Measuring pulse speed and delay
This figure provides concrete evidence that light can be slowed to very low speeds. A reference pulse is first sent into the experimental set-up without any atoms present. It sets the zero-point on the time axis from which delays are measured. The intensity of the laser pulse after it emerges from the system is measured with a photomultiplier tube. We then let atoms into the system, cool them down and send in another probe light pulse, which we measure after it has passed through the atom cloud (red line). This pulse has been delayed by more than 7 µs in a cloud that is only 0.2 mm long giving a light speed of 32 m s1. By varying the intensity of the coupling laser, we can control the speed of the pulse and the time it spends in the cloud. The coupling laser also enables the probe pulse to pass through what would otherwise be an opaque medium.