'Fast Light' Offers No Increase in Information Velocity

A group of researchers from Duke University in Durham, N.C., and from the University of Arizona in Tucson has experimentally confirmed that superluminal pulses of light cannot transmit information faster than c, the vacuum speed of light. The laboratory experiments, which were reported in the Oct. 16 issue of Nature, validate the predictions of special relativity and preserve notions of causality even for such examples of "fast light."

In recent years, scientists had discovered that anomalous dispersion media, which elongate shorter wavelengths and shorten longer ones, do not produce severe distortions of optical pulses traveling through them, as earlier hypotheses defending Einstein's theory had suggested. As a result, physicists had to reconsider what it means for light to carry information, following the work of Leon Brillouin in the early 1900s. Some predicted that the group velocity of a light pulse may be greater than c but that the frontal velocity of a step function on the pulse never could exceed it.

The new work confirms this prediction. Two 20-cm-long, laser-pumped potassium vapor cells yielded superluminal pulses with peaks that advanced by more than 27 ns as they passed through the apparatus. To encode the pulses with information, the researchers used an acousto-optic modulator driven by a computer-controlled arbitrary waveform generator to create high or low amplitudes in their Gaussian functions.

Although the pulses carrying this information traversed the experimental setup at superluminal velocities, it took approximately 1.5 ns longer to detect the information in the received signal than it did when using pulses that traveled through a vacuum, indicating a "speed of information" much slower than c. The advanced peaks that emerged from the cells thus were causally unrelated to the peaks that entered.