corresponding exactly solvable semiclassical model, in which the quantum mode is replaced with an external classical field, and with that of the Maxwell–Bloch model, in which the ladder and field variables are decorrelated in the lowest nontrivial order. The semiclassical limit of the quantum system is also studied.

DOI
10.1364/josab.9.000595

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We investigate a multilevel system coupled to a quantum-field mode, neglecting the damping. The model is shown to be exactly solvable in the case of equal couplings and detunings. The eigenvalues and eigensolutions are calculated. We compute the time evolution of the level populations. The behavior is compared with that of the corresponding exactly solvable semiclassical model, in which the quantum mode is replaced with an external classical field, and with that of the Maxwell–Bloch model, in which the ladder and field variables are decorrelated in the lowest nontrivial order. The semiclassical limit of the quantum system is also studied.
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Based on the vanishing-damping continued-fraction method which I developed previously for the analysis of the resonance behavior of a two-level atom in a two-frequency field with equal intensities in the two field modes [W. M. Ruyten, Phys. Rev. A 40, 1447 (1989)], I analyze here the related problem in which the two field intensities are unequal. Several analytical results are derived, namely, expressions for the lowest-order Bloch-Siegert shifts of all resonances, and an expression for the positions of the resonances in the limit of one strong and one weak field. It is shown that, in this limit, the resonances exhibit level crossings of the field-atom system.
Formal expressions are given for the elements of the density matrix of an nnn-level quantum system interacting with a polychromatic electromagnetic field and coupled to the modes of a photon reservoir. The interaction with the polychromatic field is treated in both the semiclassical and quantum approaches, and the expressions obtained for the density matrix in the two cases are compared. It is shown that in most physical situations one can use equivalently the semiclassical or the quantum approach with a simple form of the density matrix of the field at initial time.
One of the simplest nontrivial problems involving the atom–field interaction is the coupling of a two-level atom with a single mode of the electromagnetic field. A two-level atom description is valid if the two atomic levels involved are resonant or nearly resonant with the driving field, while all other levels are highly detuned. Under certain realistic approximations, it is possible to reduce this problem to a form which can be solved exactly; allowing essential features of the atom-field interaction to be extracted. In this chapter we present a semiclassical theory of the interaction of a single two-level atom with a single mode of the field in which the atom is treated as a quantum two-level system and the field is treated classically. A fully quantum mechanical theory will be presented in Chapter 6. A two-level atom is formally analogous to a spin-1/2 system with two possible states. In the dipole approximation, when the field wavelength is larger than the atomic size, the atom–field interaction problem is mathematically equivalent to a spin-1/2 particle interacting with a time-dependent magnetic field. Just as the spin-1/2 system undergoes the so-called Rabi oscillations between the spin-up and spin-down states under the action of an oscillating magnetic field, the two-level atom also undergoes optical Rabi oscillations under the action of the driving electromagnetic field. These oscillations are damped if the atomic levels decay. An understanding of this simple model of the atom–field interaction enables us to consider more complicated problems involving an ensemble of atoms interacting with the field.
We present detailed numerical simulations of semiclassical and quantum spectra of a cavity QED system consisting of a single three-level atom in Λ-configuration interacting with a quantized cavity mode through one of its transitions while its other transition is driven by a coherent classical field. We compute the semiclassical and quantum spectra of the system under various levels of external driving field strengths. In the semiclassical approach, we neglect the correlations between the cavity and atomic operators, whereas in the quantum case no such assumption is made. Our results show that the two approaches yield identical results under sufficiently weak driving field conditions. However, the two approaches yield starkly different results at stronger driving field intensities: the fully quantum approach yields a well-defined multiphoton spectrum whereas the semiclassical approach results in a bistable spectrum. Furthermore, our calculations reveal a complex Raman structure at the position of the dark-state transition.