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Journal of Physics B Atomic Molecular and Optical Physics

Quantum interference and phase-dependent fluorescence spectrum of a four-level atom with antiparallel dipole moments

Huatang Tan, Hongxing Xia, Gao‐xiang Li

Published Jun 1, 2009• 6 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/42/12/125502

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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.
We study theoretically quantum statistical and spectral properties of the resonance fluorescence of a single atom or system with angular momentum $J=1/2\ensuremath{-}{J}^{\ensuremath{'}}=1/2$ driven by a monochromatic linearly polarized laser field, due to quantum interference among its two antiparallel $\ensuremath{\pi}$ transitions. A magnetic field parallel to the laser polarization is applied to break the degeneracy (Zeeman effect). In the nondegenerate case, the $\ensuremath{\pi}$ transitions evolve at different generalized Rabi frequencies, producing quantum beats in the intensity and the dipole-dipole, intensity-intensity, and quadrature-intensity correlations. For a strong laser and large Zeeman splitting the beats have mean and modulation frequencies given by the average and difference, respectively, of the Rabi frequencies, unlike the beats studied in many spectroscopic systems, characterized by a modulated exponential-like decay. Further, the Rabi frequencies are those of the pairs of sidebands of the Mollow-like spectrum of the system. In the two-time correlations, the cross contributions, i.e., those with products of probability amplitudes of the two $\ensuremath{\pi}$ transitions, have a lesser role than those from the interference of the probability densities. In contrast, there are no cross terms in the total intensity. We also consider nonclassical and non-Gaussian properties of the phase-dependent fluorescence for the cases of weak to moderate excitation and in the regime of beats. The fluorescence in the beats regime is nonclassical, mainly from third-order dipole fluctuations, which reveal them to be also strongly non-Gaussian, and their quadrature spectra show complex features around the Rabi frequencies. For small laser and Zeeman detunings, a weak to moderate laser field pumps the system partially to one of the ground states, showing slow decay in the two-time correlations and a narrow peak in the quadrature spectra.
The spectrum of quantum fluctuations in phase quadratures of resonance fluorescence of a three-level V-type atom driven by a coherent field is investigated. We find that quantum interference between two decay channels of the atom can greatly modify the spectrum. We show that for weak excitation, quantum interference can greatly enhance squeezing and change the phase quadrature in which the maximum squeezing occurs. We also show that for strong and far-off-resonance excitation two deeply squeezing peaks appear at the Rabi sideband frequencies and quantum interference broadens the squeezing peaks.
In this paper we investigate the resonance fluorescence, the squeezing, and the absorption spectra of a laser-driven four-level atom consisting of three closely spaced upper levels decaying to a common lower level. The three upper levels are coupled by the same vacuum modes to the lower level leading to spontaneously generated coherence effects. High population inversion and extremely narrow emission lines are obtained with moderate Rabi frequencies, which are a direct consequence of decay-induced interferences. The fluorescent field is shown to exhibit two-mode squeezing in the in-phase quadrature. Squeezing in both quadratures simultaneously at different frequencies is greatly enhanced when quantum interference is accounted for. We also examine the absorption spectrum of a weak field and demonstrate that quantum interference can induce two prominent and nearly transparent holes where the slope of the refractive index is very steep. This special situation could allow the simultaneous propagation of two weak pulses with different frequencies, which could be feasibly exploited towards the realization of the entanglement of two photons.
We investigate steady-state squeezing in the phase quadratures of the resonance fluorescence emitted from a coherently driven four-level atom of $\ensuremath{\Lambda}$ configuration. Quantum fluctuation spectra of the optimum-phase quadratures are calculated. We show that if quantum interference between spontaneous emission channels from closed excited states to lower levels of the atom takes place, squeezing in the optimum-phase quadrature can occur in a very wider region of the parameter space than that predicted by previous studies. We also show that the quantum interference can greatly change the atomic coherences and then lead to this interesting result.
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 $\ensuremath{\pi}$ 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 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 $\ensuremath{\pi}$ 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.
Phase-dependent squeezing spectrum and photon antibunching of resonance fluorescence from a three-level V-type atom driven by a coherent field are investigated. We show that for weak excitation intensities quantum interference can greatly enhance squeezing and change the phase quadrature at which the maximum squeezing occurs. We also notice that the maximum squeezing can appear not only in either the in-phase or the out-of-phase quadrature but also in any phase quadrature, and at which phase quadrature the maximum squeezing display can be controlled by choosing the detuning of the driving field from the atomic transition. However, quantum interference can't enhance antibunching effect greatly.