Light Pulse Breaks

A team at NEC Research Institute in Princeton, N.J., has reported the gain-assisted propagation of a light pulse through a gas cell at 310 times the speed of light in a vacuum. The researchers stressed that, while the pulse effectively left the cell before it had entered, the results do not violate causality or special relativity if the signal velocity of light is defined by the frontal velocity of a step-functionlike feature in the waveform.

The researchers, who published the results of their experiment in the July 20 issue of Nature, sent a 3.7 µs pulse from a diode laser through a Pyrex cell filled cesium and placed ina 1-G magnetic field. They measured a lead time of roughly 62ns, the equivalent of the pulse traveling nearly 20 m from the exit point before it had entered the cell.