© Reprinted by permission from Macmillan Publishers Ltd: Nature 397, 594-598 (18 February 1999).
A probe light pulse propagates through the cold atom cloud shown in Figure 13 (Section 6) and is then measured with a photomultiplier. The data are recorded with an oscilloscope, and the result is the red pulse shown in the figure. The blue pulse is recorded with no atoms in the system and simply sets the zero point for the time axis. Since the light pulse takes 7 microseconds to travel though an atom cloud that is only 229 micrometers long, we can immediately conclude that light has been slowed to only 32 meters/second—slowed by a factor of 10 million! (Unit: 7)