Read the "News and Views" article by Jon Marangos in this week's
Nature. It explains the phenomenon in terms of anomalous dispersion.
It is clearer than the Letter by Wang et al itself.
If I understand the article this phenomenon is less startling than
it appears at first. From Marangos:
"When they sent a 3.7 microsecond pulse through the [six centimetre
long caesium filled optical cell], it appeared at the exit of the
cell before it arrived at the entrance. although the pulse itself
is only shifted forward in time by a modest fraction (1.7 percent)
of its width, this corresponds to the wavepacket leaving the cell
62 nanoseconds before it arrives - in other words, travelling
nearly 20 metres away from the cell before the incoming pulse
enters it."
I note that if these two wavepackets are plotted in space the
overlap is enormous, since the pulses are both more than a kilometer
long. Stiil, if one does the calculation in a mindless manner, the
group velocity within the cell appears to be vg = - c/310, since
the peak of the pulse required -62 nanoseconds to traverse a
distance which would be covered by light in vacuum in 0.2 ns.
I'm a big fan of Nature because it publishes these "News and Views"
summaries. "Science" is good that way, too. Without these I would
find it much more difficult to understand what is going on. Students
come to me several times each week (especially on Thursday mornings)
to ask about one or another startling breakthrough in astrophysics
or low temperature physics or what have you after reading about it
in a newspaper or hearing it on radio or televisioon. They expect me
to know! Fortunately "Nature" is now on line, so I can ungarble the
media presentations for them in many cases. I don't have similar
access to "Science", but the Friday press embargo usually saves me
there.