Optical precursors

This article reviews the history of the search for optical precursors over the past 100 years and the most recent progress. Although Einstein’s special relativity based on the principle of light speed invariance in vacuum has been accepted for more than a century, there are still different pictures about the speed of light in a dispersive medium, especially with regard to information velocity and the motion of a single photon. The study of optical precursors aims to answer these questions. Recently, we found clear signatures of precursors in optical pulse propagation, and for the first time, observed the precursor of a single photon. Our results confirm that information carried by light cannot travel faster than the speed of light in vacuum, and a single photon obeys the speed limit, even in a so-called“superluminal”medium (where the group velocity is faster than the speed of light in vacuum).