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arXiv:quant-ph/0603080 · Physical Review A

Interference in the resonance fluorescence of two incoherently coupled transitions

Martin Kiffner, Jörg Evers, Christoph H. Keitel

Published Jun 16, 2006• 22 citations
arXivHTMLPDF

Abstract

The fluorescence light emitted by a four-level system in $J=1∕2$ to $J=1∕2$ configuration driven by a monochromatic laser field and in an external magnetic field is studied. We show that the spectrum of resonance fluorescence emitted on the $\ensuremath{\pi}$ transitions shows a signature of spontaneously generated interference effects. The degree of interference in the fluorescence spectrum can be controlled by means of the external magnetic field, provided that the Land\'e $g$ factors of the excited and the ground state doublet are different. For a suitably chosen magnetic field strength, the relative weight of the Rayleigh line can be completely suppressed, even for low intensities of the coherent driving field. The incoherent fluorescence spectrum emitted on the $\ensuremath{\pi}$ transitions exhibits a very narrow peak whose width and weight depend on the magnetic field strength. We demonstrate that the spectrum of resonance fluorescence emitted on the $\ensuremath{\sigma}$ transitions shows an indirect signature of interference. A measurement of the relative peak heights in the spectrum from the $\ensuremath{\sigma}$ transitions allows us to determine the branching ratio of the spontaneous decay of each excited state into the $\ensuremath{\sigma}$ channel.

DOI
10.1103/physreva.73.063814

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The fluorescence light emitted by a four-level system in J=1∕2J=1∕2J=1∕2 to J=1∕2J=1∕2J=1∕2 configuration driven by a monochromatic laser field and in an external magnetic field is studied. We show that the spectrum of resonance fluorescence emitted on the \ensuremathπ\ensuremath{\pi}\ensuremathπ transitions shows a signature of spontaneously generated interference effects. The degree of interference in the fluorescence spectrum can be controlled by means of the external magnetic field, provided that the Land'e ggg factors of the excited and the ground state doublet are different. For a suitably chosen magnetic field strength, the relative weight of the Rayleigh line can be completely suppressed, even for low intensities of the coherent driving field. The incoherent fluorescence spectrum emitted on the \ensuremathπ\ensuremath{\pi}\ensuremathπ transitions exhibits a very narrow peak whose width and weight depend on the magnetic field strength. We demonstrate that the spectrum of resonance fluorescence emitted on the \ensuremathσ\ensuremath{\sigma}\ensuremathσ transitions shows an indirect signature of interference. A measurement of the relative peak heights in the spectrum from the \ensuremathσ\ensuremath{\sigma}\ensuremathσ transitions allows us to determine the branching ratio of the spontaneous decay of each excited state into the \ensuremathσ\ensuremath{\sigma}\ensuremathσ channel.
We investigate the effects of spontaneously generated interference on the spectrum of quantum fluctuations in phase quadratures of resonant fluorescence emitted from the transition of j = 1/2 to j = 1/2 of a monochromatically driven four-level atom with antiparallel dipole moments. It is found that the spectrum exhibits obvious characteristics of quantum interference in both cases of weak and strong laser fields. For the Zeeman-degenerate system, strong radiation squeezing in weak atomic excitation can occur at the central frequency of the spectrum, which is quite different from the case without consideration of the interference. Moreover, when we apply an external magnetic field to the atom, we find that the quantum interference can lead to a narrowing spectrum with much stronger squeezing by adjusting the magnetic field strength. These results suggest that the phase-dependent fluorescence spectrum can serve as a probe of the quantum interference effects in a realistic system such as a trapped 198Hg+ ion or charged quantum dots.
The interaction of a four-level atom consisting of one excited state, two upper levels and one ground state is considered. We use one field to drive the transition between the excited state and the ground state, and simultaneously apply another field to couple two transitions from the upper-level doublet to the ground state. We investigate how to control the central linewidth of resonance fluorescence spectrum of the driven transition via quantum interference between two transitions from the upper doublet to the ground state of the atom. In the case of orthogonal dipole moments for the upper doublet and the ground state the widths of the central line and the sidebands of the resonance fluorescence spectrum are totally controlled by the spontaneous emission rate of the upper doublet when the applying field is sufficiently strong. When the dipole moments are parallel, the central linewidth can be much narrower than the linewidths of spontaneous emission of the excited state and of the upper doublet due to the quantum interference. The smaller the upper doublet splitting, the narrower the central linewidth. We also notice that when the upper levels are degenerate the resonance fluorescence spectrum may depend on the initial condition because of the quantum interference.
We experimentally study the resonance fluorescence from an excited two-level atom when the atomic upper level is coupled by a nonresonant field to a higher-lying state in a rubidium atomic beam. The heights, widths and positions of the fluorescence peaks can be controlled by modifying the detuning of the auxiliary field. We explain the observed spectrum with the transition properties of the dressed states generated by the coupling of the two laser fields. We also attribute the line narrowing to the effects of Spontaneously Generated Coherence between the close-lying levels in the dressed state picture generated by the auxiliary field. And the corresponding spectrum can be viewed as the evidence of Spontaneously Generated Coherence. The experimental results agree well with calculations based on the density-matrix equations.
Abstract The resonance fluorescence of a four-level atom in J = 1/2 to J = 1/2 transition driven by two coherent fields is studied. We find that the incoherent fluorescence spectrum shows a direct indication of vacuum-induced coherence in the atomic system. We show that such coherence manifests itself via an enhancement or suppression of the spectral peaks in the π -polarized fluorescence. The effect of the relative phase of the driving fields on the spectral features is also investigated. We show that phase-dependent enhancement or suppression of the fluorescence peaks appears in the incoherent spectrum emitted along the σ transitions. It is found that this phase dependence occurs because of the polarization-detection scheme employed for the observation of the fluorescence light. We present an analytical explanation, based on dressed-states of the atom-field system, to interpret the numerical results.
We describe the resonance fluorescence spectrum of an atomic three-level system where two of the states are coupled by a single monochromatic laser field. The influence of the third energy level, which interacts with the two laser-coupled states only via radiative decays, is studied in detail. For a suitable choice of parameters, this system gives rise to a very narrow structure at the laser frequency in the fluorescence spectrum which is not present in the spectrum of a two-level atom. We find those parameter ranges by a numerical analysis and use the results to derive analytical expressions for the additional narrow peak. We also derive an exact expression for the peak intensity under the assumption that a random telegraph model is applicable to the system. This model and a simple spring model are then used to describe the physical origins of the additional peak. Using these results, we explain the connection between our system, a three-level system in the V configuration where both transitions are laser driven, and a related experiment that was recently reported.
We discuss quantum interference effects in the resonance fluorescence spectrum of a \ensuremath{\Lambda} three-level atom when the lower-level doublet is driven by a coherent field. The interfering pathways that lead to the same final state involve both spontaneous decays and stimulated transitions, and differ from one another by an odd number of stimulated processes induced by the driving field. As a consequence, the interference structures depend upon the phase of the coherent field, an effect that is absent in other resonance fluorescence phenomena. The phase dependence of the quantum interference contribution is especially significant when the level splitting of the driven doublet is comparable to the spontaneous decay rates of the competing optical paths.