This is a summary from Physics World of the paper: L J Wang et
al. 2000 Nature 406 277-- "Wang and colleagues begin by using a
third continuous-wave laser to confirm that there are two peaks
in the gain spectrum and that the refractive index does indeed
change rapidly with wavelength in between. Next they send a
3.7-microsecond long laser pulse into the caesium cell, which is
6 centimetres long, and show that, at the correct wavelength, it
emerges from the cell 62 nanoseconds sooner than would be
expected if it had travelled at the speed of light. 62
nanoseconds might not sound like much, but since it should only
take 0.2 nanoseconds for the pulse to pass through the cell,
this means that the pulse has been travelling at 310 times the
speed of light. Moreover, unlike previous superluminal
experiments, the input and output pulse shapes are essentially
the same." I realise that the velocity of light in a medium is
comprised of the phase velocity, the group velocity, and the
front velocity. While the group veloc