A scheme for controlling light speed from slower-than-c to faster-than-c inan atomic system is presented in this paper. The scheme is based on fardetuning Raman effect. Two far detuning coupling fields with small frequencydifference will produce two absorptive peaks for the probe field in a $\Lambda$structure, and an optical pump between the two ground states can change theabsorptive peaks into enhanced peaks, which makes the normal dispersion betweenthe two peaks change into anomalous dispersion, so the probe field can changefrom slow light to superluminal propagation.
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