© Reprinted by permission from Macmillan Publishers Ltd: Nature 397, 594-598 (18 February 1999).
The cigar-shaped, nanokelvin-cold atom cloud is held by the electromagnet in the middle of the vacuum chamber. The cloud is first illuminated from the side by the coupling laser beam, and then a probe laser pulse is injected. We wait behind the vacuum chamber for the light pulse to come out and measure the arrival time with a photomultiplier (PMT). To figure out the light's speed, we also need to know how long the atom cloud is. We send a third laser beam (imaging beam) up through the cloud and the atoms create an absorption shadow in the beam that is imaged onto a camera. We now take a snapshot of the atom cloud that is seen at the top right of the figure. The atom cloud in this case is 229 micrometers long. (Unit: 7)