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Physical Review A

Strong correlation and anticorrelation via quantum coherence in a cascade atom with split metastable states

Xiangming Hu, Fei Wang

Published Jun 26, 2007• 7 citations
Publisher

Abstract

We show that strongly correlated and anticorrelated photons can be produced from two fluorescent fields emitted by a driven cascade atom with split metastable states. Depending on the choice of parameters, we have strong correlations for both fluorescent fields, anticorrelations for both, and strong correlation for one and anticorrelation for the other. In the presence of level splitting, we have two interfering cascade-dressed transitions from the top state to the antisymmetric superposition of the metastable states. The interference between the two cascade-dressed transitions is responsible for the strong correlations and anticorrelations.

DOI
10.1103/physreva.75.065802

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We show that strongly correlated and anticorrelated photons can be produced from two fluorescent fields emitted by a driven cascade atom with split metastable states. Depending on the choice of parameters, we have strong correlations for both fluorescent fields, anticorrelations for both, and strong correlation for one and anticorrelation for the other. In the presence of level splitting, we have two interfering cascade-dressed transitions from the top state to the antisymmetric superposition of the metastable states. The interference between the two cascade-dressed transitions is responsible for the strong correlations and anticorrelations.
We examine the intensity correlation functions of the two fluorescent fields that are emitted from the top and middle states of a doubly driven three-level atom in the cascade configuration. Novel interference effects are shown. (i) Both of the fluorescent fields have anticorrelations which can exist for long times when the applied fields are on the two-photon resonance and far off one-photon resonances. (ii) Both of the fluorescent fields have strong correlations when the applied fields are far off one- and two-photon resonances. In particular, the extremely strong correlation occurs for the photons emitted from the top state. The above phenomena are traced to the multiple interference mechanisms.
In this paper, we investigate the spontaneously generated coherence (SGC) effects on the fluorescence fields intensity-intensity correlation in a resonant driven four-level Y-type atomic system. By using a strong control field, in the absense of SGC effects, strong correlation and anticorrelation fluorescence photons can be produced from two ladder transitions in this system respectively. However, in the presence of SGC effects, the fluorescence fields correlation properties are reversed for the two transitions. The above phenomena can be traced to the quantum destructive or constructive interference in terms of dressed states.
We investigate the two-time intensity correlation functions of the fluorescence field emitted from a VVV-type three-level atom. We are particularly interested in the manner in which the atom emits photons in the presence of quantum interference. We show that under strong-field excitation quantum interference leads to anticorrelations of photons emitted from the atomic excited levels which can exist for extremely long times. This indicates that the excited atomic levels are not the preferred radiative states. We find that the atom spends most of its time in a superposition of the excited atomic levels from which it emits strongly correlated photons. The strong correlations are present only for a nonzero splitting between the excited levels, and for degenerate levels the correlations reduce to that of a two-level atom. Moreover, we find that the transition from the ground level to the symmetric superposition of the excited levels does not saturate even for a strong driving field. We also calculate the correlation functions for a weak driving field, and find that in this case the photon correlations are not significantly affected by quantum interference, but the atom can emit a strongly correlated pair of photons produced by a three-wave mixing process. Under appropriate conditions, with near-maximal quantum interference, it is possible to make the maximum value of the correlation function extremely large, in marked contrast with the corresponding case with no quantum interference.
Correlations between the emission of photons from an excited three-level atom with one transition more strongly driven than the other are studied. Antibunching, anticorrelation, and superbunching are exhibited in the four possible configurations: ``vee'' (V), ``lambda'' (\ensuremath{\Lambda}), and two cascades with the upper or lower transition strongly driven. We pay particular attention to the projection of the atom into particular states by the measurement of fluorescence, and its significance for detecting quantum jumps in the weak transition by observing the fluorescence from the strong transition. For incoherent excitation we find that cross antibunching (the anticorrelation between emissions of different types of photons) manifests itself as prolonged periods of darkness. For the V and the cascade configuration with a strongly driven lower transition, these periods herald the emission of a weak-transition photon; for the other cascade and the \ensuremath{\Lambda} configuration the periods are heralded by the emission of a weak-transition photon. For on-resonance coherent excitation, dark periods are extremely infrequent for the former two configurations, and are extremely long or infinite for the latter two.
The fluorescence photon correlations in a driven four-level atom consisting of three upper closely lying excited states and a single lower state are investigated. It is found that in the presence of triple spontaneously generated coherence (SGC) effects, which are created by three degenerate levels coupling to the common ground state via the same vacuum mode, the correlation properties of the three fluorescence fields can be switched from strong correlation to anticorrelation or vice versa by modifying the detunings. In addition, such a system is very sensitive to the relative phases of the three fields, which can provide an effective way to control the photon correlation. Physically, these interesting phenomena can be interpreted in terms of dressed state analysis.
We study the efficiency of different two-photon states of light to induce the simultaneous excitation of two atoms of different kinds when the sum of the energies of the two photons matches the sum of the energies of the two atomic transitions, while no photons are resonant with each individual transition. We find that entangled two-photon states produced by an atomic cascade are indeed capable of enhancing by a large factor the simultaneous excitation probability as compared to uncorrelated photons, as predicted some years ago by Muthukrishnan et al., but that several unentangled, separable, correlated states, produced either by an atomic cascade or parametric down-conversion, or even appropriate combinations of coherent states, have comparable efficiencies. We show that the key ingredient for the increase of simultaneous excitation probability is the presence of strong frequency anticorrelation and neither time correlation nor time-frequency entanglement.