Slow and fast light effects induced by interference with a control light field

The group velocity of light is governed by the dispersive properties of the optical material. Controlling these properties has led to concepts such as fast and slow light. In a vacuum, the group velocity of light is equal to c, excluding a priori the possibility of controlling the propagation time of a light pulse. Here, we prove that to some extent, this limitation can be circumvented by exhibiting a specific class of periodic light pulses that can be advanced or delayed by simple linear interference with a monochromatic light field. We show that such pulses are easily generated in a recirculating frequency shifting loop, and we experimentally demonstrate the possibility of advancing or delaying these pulses by times exceeding the nanosecond.