The manipulation of light pulse from subluminal to superluminal propagation in a degenerate two-level Cs atomic system

We experimentally demonstrate the propagation of light pulse from subluminal to superluminal light based on quantum coherence in a degenerate two-level atomic system in a Cs vapor cell. It is shown that the group velocity of light pulse can be switched from subluminal to superluminal propagation via changing the coupling field from a traveling wave to a standing wave, while can also be continuously manipulated by varying the intensity of two waves superposed to form a standing wave. The observed maximum delay and advance times are about 0.45 and 0.54 μs, corresponding to the group velocity of V _s = 168 km/s and v _g =?138 km/s, respectively. This investigation may have the practical applications of devices for optical tunable delay lines, optical switching and optical buffering.