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arXiv:quant-ph/9810051 · Physical Review A

Cavity-induced coherence effects in spontaneous emissions from preselection of polarization

Anil K. Patnaik, G. S. Agarwal

Published Apr 1, 1999• 92 citations
arXivHTMLPDF

Abstract

Spontaneous emission can create coherences in a multilevel atom having close-lying levels, subject to the condition that the atomic dipole matrix elements are nonorthogonal. This condition is rarely met in atomic systems. We report the possibility of bypassing this condition and thereby creating coherences by letting the atom with orthogonal dipoles interact with the vacuum of a preselected polarized cavity mode rather than the free-space vacuum. We derive a master equation for the reduced density operator of a model four-level atomic system, and obtain its analytical solution to describe the interference effects. We report the quantum beat structure in the populations.

DOI
10.1103/physreva.59.3015

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John Wiley & Sons, Inc. eBooks•Apr 2, 2003•6 citations
Spontaneous emission can create coherences in a multilevel atom having close-lying levels, subject to the condition that the atomic dipole matrix elements are nonorthogonal. This condition is rarely met in atomic systems. We report the possibility of bypassing this condition and thereby creating coherences by letting the atom with orthogonal dipoles interact with the vacuum of a preselected polarized cavity mode rather than the free-space vacuum. We derive a master equation for the reduced density operator of a model four-level atomic system, and obtain its analytical solution to describe the interference effects. We report the quantum beat structure in the populations.
We show that radiative coupling between two multilevel atoms having near-degenerate states can produce new interference effects in spontaneous emission. We explicitly demonstrate this possibility by considering two identical V systems each having a pair of transition dipole matrix elements that are orthogonal to each other. We discuss in detail the origin of the new interference terms and their consequences. Such terms lead to the evolution of certain coherences and excitations that would not occur otherwise. The special choice of the orientation of the transition dipole matrix elements enables us to illustrate the significance of vacuum-induced coherence in multiatom multilevel systems. These coherences can be significant in energy transfer studies.
The earliest studies on the subject of spontaneous emission from an excited atom via interaction with the vacuum modes of the radiation field go back to the early days of quantum mechanics. In view of the fundamental nature of the vacuum fluctuations, it was long thought that spontaneous emission could not be controlled. However, in 1946 Purcell noted that the spontaneous emission rate of the atom inside a resonant cavity can be substantially enhanced (or suppressed) over its free-space value. This is due to the dramatic change of the density of modes of the vacuum field in a cavity. The enhancement and suppression of spontaneous emission in resonant and off-resonant cavities were observed experimentally in the 1980s. It was also realized in the 1950s and 1960s that coherent driving fields can dramatically change the characteristics of the spontaneous emission. The Autler-Townes doublet and Mollow three-peak spectrum are the examples of such behavior. A dramatic narrowing of the spectral peaks were discovered in driven multilevel atomic systems in the early 1990s. The role of atomic coherence in quenching spontaneous emission fluctuations in the context of laser physics was studied in the 1980s, via the correlated spontaneous emission laser (CEL). In this case atomic coherence results in the correlation of two spontaneous emission noise events so that the spontaneous emission quantum noise is eliminated in the relative linewidth. Later, it was “discovered” that the spontaneous emission can be suppressed through quantum interference between decay channels. Properly stated, this was rediscovered since it was first discovered and explained by Lamb in the 1940s. If the dipoles of two transitions from one upper level to two lower levels are parallel, the spontaneous emission in an ordinary vacuum can be cancelled. In a nonuniform vacuum, e.g., a cavity, the quenching of spontaneous emission can be achieved, even for the orthogonal dipoles.
Possibility of coherent control of spontaneous emission from four- and five-level systems in the laser radiation field is studied. The four-level system consists of two levels resonantly driven by laser radiation where either of levels may decay to a separate level. For such a system we show that the presence of the second decay channel may deteriorate the destructive interference occurring because of Autler-Townes effect in case of one decay channel. The five-level system represents two two-level resonantly driven systems with the upper levels decaying to a common level. For this case, the interference between the two decay channels is partially or completely destructive or constructive depending on the initial conditions and on the mutual orientation of the transition dipole moments. It is shown that population transfer takes place by the same quantum vacuum via spontaneous emission. The populations are shown to have damping oscillatory nature.
We study the spontaneous-emission properties of a coherently driven four-level atom, and show a few interesting phenomena such as fluorescence quenching, spectral-line narrowing, spectral-line enhancement, and spectral-line elimination. These phenomena can be observed in experiment since the rigorous condition of near-degenerate levels with nonorthogonal dipole moments is not required here. Qualitatively, these phenomena can be attributed to the quantum interference between competitive spontaneous-decay channels in the bare-state picture or the spontaneously generated coherence between two close-lying levels in the dressed-state picture.
Quantum interference and coherence effects in the interaction of atoms with the quantized electromagnetic field are investigated theoretically. A general master equation for the description of atom-field interactions is introduced. The interplay of the concepts of complementarity and interference in the time-energy domain are studied on the basis of the fluorescence light emitted by a single laser-driven atom, where the coherence of spontaneous processes gives rise to quantum interference in the spectrum of resonance fluorescence. The vacuum-induced dipole-dipole interaction in pairs of multi-level atoms is analyzed. It is shown that the interaction between orthogonal transition dipole moments of different atoms does not only influence the system dynamics crucially, but implies that the few-level approximation in general cannot be applied to near-degenerate Zeeman sublevels of the atomic level scheme. Potential applications of dipole-dipole interacting multi-level atoms for the implementation of decoherence-free subspaces and the generation of entanglement between atomic states are examined. The generation of an entangled state of the radiation field with a macroscopic number of photons is discussed on the basis of a single-atom laser.
In this review the author discusses the major effects resulting from the modification of spontaneous emission by quantum interference. He begins in Section II by presenting elementary concepts and definitions of the first- and second-order correlation functions, which are frequently used in the analysis of the interference phenomena. Section III describes the master equation approach used for analyzing quantum interference effects in atomic and molecular systems. In Sections IV and V the author reviews the fundamental quentum interference effects in spontaneous emission and in coherently driven systems. He discusses different schemes for suppression of spontaneous emission and population trapping and gives an alternative explanation of these effects in the dressed-atom picture. Section VI deals with the problem of amplification on dark transitions, which are important for information processing without spontaneous emission. The effect of quantum interference on photon correlations is discussed in Section VII. Multilevel systems composed of two transitions with parallel dipole moments and methods for generating two transitions with nonorthogonal dipole moments are briefly discussed in Section VIII. The final section (Section IX) deals with the experimental demonstration of constructive and desctructive interference effects in a driven molecular system.