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Physical Review A

Origin and restoration of missing interference in emission in a laser-driven V system

G. S. Agarwal

Published Jul 1, 1998• 9 citations
Publisher

Abstract

We present physical reasoning for the missing interference [Zhu, Narducci, and Scully, Phys. Rev. A 52, 4791 (1995)] in emission in a laser-driven V system. We demonstrate how the interference effects can be restored by considering an additional channel of spontaneous emission. Analytical results are given to identify the various pathways contributing to this interference. The interference terms show up in the form of dispersive contributions to the line shape.

DOI
10.1103/physreva.58.686

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We present physical reasoning for the missing interference [Zhu, Narducci, and Scully, Phys. Rev. A 52, 4791 (1995)] in emission in a laser-driven V system. We demonstrate how the interference effects can be restored by considering an additional channel of spontaneous emission. Analytical results are given to identify the various pathways contributing to this interference. The interference terms show up in the form of dispersive contributions to the line shape.
We show how control lasers lead to subnatural linewidths in spontaneous emission as well as to the quenching of the quantum noise. We trace this quenching to the dispersive contributions to the line shape. We obtain analytical expressions for linewidths that for $\ensuremath{\wedge}$ systems are half the natural linewidths with the possibility of much more reduction for detuned control laser. We further present a simple physical analysis in terms of a two-photon Fermi golden rule to understand cancellation of spontaneous emission. Finally, we relate our results to recent experiments.
We show that a dynamical suppression of spontaneous emission, predicted for a three-level atom [S.-Y. Zhu and M. O. Scully, Phys. Rev. Lett. 76, 388 (1996)] can occur in a two-level atom driven by a polychromatic field. We find that the quantum interference, responsible for the cancellation of spontaneous emission, appears between different channels of transitions among the dressed states of the driven atom. We discuss the effect for bichromatic and trichromatic (amplitude-modulated) fields and find that these two cases lead to the cancellation of spontaneous emission in different parts of the fluorescence spectrum. Our system has the advantage of being easily accessible by current experiments.
We study spontaneous emission from an atom under the action of laser fields. We consider two different energy level diagrams. The first one 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 in case of one decay channel because of Autler-Townes effect. The second diagram represents two two-level resonantly driven systems with the upper levels decaying to a common level. For this diagram we show that there is no interference between the two decay channels when the laser fields are described in the Fock representation, while in case of definite-phase classical fields such interference takes place and is partially or completely destructive or constructive depending on the initial conditions and on the mutual orientation of the spontaneous emission dipole moments.
We study the influence of quantum interference on the spontaneous emission from an excited two-level atom when either the atomic upper or lower level is coupled by a coherent field to a third, usually higher-lying, state. In the case of the upper level coupling, destructive quantum interference between two competing decay amplitudes produces a dark line in the emission spectrum, a phenomenon that should not be confused with the well known population trapping, and a narrowing of one of the two side lobes that make up the spectral profile. Quantum interference is absent, instead, in the case of the lower level coupling, and the spectrum modified by the external driving field is just the incoherent superposition of two Lorentzian lines. We suggest a physical interpretation of these results. In addition, we compare the analytic predictions of the simplest nontrivial versions of these models with more realistic but nonanalytic descriptions and show, numerically, that the interference effects persist in the upper level case even when additional complications are taken into account.