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arXiv:quant-ph/0010066 · Physical Review A

Vacuum-induced coherences in radiatively coupled multilevel systems

G. S. Agarwal, Anil K. Patnaik

Published Mar 12, 2001• 52 citations
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Abstract

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.

DOI
10.1103/physreva.63.043805

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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.
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.
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.
When a four-level Y\mathsf{Y}Y-type atom with two highest nearly degenerate lying levels interacts with the same vacuum radiation field, its excited levels have vacuum-induced coherence due to the quantum interference between the spontaneous decay channels. We consider the effects of the vacuum-induced coherence on the electromagnetically induced transparency against single- and two-photon absorption in the atomic system. We find that the vacuum-induced coherence can lead to probe gain without incoherent pumping. On the other hand, the coherence can suppress the two-photon transparency, even in some cases where it can enhance the two-photon absorption. Another important result is the finding of the crucial role played by the relative phase between the probe and coupling fields: the two-photon transparency and the probe gain spectra can be modulated by this phase.
We investigate cooperative light scattering by an ordered chain of multilevel atoms, which possess two quasi-resonant transitions with parallel dipole moments. Interference between dipole transitions induced by coupling with the vacuum gives rise to so called cross-damping and cross-shifts, that modify the incoherent and coherent dynamics and can be manifest in the spectroscopic properties of the emitted light. We determine the excitation spectrum when the atomic chain is driven by an external laser in the limit in which the dipolar transitions can be described by harmonic oscillators and the scattering is coherent. We show that the interplay of multilevel interference and superradiance can give rise to measurable effects in chains of alkali-metal atoms.
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.
In this paper we reveal that the constructive interference of atomic coherence on absorption leads to electromagnetically induced absorption,which is in sharp contrast to the case in which destructive interference of atomic coherence gives rise to electromagnetically induced transparency. A four-level atomic system is employed in which three transitions are in N configuration and the middle transition is used as a probe transition. In the dressed states representation the probe transition is split into four transitions that are coupled to each other. The coupling originates from the spontaneous emission of the atomic medium and the spontaneous transfer from excited state coherence to ground state coherence. The former leads to couplings between all transitions with the same sign and to the absorption reduction. The latter results in couplings,of which t wo terms have positive sign and another two have negative sign. It is due to the couplings with positive and negative signs that significant modification of the contribution of dressed coherence to absorption occurs,i.e., absorption inhibition is changed to absorption enhancement.