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Communications in Theoretical Physics

Storage and Retrieval of a Weak Optical Signal Improved by Spontaneously Generated Coherence in an Atomic Assemble

Tao Zheng, Tianhui Qiu, Guojian Yang

Published Mar 1, 2012• 2 citations
Publisher

Abstract

We investigate the reversible storage of a weak single-mode light signal in a Λ-type three-level atomic system with spontaneously generated coherence (SGC) and pumped by an incoherent field. The scheme is phase-dependent. If the phase of the controlling field is set oppositely to that of the signal field, the combination of SGC with weak pump overcomes the drawback of incompletely retrieval in the conventional scheme, leading to 100% retrieval fidelity or even to the amplification of the retrieved signal with respect to its initial one.

DOI
10.1088/0253-6102/57/3/19

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We investigate the reversible storage of a weak single-mode light signal in a Λ-type three-level atomic system with spontaneously generated coherence (SGC) and pumped by an incoherent field. The scheme is phase-dependent. If the phase of the controlling field is set oppositely to that of the signal field, the combination of SGC with weak pump overcomes the drawback of incompletely retrieval in the conventional scheme, leading to 100% retrieval fidelity or even to the amplification of the retrieved signal with respect to its initial one.
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Abstract Owing to interference induced by spontaneous emission, the density-matrix equations in a three-level Λ-type system have an additional coherence term, which plays a critical role in modulating the inversionless gain and electromagnetically induced transparency effect. In addition, it is shown that spontaneously generated coherence (SGC) has an effect on the entanglement between an atom and a photon of the coupling laser field by calculating the degree of entanglement (DEM) of the atomic system. In this paper, we investigate the influence of the SGC effect on atom-photon entanglement in a Λ-type system, which generally remains a high entangled state. When an incoherent pump source is introduced, we find that the SGC effect could exert considerable influence on the atom reduced entropy under certain conditions for both transient and steady states. More interestingly, such an incoherent pump field could actively affect the short-time dynamic behaviors of the transient quantum entangled state at a certain range of pump rate as a typical coherent case.
The probe response and dispersion of a three-level closed ? system considering spontaneously generated coherence (SGC) have been investigated with and without an incoherent pumping. One can get electromagnetically induced transparency (EIT) whatever be the value of the SGC parameter and phase difference when an incoherent pump is absent. But by turning the incoherent pump rate on, one can control the light propagation from subluminal to superluminal and also the nonlinear response from EIT to electromagnetically induced absorption (EIA) by controlling the SGC parameter and phase difference. Using the incoherent pump rate as an optical tool, the desired configuration around the resonance can be realized. Exact analytical expressions for the coherences in the steady-state limit have been derived in the absence of an incoherent pump rate (keeping all orders of the system parameters) to generalize the analysis, and any restriction on the system parameters has been avoided to make it applicable to various atomic and molecular systems.
The coherence of a four-level Λ-type system with spontaneously generated coherence(SGC) is studied.With a weak probe and a strong coupling fields,whether the atomic system is on two-photon resonance or not,there is light without population inversion(LWI) on the probe field.On two-photon resonance,LWI is strengthened by SGC.On non-two-photon resonance,LWI can be controlled simply by adjusting the parameter of SGC and the two-upper-level separation.Meanwhile,the analytical solutions to the SGC parameter where LWI occurs are given in both cases.
The effects of spontaneously generated coherence (SGC) on the propagation of a weak probe pulse are investigated in a ladder-type atomic system. Due to SGC, the properties of the dispersion and the absorption of a three-level ladder-type system are changed greatly. With proper parameters, the system can produce normal and anomalous dispersion regions in the gain region. The effects of the incoherent pumping and the relative phase on the group velocity are also studied. They all can be used as switches to manipulate the group velocity of the weak probe field from gain-assisted sub- to superluminance.