Direct Measurement of the Group Velocity of Light

We propose and demonstrate a method to directly measure with high accuracy the group velocity of light. We make use of the natural fluctuations of light and determine in a correlation experiment the coincidence of fluctuations of light passing through a sample path and a reference path with known delay. It is shown that this coincidence can be detected by using amplitude interferometers such as a Michelson interferometer, or intensity interferometers. Accuracies of the order of 10^-6 are practically possible. That is orders of magnitude better than previous methods allow. Using a Nd:glass laser as light source and an intensity interferometer for detecting coincidence, we demonstrate our method by measuring the group velocity of light in water within 2 ×{} 10^-4 using a sample length of only 15 cm. The limitations of this new method caused by second-order dispersion are discussed quantitatively; higher-order effects are discussed qualitatively.