Three laser beams and an ultracold cloud of sodium atoms (size exaggerated) in a high vacuum lie at the heart of the slow-light experiment. The coupling beam interacts with the cloud, making it transparent but molasseslike to a pulse of the probe beam. A photomultiplier tube measures the pulse's time of arrival to better-than-microsecond precision. The imaging beam then measures the length of the cloud by projecting its shadow onto a camera. Not shown are the system that delivers and cools a new ultracold cloud for each pulse, electromagnets whose combined field holds the atoms in place, and additional details of the optics.